Respiration in Plants
Pyruvic dehydrogenase
multienzyme complex→
CH 3CO.COOH + CoA.SH + NAD **TPP CH3.CO.S.CoA+ NAD.2H + CO2
(Pyruvic acid) (CoA) ** LAA (acetyl -S-CoA)
**TPP=Thiamine pyrophosphate
**LAA=Lipoic acid amide
Acetyl CoA is a common intermediate of carbohydrate and fat metabolism. Latter this acetyl CoA from both
the sources enters Kreb's cycle. This reaction is not a part of Kreb's cycle.
(2) Kreb's cycle / TCA cycle / Citric acid cycle
(i) Discovery : This cycle has been named after the German biochemist in England Sir Hans Krebs who
discovered it in 1937. He won Noble Prize for this work in 1953. Krebs cycle is also called the citric acid cycle
after one of the participating compounds.
(ii) Site of occurrence : It takes place in the mitochondrial matrix.
(iii) Kreb’s cycle Fat Proteins
β-Oxidation Pyruvic acid Alanine Amino acids
Glycine
2 Acetyl CoA Pyruvic NAD
dehydrogenase NADH2
1/2 O2 CO2
H2O
Aceto acetyl CoA CoA
Acetyl CoA
Synthetase
Aspartic acid H2O
CoA
Oxalo acetic acid
Citric acid
NADH2 Malic
NAD dehydrogenase Aconitase Fe++
H2O
H2O←1/2 O2
Malic acid Cis-aconitic acid
H2O Aconitase H2O
Fumarase Fe++
Fumaric acid 33 Isocitric acid 6C
H2O←1/2 O2 Succinic ATP 3 1/2 O2→H2O
dehydrogenase 2 POOL
FADH2
FAD 13 NAD
NADH2
Succinic acid Isocitric
dehydrogenase Mn++
H2O
ATP Succinic
ADP GTP thiokinase
GDP
4C Succinyl CoA Oxalo succinic acid 6C
Decarboxylase
Chapter 20 Ketoglutaric Mn++
dehydrogenase CO2
5C
CO2 NADH2 352
NAD
H2O←1/2 O2 α-Ketoglutaric acid
Respiration in Plants
(iv) Enzymes of Kreb's cycle
Step Enzyme (Location in mitochondria) Coenzyme(s) and Inhibitor(s) Type of reaction
cofactor (s) catalyzed
(a) Citrate synthetase Matrix space CoA Monofluoro-acetyl- Condensation
CoA
(b) Aconitase Inner membrane Fe2+ Fluoroacetate Isomerization
(c) Isocitrate dehydrogenase Matrix space NAD+, NADP+, ATP Oxidative
Mg2+, Mn2+ decarboxylation
(d) α -ketoglutarate Matrix space TPP,LA,FAD,CoA, Arsenite,Succinyl- Oxidative
NAD+ CoA, NADH decarboxylation
dehydrogenase complex
(e) Succinyl-CoA synthetase Matrix space CoA - Substrate level
phosphorylation
(f) Succinate dehydrogenase Inner membrane FAD Melonate, Oxidation
Oxaloacetate
(g) Fumarase Matrix space None - Hydration
(h) Malate dehydrogenase Matrix space NAD+ NADH Oxidation
(v) Steps in Kreb's cycle : Kreb's cycle consists of 8 cyclic steps, producing an equal number of organic
acids. Each step is catalyzed by a specific enzyme. In Kreb's cycle, the entrant molecule is 2-carbon acetyl CoA
and the receptor molecule is 4- carbon oxaloacetate.
(a) condensation : Acetyl coenzyme A reacts in the presence of water with the oxaloacetate normally
present in a cell, forming 6-carbon citrate and freeing coenzyme A for reuse in pyruvate oxidation. The high-energy
bond of acetyl CoA provides the energy for this reaction. The reaction is catalyzed by the citrate synthetase
enzyme. The citrate has 3-carboxyl group. Hence, Krebs cycle is also called tricarboxylic acid cycle, or TCA
cycle after its first product.
Oxaloacetate + Acetyl CoA + H 2O Citrate → Citrate + CoA (Reused)
Synthetase
(b) Reorganisation (Dehydration) : Citrate undergoes reorganisation in the presence of an enzyme,
aconitase , forming 6-carbon cisaconitate and releasing water.
Citrate Aconitase → Cisaconitate + H2O
(c) Reorganisation (Hydration) : Cisaconitate is further reorganised into 6-carbon isocitrate by the
enzyme, aconitase, with the addition of water.
Chapter 20 353
Respiration in Plants
Cisaconitate + H2O Aconitase → Isocitrate
(d) Oxidative decarboxylation I : This is a two stage process :
Stage I : Hydrogen atoms from isocitric acid react with NAD to form NAD. 2H forming oxalosuccinic acid.
The pair of hydrogen atoms give two electrons and one H+ to NAD+ forming NADH +H+. The enzyme
isocitrate dehydrogenase catalyses the reaction in the presence of Mn2+. NADH generates ATP by transferring its
electron over the ETS.
Isocitric acid + NAD Isocitricdehydrogenase → Oxalosuccinic acid + NAD.2H(or NADPH.2H)
Mn2+
Stage II : Decarboxylation of oxalosuccinic acid occurs forming α -ketoglutaric acid, which is a first 5-C
carbon molecule of Kreb's cycle.
Oxalosuccinic acid Carboxylase →α − Ketoglutaric acid + CO2
Mn2+
(e) Oxidative decarboxylation II : This is also a 2 stage process :
Stage I : Coenzyme A reacts with α -ketoglutarate, forming 4-carbon succinyl-coenzyme A and releasing
CO2 and a pair of hydrogen atoms. The reaction is catalysed by α -ketoglutarate dehydrogenase complex enzyme.
the pair of hydrogen atoms pass two electrons and one H+ to NAD+, forming NADH + H+
α − ketoglutarate + CoA + NAD + α −ketoglutarate → Succinyl − CoA + CO2 + NADH + H+
dehydrogenase
Stage II : Succinyl –coenzyme A splits into 4-carbon succinate and coenzyme A with the addition of water.
The coenzyme A transfers its high energy to a phosphate group that joins GDP (Guanosine diphosphate), forming
GTP (Guanosine triphosphate). The latter is an energy carrier like ATP. This is the only high-energy phosphate
produced in the Krebs cycle. The stage 2 reaction is catalysed by succinyl-CoA synthetase enzyme. The
formation of GTP is called substrate level phosphorylation.
Succinyl − CoA + H 2O + GDP / ADP Succinyl−CoA → Succinate + CoA + GTP / ATP
Synthetase
In a plant cell, this reaction produce ATP from ADP and GTP from GDP or ITP (Inosine triphosphate) in
animals.
Oxygen to oxidize a carbon atom to CO2 is taken in steps 4 and 5 from a water molecule.
(f) Dehydrogenation : This process converts succinate into 4-carbon fumarate with the aid of an enzyme,
succinate dehydrogenase, and liberates a pair of hydrogen atoms. The latter pass to FAD+ (Flavin adenine
dinucleotide), forming FADH2. Hydrogen is carried by FAD in the form of whole atoms.
Succinate+FAD+ Succinate Fumarate +FADH2
dehydrogenase
(g) Hydration : This process changes fumarate into 4-carbon maltate in the presence of water and an
enzyme, fumarase.
Fumarate +H2O Fumarase Maltate
(h) Dehydrogenation : This process restores oxaloacetate by removing a pair of hydrogen atoms from
maltate with the help of an enzyme maltate dehydrogenase. The pair of hydrogen atoms pass two electrons and
one H+ to NAD+ , forming NADH+H+.
Maltate +NAD+ Maltate Oxaloacetate +NADH +H+
dehydrogenase
Oxaloacetate combines with acetyl coenzyme A to form citrate, and so the cycle continues.
Chapter 20 354
Respiration in Plants
(vi) Summary of Kreb's cycle
(a) All the enzymes, reactants, intermediates and products of TCA cycle also are found in aqueous solution in
the matrix, except the enzyme α-ketoglutarate dehydrogenase and succinate dehydrogenase which are located in
the inner mitochondrial membrane. Both are called mitochondrial marker enzyme.
(b) Oxidation of one mole of acetyl CoA uses 4 molecules of water and releases one molecule of water.
(c) Liberates 2 molecules of carbon dioxide.
(d) Gives off 4 pairs of hydrogen atoms.
(e) Produces one GTP/ ATP molecule during the formation of succinate.
(f) One mole of acetyl CoA gives 12 ATP during oxidation in Krebs cycle.
(g) Regenerates oxaloacetate used in last cycle for reuse.
The above summary is for one molecule of acetyl coenzyme A. There are two acetyl coenzyme A molecules
formed from one molecule of glucose by glycolysis and oxidative decarboxylation of pyruvate. The entire Krebs
cycle may be represented by the following equation –
2Acetyl coenzyme A + 8H2O + 6NAD+ + 2FAD+ + 2GDP / ADP + 2Pi
→ 4CO2 + 2H2O + 6 NADH + 2FADH2 + 2GTP / ATP + 6H +
(vii) Difference between Glycolysis and Kreb's cycle
Glycolysis Kreb’s cycle
It takes place in the cytoplasm. It takes place in the matrix of mitochondria.
It occurs in aerobic as well as anaerobic respiration. It occurs in aerobic respiration only.
It consists of 9 steps. It consists of 8 steps.
It is a linear pathway. It is a cyclic pathway.
It oxidised glucose partly, producing pyruvate. It oxidises acetyl coenzyme A fully.
It consumes 2 ATP molecules. It does not consume ATP
It generates 2 ATP molecules net from 1 glucose molecules. It generates 2 GTP/ATP molecules from 2 succinyl coenzyme A
molecules.
It yields 2 NADH per glucose molecule. It yields 6 NADH molecules and 2 FADH2 molecules from 2 acetyl
coenzyme A molecules.
It does not produce CO2. It produces CO2.
All enzyme catalysing glycolytic reactions are dissolved in cytosol.
Two enzymes of Krebs cycle reactions are located in the inner
mitochondrial membrane, all others are dissolved in matrix.
(viii) Product form during aerobic respiration by Glycolysis and Kreb’s cycle.
(a) Total formation of ATP
ATP formation by substrate ATP formation in Glycolysis Product of reactions In terms of ATP
phosphorylation 2 ATP 2 ATP
Steps 2 ATP 2 ATP
1, 3-diphosphoglyceric acid (2 moles) → Total 4 ATP
3 phosphoglyceric acid (2 moles)
Phosphoenolpyruvic acid (2 moles) →
Pyruvic acid (2 moles)
Chapter 20 355
Respiration in Plants
ATP formation by oxidative 1, 3 - disphosphoglyceraldehyde (2 moles) 2 NADH2 6 ATP
phosphorylation or ETC 1, 3 – diphosphoglyceric acid (2 moles) 10 ATP
ATP consumed in Glycolysis 4 + 6 ATP = – 1 ATP
Total ATP formed – 1 ATP – 1 ATP
2 ATP
Glucose (1 mole) → Glucose 6 phosphate (1 mole) – 1 ATP 8 ATP
Fructose 6 phosphate (1 mole) → Total
2 ATP
Fructose 1, 6-diphosphate (1 mole) 10 ATP – 2ATP
2 ATP
Net gain of ATP = total ATP formed – Total 6 ATP
ATP consumed
6 ATP
ATP formation in Kreb’s cycle 6 ATP
4 ATP
ATP formation by substrate Succinyl CoA (2 mols) → 2 GTP 6 ATP
phosphorylation Succinic acid (2 mols) 28 ATP
30 ATP
Total
38 ATP
ATP formation by oxidative Pyruvic acid (2 mols) → 2 NADH2 34 ATP
phosphorylation or ETC Acetyl CoA (2 mols)
Isocitric acid (2 mols) → 2 NADH2
Oxalosuccinic acid (2 mols) 2 NADH2
2 FADH2
α-Ketoglutaric acid (2 mols) → 2 NADH2
Succinyl CoA (2 mols)
Succinic acid (2 mols) →
Fumaric acid (2 mols)
Malic acid (2 mols) →
Oxaloacetic acid (2 mols)
Total
Net gain in Kreb’s cycle (substrate 2ATP + 28 ATP
phosphorylation + oxidative
phosphorylation)
Net gain of ATP in glycolysis Net gain of ATP in glycolysis + Net 8 ATP + 30 ATP
and Kreb’s cycle gain of ATP in Kreb’s cycle
Over all ATP production by ATP formed by oxidative 6 ATP + 28 ATP
oxidative phosphorylation or phosphorylation in glycolysis + ATP
ETC formed by oxidative phosphorylation
or ETC.
22 ATP produced by oxidation of NADH2 and FADH2 in Kreb’s cycle and 6 ATP comes from oxidative
decarboxylation of pyruvic acid.
(b) Formation and use of water
Formation of water molecules 2H2O
Formation of water molecules in 2 phosphoglyceric acid (2 mols) −H2O → 2H2O
glycolysis
2 phosphoenol pyruvic acid (2 mols)
1, 3-diphosphoglyceraldehyde −H2O →
Chapter 20 356
Respiration in Plants
Formation of water molecules in 1, 3 diphosphoglyceric acid 4H2O
kreb’s cycle 10 H2O
Total water molecules formed in glycolysis 2H2O
Use of water in Glycolysis 12 H2O
One molecule of water in each of the five oxidation reactions (these reactions 16 H2O
occur twice as there are two molecules of pyruvic acid).
Other than oxidation reaction 2H2O
Citric acid (2 mols) → Cis-aconitic acid (2 mols)
2H2O
Total water molecules formed in Kreb’s cycle 2H2O
2H2O
Total water molecules formed in aerobic respiration (Glycolysis + 2H2O
Kreb’s cycle) 2H2O
Use of water molecules 8H2O
10H2O
3-phosphoglyceraldehyde (2 mols) +H2O → 1, 3 diphosphoglyceric acid (2 6H2O
mols)
2CO2
Total water molecule used in glycolysis 2CO2
2CO2
Use of water in Kreb’s cycle Oxaloacetic acid (2 mols) +H2O → Citric acid (2 mols) 6CO2
Cis aconitic acid (2 mols) +H2O → Isocitric acid (2 mols) 1O2
Succinyl CoA (2 mols) +H2O → Succinic acid (2 mols) 5O2
6O2
Fumaric acid (2 mols) +H2O → Malic acid (2 mols)
Total water molecules used is Kreb’s cycle
Total water molecules used in aerobic respiration (Glycolysis + Kreb’s cycle)
Net gain of water molecules in Number of water molecules formed – Number of water molecules
aerobic respiration used = ( 16 H2O – 10H2O)
(c) Evolution of carbon dioxide
Pyruvic acid (2 mols) −CO2 → Acetyl CoA (2 mols)
Oxalosuccinic acid −CO2 → α ketoglutaric acid (2 mols)
α Ketoglutaric acid (2 mols) −CO2 → Succinyl CoA (2 mols)
Total CO2 molecules released in aerobic respiration
(d) Use of O2 (Oxygen) 1, 3-diphosphoglyceraldehyde (2mols) + 12O2 → 1, 3-diphosphoglyceric
acid (2 mols)
Use of oxygen in Glycolysis
Five oxidation reactions of Kreb’s cycle (2 times)
Use of oxygen in Kreb’s cycle Total O2 molecules required for aerobic respiration
Chapter 20 357
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(ix) Energy storage and energy transfer : In respiration energy released takes in the form of chemical
energy, stored in a form called ATP . Energy transfer of biological oxidation hinges on the formation of labile high
energy phosphate bonds of ATP. Nicotinamide adenine dinucleotide phosphate (NAD), Flavin adenine dinucleotide
(FAD), Guanosine triphosphate are also the product of respiration and converted to ATP by electron transport
system.
(a) Adenosine triphosphate : An energy intermediate :
There are several compounds like NAD, FAD, GTP and ATP are known as energy yielding compounds. The
best known, and probably the most important of these are adenosine triphosphate (ATP). It serves as the energy
currency of the cells.
• Structure of ATP : Adenosine triphosphate is a NN Adenine
nucleotide consisting of three main constituents; Ribose
High energy bonds
(a) A nitrogen contain purine base
O OO NN
(b) A five carbon sugar ribose
|| || ||
(c) Three inorganic phosphate groups O — P ~ O — P ~ O — P ~ O — CH2
| || O
The bonds attaching the last two phosphate to the O OO
rest of the molecule are high energy bonds (~) contain
more than twice the energy of an average chemical bond. Inorganic phosphate H HH
H
OH OH
Fig : Structure of ATP
• ATP hydrolysis : The energy is usually released from ATP by High energy bond
hydrolysing the terminal phosphate groups. Each molecule on hydrolysis yields
ADP, one inorganic phosphate group (Pi) and about 7.28 Kcal energy. Adenosine P P P
H2O Hydrolysis P +30.6 Kj mol
ADP further hydrolysed to AMP and inorganic phosphate, releasing 7.3 Kcal
energy per molecule (of ATP). Above process represented by following reactions. Adenosine P P
Work
Fig : Hydrolysis of ATP
Adenosine triphosphate hydrolysis → Adenosine diphosphate(ADP) + Pi + 7.3Kcal.....
hydrolysis
Adenosine diphosphate → Adenosine monophosphate(AMP) + Pi + 7.3Kcal.
• Phosphorylation : The ATP hydrolysis reactions are reversible because ATP are synthesized from ADP,
Pi and energy (take up for the bond formation). The addition of phosphate group to ADP and AMP called
phosphorylation. Energy required for the bond formation is equal to the energy released in hydrolysis.
PP Adenosine PP
ADP
Energy yield of
30.6 kJ or more
from respiration
Adenosine PPP
ATP
Fig : phosphorylation
Chapter 20 358
Respiration in Plants
The significant role of ATP as an intermediate energy transfer compound
• Major functions of ATP : ATP molecules receive the energy, which released in exergonic reactions and
make this energy available for various endergonic reactions. Some of the important process in which ATP is utilized
are as follows :
(a) Synthesis of carbohydrates, proteins, fats, etc.
(b) Translocation of organic food.
(c) Absorption of organic and inorganic food.
(d) Protoplasmic streaming.
(e) Growth. CO2
H2O
Respiration of carbohydrates,
fats and proteins
NADP ADP Pi ADP NADPH
Pi Synthesis of
Pi carbohydrates, fats and
proteins
Absorption and
translocation of solutes
Streaming of protoplasm
Pi
Electrical potential
Pi
Growth
Pi
Fig : Schematic representation of the role of ATP
(b) Nicotinamide dinucleotide phosphate/ Nicotinamide dinucleotide (NADP/NAD) : It is called
universal hydrogen acceptor, produced during aerobic respiration (glycolysis+ Kreb's cycle) and also in anaerobic
respiration, work as coenzyme in ATP generation Via electron transport system. NADP have one additional
phosphate.
Structure of NAD = Nicotinamide-adenine-dinucleotide (formerly called coenzyme I or CO-I
Diphosphopyridine dinucleotide = DPN) is shown below :
[Nicotinamide]
[Adenosine] Adenine O ⊕
OH N
Ribose P O P OCH2 O
OO HH
HH
Chapter 20
3O5H9OH
Fig : Reduced NAD
Respiration in Plants
It plays a crucial role in dehydrogenation processes. Some dehydrogenases do not work with NAD, but react
with NADP (Nicotinamide adenine dinucleotide phosphate). Formerly called Coenzyme II or Triphosphopyridine
nucleotide = TPN Nicotinamide is a vitamin of B group.
First NAD and NADP both functions as hydrogen acceptors. Later H ions and electrons (e_) from these are
transported through a chain of carriers and after being released at the end of a chain react with O2 and from H2O
(see Electron Transport chain). During the release of 2 electron from 2H+ atoms from NAD. 2H and their reaction
with O2 to form water, 3 ATP molecules are synthesized.
Important Tips
• Krebs cycle is the central pathway of the cell respiration where the catabolic pathways converge upon it an anabolic pathways diverge
from it, so called amphibolic pathway.
• Acetyl Co~A, also called active acetate.
• Number of oxidation steps(dehydrogenation) for pyruvic acid is 5.
• In Kreb’s cycle, acetyl CoA undergoes two decarboxylation and four dehydrogenation. Krebs cycle catabolises about 80-90% of
glucose.
• ATP discovered by Lohmann (1929), term was coined by Lohmann (1931) while ATP cycle was discovered by Lipmann (1941).
• Allosteric inhibition or negative feedback by accumulation of NADH2.
• Production of 36 ATP molecules from the oxidation of glucose is only an estimate as :
(i) NADH2 may be used in some other metabolic pathway.
(ii) NADH2 do not always produces 3 ATP molecules. More active muscle cells produces more while less active fat cell produce less
ATP molecules.
(iii) Not all the proteins are routed through F0 –F1 channel.
(iv) So realistic aerobic respiratory efficiency ranges between 22% to 38%, as realistic power limit is 21 ATP molecule.
(3) Electron transport system : The electron transmitter system is also called electron transport chain
(ETC), or cytochrome system (CS), as four out of these seven carriers are cytochrome. It is the major source of cells
energy, in the respiratory breakdown of simple carbohydrates intermediates like phosphoglyceraldehyde, pyruvic
acid, isocitric acid, α − ketoglutaric acid, succinic acid and malic acid are oxidised. The oxidation in all these
brought about by the removal of a pair of hydrogen atoms (2H) from each of them. This final stage of respiration is
carried out in ETS, located in the inner membrane of mitochondria (in prokaryotes the ETS is located in
mesosomes of plasma membrane). The system consists of series of precisely arranged seven electron carriers
(coenzyme) in the inner membrane of the mitochondrion, including the folds or cristae of this membrane. These
seven electron-carriers function in a specific sequence and are :
Nicotinamide adenine dinucleotide (NAD), Flavin mononucleotide (FMN), Flavin adenine dinucleotide (FAD),
Co-enzyme-Q or ubiquinone, Cytochrome-b, Cytochrome-c, Cytochrome-a and Cytochrome-a3,
Glucose
Glycolysis Diphospholyceraldehyde or
Diphosphoglyceric acid
Pyruvic acid Glycerol phosphate shuttle
Malate-Aspartate shuttle
Acetyl-CoA
Isocitric acid
Oxalosuccinic acid ADP+iP ATP ADP+iP
(Kreb’s cycle)
Chapter 20Oxaloacetic acid 3602e FAD
α-Ketoglutaric acid NAD2H CoQRED 2e 2Fe+++ 2Fe++ 2e 2Fe+++ 2Fe++ 2e 2Fe+++ O2
NAD+ 2 Cyt c1 2 Cyt a 2 Cyt a3
Succinal-S CoA 2e 2 Cyt b 2Fe+++ 2 Cyt c 2Fe+++
2Fe++
2H+ FADH2 2H+ CoQOX 2Fe++ 2e 2Fe++ 2e ½ O2
Malic acid
Respiration in Plants
The first carrier in the chain is a flavoprotein which is reduced by NADH2. Coenzyme passes these electron to
the cytochromes arranged in the sequence of b-c-a-a3, finally pass the electron to molecular oxygen. In this
transport, the electrons tend to flow from electro-negative to electro-positive system, so there is a decrease in free
energy and some energy is released so amount of energy with the electrons goes on decreasing. During electron-
transfer, the electron-donor gets oxidised, while electron-acceptor gets reduced so these transfers involve redox-
reaction and are catalysed by enzymes, called reductases. Oxidation and reduction are complimentary. This
oxidation-reductiion reaction over the ETC is called biological oxidation.
Electron − donor → e- + electron − acceptor
here, electron-donor and electron –acceptor form redox pair.
During the electron transfers, the energy released at some steps is so high that ATP is formed by the
phosphorylation of ADP in the presence of enzyme ATP synthetase present in the head of F1-particles present on
the mitochondrial crista. This process of ATP synthesis during oxidation of coenzyme is called oxidative
phosphorylation, so ETS is also called oxidative phosphorylation pathways.
ADP + Pi ATPSynthetase → ATP
From the cytochrome a3, two electrons are received by oxygen atom which also receives two proton (H+) from
the mitochondrial matrix to form water molecule. So the final acceptor electrons is oxygen. So the reaction
H2 + 1 O2 → H2O (called metabolic water) is made to occur in many steps through ETC, so the most of the
2
energy can be derived into a storage and usable form.
(i) Two route systems of ETC : The pairs of hydrogen atoms from respiratory intermediates are received
either by NAD+ or FAD coenzymes which becomes reduced to NADH2 and FADH2. These reduced coenzyme pass
the electrons on to ETC. Thus, regeneration of NAD+ or FAD takes place in ETC. There are two routes ETC :
(a) Route 1 : NADH2 passes their electrons to Co-Q through FAD . In route 1 FAD is the first electron carrier.
3 ATP molecules are produced during the transfer of electron on following steps :
NAD to FAD
Cyt b to Cyt c and
Chapter 20 361
Respiration in Plants
Cyt a to Cyt a3 molecules are produced during the
(b) Route 2 : FADH2 passes their electron directly to FAD. 2 ATP
transfer of electron on following steps.
Cyt b to Cyt c and
Cyt a to Cyt a3
(ii) Structure of mitochondria in relation to oxidative function : On inner side of mitochondria
elementary particles or F0-F1 complex of ATPase complex or elementary particle (oxysomes) are found. Previously it
was considered that elementary particles contain all the enzyme of oxidative phosphorylation and electron transport
chain.
Component of electron transport chain are located in the inner membrane in the form of respiratory chain
complexes. For complexes following theories are given :
(a) Four complex theory : According to Devid green electron transport chain contains 4 complexes-
Complex I : Comprises NADH dehydrogenase and its 6 Iron Sulphur centers (Fe-S).
Complex II : Consists of Succinate dehydrogenase and its 3 Iron Sulphur centers.
Complex III : Consists of cytochrome b and c, and a specific Iron-Sulphur centers.
Complex IV : Comprises cytochromes a and a3.
Outer Inner
membrane membrane
Complex-I
NADH ATP
Complex-III
Complex-IV
ATP ATP
O2
Succinate
Stalk Head piece
Outer Base-piece
chamber Complex-II
Fig. Four complexes of Oxysome
(b) Five complex theory : According to Hatefi, (1976), Complex I to Complex IV are related to the electron
transport.
Complex V related to mainly with ATP synthesis, so it is called ATPase /ATP syntheses complex.
The head piece (F1) of the oxysome consists of 5 hydrophobic subunits (α, β,γ ,δ ,ε ), which are
responsible for ATPase functioning.
The stalk (F0) contain F5 (oligomycin sensitivity conferring protein) i.e. CSCP and F6. F0 are related to the
proton channel and embeded fully in thickness of inner mitochondrial membrane.
Five complex i.e. I, II, III, IV, V, have been isolated from mitochondrial membrane by chemical treatment.
Complex I : NADH/NADPH : CoQ reductase
Complex II : Succinate : CoQ reductase
Complex III : Reduced CoQ (CoQH2) : cytochrome C reductase
Complex IV : Cytochrome C oxidase
Chapter 20 362
Respiration in Plants
Complex V : ATPase
Cytochrome C and Q are mobile components of the respiratory chain.
C-side
Base piece
Stalk F6 M-side
F5
CSCP
δγ
βα
Head piece ATPase
(F1 ATPase proper) inhibitor protein
(iii) Oxidative phosphorylatioFnig.:FiTvehceompprolecxeesssofoOfxyAsoTmPe synthesis Intermembrane Inner mitochondrial Matrix
space membrane NADH
during oxidation of reduced coenzymes in ETC is called oxidative +H-+
NAD+
phosphorylation. Peter Mitchell (1961) proposed the chemiosmotic 2H
2H+ 2H FMN
mechanism of ATP synthesis (Noble prize in 1978) which states that ATP
synthesis occurs due to H+- flow through a membrane. It involves two 2e– FeS
steps : FeS
(a) Development of proton gradient. At each step of ETC, the 2H+ QH2 2e– 2H+
electron- acceptor has a higher electron –affinity than the electron-donor. O
The energy from electron-transport is used to move the proton (H+) from
the mitochondrial matrix to inter-membranous or outer chamber. Three 2e– 2e–
Cy b FeS
pairs of protons are pushed to outer chamber during the movement of 2H+ QH2 2e–
electrons along route I while two pairs of protons are moved to outer 2H+
chamber during the movement of electrons along route–II. This generates a 2e– 2e– 1/2O2
pH-gradient across the inner mitochondrial membrane with protons (H+) Cy Cya-a3 2H+
concentration higher in the outer chamber than in the mitochondrial matrix. Cy c H2O
This difference in H+ concentration across the inner mitochondrial
Fig : Proton flow
membrane is called proton-gradient( ∆ pH). Due to proton gradient, an electrical potential (∆ψ) is developed
across the inner mitochondrial membrane as the matrix is now electronegative with respect to the intermembranous
(outer) chamber. The proton gradient and membrane electric potential collectively called proton motive force.
(b) Proton flow : Due to proton-gradient, the protons returns to the matrix while passing through proton
channel of F0-F1 ATPase. This proton gradient activates the enzyme ATP synthetase or F0 – F1 ATPase
ATP synthetase controls the formation of ATP from ADP and inorganic phosphate in the presence of energy.
Matrix 2H+ Inner membrane
ATP particle
Pi
F1 ADP
Chapter 20 3F60 3
2H+
Respiration in Plants
Important Tips
• Cytochromes were discovered by MacCunn and term cytochrome given by K.P.Kailin.
• Iron-Sulphur is the component of ETC (complex I, II, III) and helps in transfer of electrons from FMNH2 to coenzyme Q. Thus,
deficiency of iron direct affect ETC or oxidative phosphorylation.
• Cytochromes are Iron-containing (Iron porphyrin protein) electron transferring (electrons picked up and release by Fe) except
cytochrome a3. Cytochrome a3 contains both Iron and Copper, in this Fe picks the electrons and through Cu it hands over electron to
oxygen, so cytochrome a3 called terminal electron donar.
• Cytochrome P-450 : It occurs in E R and takes part in hydroxylation reaction.
• ETC inhibitors
(i) Dinitrophenol (2,4-DNP) : It prevents synthesis of ATP from ADP because it directs electrons from CoQ to Q2.
(ii) Cyanide : It prevents flow of electrons from Cyt a3 to oxygen.
(iii) Carbon monoxide : It functions like cyanide.
(iv) Antimycin A: Transfer of electron from Cyt b to Cyt c1 is prevented.
(v) Rotenone : It checks flow of electrons from NADH /FADH2 to CoQ.
• Action of ATPase needed Na+ and K+.
• Amount of energy released in ETC :
(i) 12.2 Kcal during transfer of electrons from NAD to FMN.
(ii) 15.2 Kcal during transfer of electrons from Cyt b to Cyt c.
(iii) 24.5 Kcal during transfer of electrons from Cyt a to Cyt a3.
(iv) Role of shuttle system in energy production : Glycolysis occurs in the cytoplasm outside the
mitochondrion in which 2NADH2 molecules are produced but ETC is located along inner mitochondrial membrane,
so NADH2 of glycolysis must enter inside the mitochondrion to release energy. But the inner mitochondrial
membrane is impermeable to NADH2. In mitochondrial membrane, there are 2 shuttle-system, each formed of
carrier-molecule.
These shuttle systems are :
(a) Malate-Aspartate shuttle : It is more efficient and results in the transfer of electron from NAD. 2H in
cytosol to NAD inside the mitochondrion, via NAD. 2H dehydrogenase as follows :
Electrons are transferred from NAD. 2H in cytosol to malate which traverses the inner mitochondrial
membrane and reoxidised to form NAD. 2H thus resulting in the formation of oxaloacetate . Oxaloacetate does not
readily cross the inner mitochondrial membrane and so a transamination reaction is needed to form aspartate which
Chapter 20 364
Respiration in Plants
does traverse this barrier. As a result 3 ATP molecules are generated for each pair of electrons. Thus if this shuttle is
predominant there is a gain of 38 ATP molecules by complete oxidation of one molecule of glucose.
Cytosol Outer Inner Matrix
membrane membrane
NAD.2H NAD Malate
dehydrogenase
Oxaloacetate Malate Malate NAD
Aspartate Oxaloacetate NAD.2H
Aspartate
Fig : Malate-Aspartate shuttle
(b) Glycerol-Phosphate shuttle : It is less efficient and results in the reduction of FAD inside the
mitochondrion.
If this shuttle predominates the electrons from NAD. 2H are transferred to FAD inside the mitochondrion as
follows. NAD. 2H reacts with dihydroxyacetone phosphate (DHAP) in cytosol to form glycerol phosphate which
diffuses through outer mitochondrial membrane to the outer surface of inner membrane. There glycerol phosphate
reacts with membrane dehydrogenase to form dihydroxyacetone phosphate (DHAP) which returns to cytosol. In
this process FAD is reduced to FADH2. Electrons from FADH2 directly pass to Q and other components of ETC and
results in the synthesis of 2 ATP for each molecule of FADH2. In this case complete oxidation of glucose will result in
a gain of 36 ATP molecule.
Cytosol Outer Inner
membrane membrane Matrix
NAD.2H NAD Glycerol
phosphate
Glycerol FAD dehydrogenase
phosphate
DHAP Glycerol
phosphate
DAPH
FADH2
Fig : Glycerol-Phosphate shuttle
Which shuttle predominates depends on the particular species and tissues envolved, for example : 38 ATP are
formed in kidney, heart and liver cell while 36 ATP molecules are formed in muscle cells and nerve cells. In these
cells glycerol-phosphate shuttle is predominant and 2 ATP formed from NADH2.
Other pathways of glucose oxidation.
(1) Entner-Doudoroff pathway
(i) Discovery : Entner-Doudoroff path discovered by Entner & Doudoroff. This pathway is also called
glycolysis of bacteria.
Certain bacteria such as Pseudomonas sacchorophila, P. fluorescens, P. lindeneri and P. averoginosa lack
phosphofructokinase enzyme. They can not degrade glucose by glycolytic process.
Chapter 20 Glucose (6C) 365 Glyceraldehyde
ATP 3-phosphate
Pyruvic acid
ADP (3C)
Glucose-6-phosphate
(6C)
Respiration in Plants
(ii) Description : In this pathway the glucose molecule first phosphorylated to Glucose-6-phosphate by ATP.
Then it oxidised to 6-phosphogluconic acid by NADP which itself reduced to NADPH2 by the electrons released.
The NADPH2 is channeled through ETS system to produce 3-molecules of ATP per NADPH2 molecule through ETS
system to produce 3 molecules of ATP per NADH2 molecule and 1,6-phosphogluconic acid is channeled to pyruvic
acid. The main reaction are :
(a) Glucose - 6 - phosphate + NADP → 6 - Phosphogluconolactone + NADPH 2
(b) 6-phosphoglyconolactone → 2-Keto-3-deoxyphospho-6-gluconic acid
(c) 2-Keto-3-deoxyphosphogluconic acid → Pyruvic acid + Glyceraldehyde-3-phosphate
(d) Glyceraldehyde-3-phosphate → Pyruvic acid
The glyceraldehyde 3-phosphate by EMP pathway gets converted into pyruvic acid which can be further used
up in the process.
(2) Pentose phosphate pathway
(i) Discovery : It is also called as Hexose monophosphate (HMP) shunt or Warburg Dickens pathway
or direct oxidation pathway. It provides as alternative pathway for breakdown of glucose which is
independent of EMP pathway (glycolysis) and Krebs cycle. Its existence was suggested for the first time by Warburg
et al. (1935) and Dickens (1938). Most of the reaction of this cycle were described by Horecker et al.(1951) and
Racker (1954).
(ii) Occurrence : Pentose phosphate pathway that exists in many organisms. This pathway takes place in
the cytoplasm and requires oxygen for its entire operation.
ADP NADPH+H+ NADPH+H+
ATP NADP+ NADP+
Glucose Glucose-6-P 6-Phosphogluconic acid Ribulose-5-P
Dihydroxyacetone-P CO2
Fructose-1, 6-diP
Glyceraldehyde-3-P Fructose-6-P Glyceraldehyde-3-P Xylulose-5-P
Xylulose-5-P Erythrose-4-P Sedoheptulose-7-P Ribose-5-P
Chapter 20 366Fig : Hexose monophosphate shunt
Respiration in Plants
(iii) Description : There are two types of evidences is support of the existence of such an alternative
pathway-works on the inhibiting action of malonic acid on the Krebs cycle and studies with the radioactive (C14).
Twelve molecules of NADH2 formed in the reaction can be oxidised back to 12 NADP with the help of the
cytochrome system and oxygen of the air.
12 NADPH 2 + 6O2 Cytochrome → 12H 2 O + 12NADP
System
In this electron transfer process, 36 molecules of ATP are synthesized.
The reaction can be summarised as follows :
6 Glucose + 6 ATP hexokinase → 6 Glucose - 6 - phosphate
6 Glucose - 6 - phosphate + 6 NADP + 6H2O dehydrogenase → 6 phosphogluconic acid + 6NADPH2
oxidative
6 Phosphogluconic acid 6NADP decarboxylation → 6 ribulose - 5 - phosphate 6 CO 2 6NADPH 2
+ dehydrogenase + +
2 Ribulose - 5 - phosphate isomerase → 2 ribose - 5 - phosphate
5 Ribulose - 5 - phosphate isomerase → 4 xylulose - 5 - phosphate
2 Xylulose - 5 - phosphate + 2 ribose - 5 - phosphate
transketolase → 2 sedoheptulose − 7phosphate + 2 glyceraldehyde - 3 - phosphate
2 Sedoheptulose - 7 - phosphate + 2 glyceraldehyde 3 - phosphate
transketolase → 2 fructose - 6 - phosphate + 2 erythrose - 4 - phosphate
2 Erythrose - 4 - phosphate + 2 xylulose - 5 - phosphate
transketolase → 2 fructose - 6 - phosphate + 2 glyceraldehyde 3 - phosphate
2 Glyceraldehyde - 3 - phosphate + H 2O aldolase → fructose - 6 - phosphate + H 3PO 4
Sum total of the reaction :
6 Glucose-phosphate + 12 NADP+7H2O → 5 Fructose-6-phosphate+6 CO2 + 12NADPH2 +H2 PO4
(iv) Significance of PPP
It is the only pathway of carbohydrate oxidation that gives NADPH2, Which is needed for synthetic action
like synthesis of fatty acid (in adipose tissues) and amino acids (in liver).
Chapter 20 367
Respiration in Plants
It synthesizes 3C-glyceraldehyde-3-P, 3C-dihydroxy acetone phosphate, 4C-erythrose-4-P, 5C-ribulose
phosphate, 5C-xylulose phosphate, 5C-ribose phosphate, 6 C-Fructose 6-phosphate, 7C-sedoheptulose-7-
phosphate.
It is the major pathway by which necessary ribose and deoxyribose are supplied in the biosynthesis of
nucleotides and nucleic acid.
Erythrose 4 phosphate for the synthesis of lignin, oxine, anthocyanine and aromatic amino acid
(phenylalanine, tyrosine, and tryptophan).
Young growing tissues appears to use to the Krebs cycle as the predominant pathway for glucose
oxidation, while aerial parts of the plants and other tissues seem to utilise the PPP as well as the Krebs
cycle.
It gives 6 CO2, required for photosynthesis.
Ribulose five phosphate is used in photosynthesis to produce RuBP which act as primary CO2 acceptor in
C3 cycle.
(3) Comparison of the different pathway of glucose metabolism
ATP Glucose 2H
ATP
Glucose 6-Phosphate Phosphogluconic acid
Fructose 1,6-diphosphate
Pentose Phosphate + CO2
Phosphoglyceraldehyde Phosphoglyceraldehyde + Ethanol Phosphoglyceraldehyde + Pyruvic acid
4ATP 4H 2ATP 2H 2ATP 2H
2 Pyruvic acid Pyruvic acid Pyruvic acid
EMP Pentose E.D.
Pathway phosphate pathway
(Glycolysis) pathway
Fig : A comparison of the different pathways of glucose metabolism
(4) Cyanide resistant pathway : Cyanide-resistant respiration seems to be widespread in higher plant
tissues. Cyanide prevents flow of electron from Cyt a3 to oxygen, so called ETC inhibitor. In these plant tissues
resistance is due to, a branch point in the ETS preceeding the highly cyanide-sensitive cytochromes. The tissues
lacking this branch point, or alternate pathway and blockage of cytochromes by cyanide, inhibits the electron flow.
Significance
(a) The role of alternative pathway is that it may provide a means for the continued oxidation of NADH and
operation of the tricarboxylic acid cycle, even through ATP may not be sufficiently drained off.
Kreb's cycle substrates
NADH FADH2 Q Flow Cytochrome
Chapter 20 Flow
368 H2O2 ?
Alternate pathway
X O2
Respiration in Plants
(b) It is significant in respiratory climateric of ripening fruits and leads to the production of hydrogen peroxide
and super oxide, which in turn enhances the oxidation and breakdown of membranes.
(c) Necessary activities in the ripening process because peroxides are necessary for ethylene biosynthesis.
Anaerobic respiration.
Anaerobic respiration first studied by Kostychev (1902), Anaerobic respiration is an enzyme-controlled,
partial break down of organic compounds (food) without using oxygen and releasing only a fraction of the energy.
It is also called intra-molecular respiration (Pfluger, 1875). Anaerobic respiration occurs in the roots of some water-
logged plants, certain parasitic worms (Ascaris and Taenia), animal muscle and some microorganisms (bacteria,
moulds). In microorganisms anaerobic respiration is often called fermentation.
Higher organism like plants can not perform anaerobic respiration for long. It is toxic because accumulation of
end products, insufficient amount of available energy and causes stoppage of many active process.
(1) Process of anaerobic respiration : In this process pyruvate which is formed by glycolysis is metabolised
into ethyl alcohol or lactic acid and CO2 in the absence of oxygen. Glycolysis is occurs in cytoplasm so the site of
anaerobic respiration is cytoplasm.
C6H12O6 → 2C2H5OH + 2CO2 + 52 Kcal/218.4 kJ
(i) Formation of ethyl alcohol : When oxygen is not available, yeast and some other microbes convert
pyruvic acid into ethyl alcohol. This is two step process as explained below
Glucose
C6H12O6
2ADP+2(Pi)
2ATP (Net gain)
2×pyruvic acid In the presence Kreb’s cycle
CH3COCOOH of oxygen
In absence of oxygen
Anaerobic Respiration
Animals Plants
CO2
NAD+
NADH+H+
Lactic acid Acetaldehyde
2×CH3CHOHCOOH 2×CH3CHO
NAD+ Alcoholic
fermentation
NADH+H+
Ethanol
2×CH3CH2OH
Fig : Summary of anaerobic respiration pathways
Chapter 20 369
Respiration in Plants
(a) In the first step pyruvic acid is decarboxylated to yield acetaldehyde and CO2. In the presence of Mg++ and
TPP (Thiamine pyrophosphate) pyruvate carboxylase.
CH 3COCOOH Pyruvic → CH 3CHO+ CO2
(Pyruvic acid) Carboxylase
Mg 2+ +TPP (Acetaldehyde)
(b) In the second step acetaldehyde is reduced to ethyl alcohol by NADH2 formed in the glycolysis.
CH 3CHO+ NADH 2 Alcoholic → C2 H5OH + NAD
(Acetaldehyde) dehydrogenase (Ethyl alcohol)
(ii) Production of lactic acid : In this process hydrogen atoms removed from the glucose molecule during
glycolysis are added to pyruvic acid molecule and thus lactic acid is formed.
CH 3COCOOH → CH 3CHOH.COOH+ NAD
(Pyruvic acid) (Lactic acid)
Lactic acid is produced in the muscle cells of human beings and other animals. During strenuous physical
activity such as running, the amount of oxygen delivered to the muscle cells may be insufficient to keep pace with
that of cellular respiration. Under such circumstances lactic acid is formed which accumulates in the muscle cells and
causes muscle fatigue.
(2) Pasteur effect : Two types of respiration –anaerobic and aerobic respiration produce carbon dioxide in
the ratio of 1:3 as shown in the equation.
Anaerobic Respiration : C6 H12O6 → 2C2 H5OH + 2CO2
(Two molecules of CO2 are produced)
Aerobic Respiration : C6H12O4 + 6O2 → 6H2O + 6CO2
(Six molecules of CO2 are produced)
Pasteur noted that when oxygen is given to the running anaerobic respiration the output of CO2 is not similar
to aerobic respiration, i.e. during aerobic respiration the ratio 1:3 does not always appear to be true. In several
cases the amount of carbon dioxide is much less in comparison to normal aerobic respiration as shown above. For
such cases it is considered that the presence of oxygen may sometimes lower down the rate of breakdown of sugar.
The phenomenon is named as 'Pasteur's effects' after the name of great scientist and the process may be defined as
"the inhibition of sugar breakdown due to the presence of oxygen under aerobic condition" and the reaction is called
Pasteur reaction. Dixon (1937) stated that the Pasteur effect is the action of oxygen is checking the high rate of loss
of carbohydrate and in suppressing or diminishing the accumulation of products of fermentation."
Pasteur's effect is said to occur due to many reasons. Some of them are :
(i) Pasteur reaction inhibits some glycolytic enzyme and stops glycolysis.
(ii) Formation of excess of CO2 from degradation of compounds other than respiratory substrate.
(iii) Increased glycolysis with decreased oxygen tension.
(iv) Occurrence of partial oxidative glycolytic products and oxidative anabolism (resynthesis, a process
corresponding HMP pathway).
(3) Connection between aerobic and anaerobic respiration
Carbohydrate
ic respiration Glycolysis or Glucose (C6H12O6) Glycolysis or
Takes place in cytoplasm EMP pathway EMP pathway
370
Chapter 20 Anaerobic resp
2 mols of pyruvic acid Takes place in cy
Oxidation of pyruvic acid
Respiration in Plants
The glycolysis is the common phase and its products pyruvic acid is the common intermediate of the aerobic
and anaerobic respiration.
(4) Fermentation : Fermentation is a kind of anaerobic respiration carried out by microorganisms fungi and
bacteria. In microorganism the term anaerobic respiration is replaced by fermentation (Cruickshank, 1897) ; which
is known after the name of its major product, e.g., alcohol fermentation, lactic acid fermentation.
Gay Lussac was the first to provide following reaction for the fermentation of sugar.
C6H12O6 = 2CO2+2CH3CH2OH+ 52 Kcal
Louis Pasteur (1822-1895) supported Gay Lussacs reaction and concluded that fermentation occurred and
concluded that fermentation occurred only when living Yeast cells were present. Buchner (1897) found that yeast
extract could perform fermentation of sugary solution. The enzyme complex present in yeast which could perform
fermentation was named as zymase. Because of the latter, fermentation is also called zymosis. Besides zymase yeast
cells also contain enzymes like sucrose and maltose which can ferment sucrose and maltose respectively. Direct
fermentation of starch by yeast is not possible as it lacks amylase enzyme.
The fermentation is of two types : Homofermentive (one product) and heterofermentive (two or more than
two types of products). Alcoholic fermentation may occur in almost, any moist sugar containing medium or sugar
solution, such as fruit juice, which is inoculated with yeast or which is left exposed to air. The examples of
fermentation :
(i) Butyric acid fermentation : It occurs in butter which has turned rancid. Bacteria like Clostridium
butyricus and Bacillus butyricus are responsible for fermenting sugars and lactic acid into butyric acid to the
following equation :
C6H12O6 → C4 H8O2 + 2H2 + 2CO2
(hexose) (butyric acid)
2C3H6O3 → C4 H8O2 + 2H2 + 2CO2
(lactic acid) (butyric acid)
(ii) Lactic acid fermentation : In this process the lactose sugar, present in the milk, is converted into lactic
acid which provides a distinctive sour taste to the milk. Two bacteria viz., Bacterium lactic acidi and B.acidi lactici
take part in this process.
C12H22O11 + H2O → C6H12O6 + C6H12O6
(lactose) (glucose) (galactose)
C6H12O6 → 2C3H6O3
(hexose) (lactic acid)
Chapter 20 371
Respiration in Plants
(iii) Acetic acid fermentation : It is different from other types of fermentation as it utilises atmospheric
oxygen. Acetic acid fermentation is catalysed by Acetabacter aceti, and A. xylinum which oxidised ethyl alcohol into
acetic acid.
C2 H5OH+ O2 → CH 3COOH+ H 2O + energy
(ethyl alcohol) (acetic acid)
(iv) Importance of fermentation : Anaerobic respiration is advantageous in many ways :
(a) It supplement, the energy provided by aerobic respiration during intense muscular activity.
(b) Brewing industry produces beers and wines by fermentation of sugary solution with yeast (saccharomyces
cerevisiae).
(c) Baking industry uses CO2 released by Yeast cells in alcoholic fermentation in raising the dough and making
bread spongy.
(d) Dairy industry produces yogurt, cheese and butter by fermenting milk sugar lactose to lactic acid with lactic
acid bacteria (Streptococcus lactis). Lactic acid coagulates the milk protein casein and fuses droplets of milk fat.
(e) Tea and tobacco leaves are cured (freed of bitterness and imparted pleasant flavours) by fermentation with
certain bacteria.
(f) Vinegar is produced by fermenting molasses with yeast to ethyl alcohol which is then oxidised to acetic acid
with aerobic acetic acid bacteria (Acetobacter aceti).
(g) Bacterial fermentation is also used for tanning hides (removal of fat, hair and other tissues).
(h) Retting of hemp fibers is achieved by fermentation with Pseudomonas fluroscense.
(i) Ensilase, a nutritive fodder for cattle, is prepared by fermentation with bacteria in air-tight-chambers called
silos.
(5) Efficiency of respiration
(i) Efficiency of aerobic respiration : We used the generally accepted amount of 12,000-14,000 calories
per mole of ATP approximately 456,000-532,000 calories are generated from one mole of glucose. One mole of
glucose contains about 686,000 calories(686 Kcal) of energy in the form of bonds. When one molecule of glucose is
oxidised to carbon dioxide and water 673,000 calories of energy released.
However the actual amount of energy available from each ATP (rest of energy is lost as heat, and so on) is
approximately 7,3000 calories (7.28 Kcal) or–34 kJ.
Therefore, actual energy yield from one mole of glucose is :
= 38×7.3=277.4 Kcal
So the percent of aerobic respiration = 277.4 × 100
686
= 40.43%
Thus, efficiency of aerobic respiration = 40% approx.
Out of 686 Kcal. of one mole glucose, only 277.4 Kcal. is trapped in the form of ATP.
(ii) Efficiency of anaerobic respiration : In anaerobic respiration of carbohydrate by glycolysis apparently
2ATP molecules are formed per glucose molecule. Therefore, efficiency of anaerobic respiration will be :
C6H12O6 → 2C2H5OH + 2CO2 + 52 Kcal.
Percent of anaerobic respiration = 2 × 7.3 × 100 = 28.07%
52
Chapter 20 372
Respiration in Plants
(iii) Efficiency of alcoholic fermentation : By Yeast only two molecules of ATP are generated per glucose
molecule and efficiency will be, therefore,
C6H12O6 → 2C2H5OH + 2CO2 + 56Kcal.
Percent of fermentation = 2 × 7.3 ×100 =26.07%
56
(6) Difference between respiration and fermentation
Respiration Fermentation
It may occur both in the presence and absence of oxygen. It does not require oxygen.
Occurs only in living cells. It does not occur with in the living cells. It requires only enzymes
and substrate.
Sugar is oxidised and CO2 and H2O are formed as end products. Different substances oxidise to form alcohol or organic acids.
Complete oxidation of substrate occurs, hence produces large Incomplete oxidation of substrate occurs and hence less energy is
amount of energy. produced.
It can occur in any living cell. It occurs mainly with the help of yeast or bacterial cells.
Many microbiologists have distinguished Aerobic respiration, Anaerobic respiration and Fermentation from
one another principally by the means through which hydrogen is removed from various substrates (the hydrogen
donor) and by nature of ultimate substrate accepting this hydrogen (the hydrogen acceptor).
Aerobic Respiration Anaerobic Respiration Fermentation
Molecular oxygen is the ultimate electron The ultimate electron acceptor is an The final electron acceptors are organic
acceptor for biological oxidation. The ETS inorganic compound other than oxygen. The compounds. Both electron donors
serves to transfer electrons from oxidisable compounds accepting the hydrogen (oxidizable substrate) and electron
donor to molecular oxygen. The early (electrons) are nitrates, sulphates, carbonates acceptors (oxidizing agent) are organic
enzymatic steps involve dehydrogenation or CO2. Anaerobic respiration produces ATP compounds and usually both substrates
whereas the final steps are mediated by a through phosphorylation reaction involving arise from same organic molecules during
group of enzyme called cytochromes. electron transfer systems. (mechanism not metabolism. Thus part of the nutrient
Ultimately the electrons are transferred to known) molecule is oxidised and part reduced and
oxygen which is reduced to water. During the metabolism results in intramolecular
aerobic respiration ATP is generated by electron rearrangement. ATP is generated
coupled reaction by substrate level phosphorylation. This
reaction differs from oxidative
phosphorylation because oxygen itself is
not required for ATP generation.
Fat, Protein and Salt respiration.
(1) Fat respiration : Fats are stored as triglycerides in cells, (in animal-adipose tissues and in plants-seeds).
They break up into fatty acids and glycerol in the cytoplasm before use in respiration. Glycerol converted into
Dihydroxy acetonephosphate and enters into glycolysis. The conversion of fatty acid into carbohydrate is called
β oxidation. It convert in acetate units of acetyl CoA to glyoxylate and malate (malic acid) takes place in
microbodies, termed glyoxysomes. The glyoxysomes are contain all the necessary enzymes for β oxidation of fatty
acid to acetyl CoA and subsequent conversion of the acetate units to malic acid (malate) and succinic acid
(succinate), the cycle is known as Glyoxylate cycle.
Chapter 20 373
Respiration in Plants
(i) Energy output : A molecule of 18-carbon stearic acid on complete oxidation produces 147 high-energy
phosphates. A 6-carbon glucose molecule yields 36 or 38 ATP. With this rate, an 18-carbon molecule is expected to
give 3 times more energy (36 or 38×3=108 or 114 ATP) but it provides about 4 times more energy (36 or 38
×4=144 or 152 ATP) than 6-carbon glucose produces.
(ii) Glyoxylate cycle
(a) Discovery : Kornberg and Krebs discovered first this cycle in the bacterium Pseudomonas. Later, the
reaction of β -oxidation of fatty acids and its conversion of acetyl CoA to glyoxylate and malate occurs in
glyoxysomes given by Beevers.
(b) Occurrence : It occurs in seed rich in fats convert stored fats to carbohydrates during germination. The
cycle does not appears to be present in those seeds that store starch rather than fat. Glyoxylate activity in
germination seeds ceases as soon as the fat reserves have been used up. The fact that plants convert fatty acid to
carbohydrates is due to operation of two unique glyoxysome enzyme not known to be present in animals : isocitrate
lyase and malate synthetase.
Oleosome Glyoxysome Mitochondrion
Lipids Fatty acids
Lipase
Fatty acids
NAD+ β-oxidation
NADH + H+
Acetyl CoA
Citrate
Isocitrate
Glyoxylate Succinate Succinate
Malate FAD+ β-oxidation
synthetase FADH2 Fumarate
L-malate
FAD+
NADH + H+ L-malate
Oxaloacetate
Oxaloacetate
H2O PEP ATP
CoA carboxykinase ADP
Phosphoenol
pyruvate
Chapter 20 Hexose
phosphates
Fig : Conversion of storage fat to carbohydrates in germinating
374seed via glyoxylate cycle.
Respiration in Plants
(c) Description : The cycle starts with fatty acids are derived from lipase-mediated enzymolysis of
triglycerides occurring in lipid bodies called oleosomes. The fatty acids undergo β oxidation in the glyoxysome with
the formation of acetyl CoA.
The acetyl CoA reacts with oxaloacetate to form citrate and then isocitrate. The isocitrate is cleaved to
succinate and glycoxylate. This reaction is catalyzed by isocitrate lyase. The glyoxylate combines with acetyl CoA to
form malate, the reaction being catalyzed by malate synthetase. The malate in the glyoxysome is oxidized to
oxaloacetate, which initiates the cycle again by combining with acetyl CoA derived from β oxidation of fatty acids.
The succinate produced moves out of the glyoxysome and into the mitochondrion, where it is converted through
the conventional Krebs cycle reactions to oxaloacetate.
The increase of oxaloacetate (OAA) provides ample substrate for amino acid production and carbohydrate
formation by reverse glycolysis. Conversion of OAA to phosphoenolpyruvic acid and other glycolytic intermediates
takes place in the cytoplasm.
(2) Protein respiration : The proteins split into amino acids in the cytoplasm for use in respiration. The
amino acids enter respiratory routes in two ways : Deamination and Transamination.
(i) Deamination : In deamination, an amino acid loses its amino group (-NH2) and changes into a keto acid.
The latter may further change into a pyruvic acid or acetyl coenzyme A. Pyruvic acid is oxidised to acetyl coenzyme
A. The latter enters the Krebs cycle.
(ii) Transamination : In transamination, an amino group of an amino acid is transferred to an appropriate
keto acid, forming a new amino acid and a new keto acid. The keto acids so formed are normal participants of
glycolysis or Krebs cycle. Of the all amino acids of plant cell only glutamic acid is believed to be oxidised directly by
the enzyme, glutamic acid dehydrogenase into α-ketoglutaric acid and ammonia in the presence of NAD. α-
ketoglutaric acid enters the Kreb's cycle to undergo cyclic degration and oxidation.
Fats Carbohydrates Proteins
Glycogen Glucose Disaccharides
Glucose phosphate
Fructose Fructose phosphate
Fructose diphosphate
Lactic acid
Glycerol Phosphoglyceraldehyde
Phosphoglyceric acid
Pyruvic acid Keto acids Deamination Amino
NH3 acids
CO2 375
Chapter 20Oxidation
Acetyl coenzyme A
Fatty acids
Respiration in Plants
(3) Salt respiration : This respiration is discovered by Lundegarth and Burstram monovalent chlorides of
Na, K and divalant chlorides of Li, Ca and Mg are responsible for salt respiration. According to Lundegarth amount
of anion absorbed by plant cells rather than to the absorption of cations of salts, so it is also called anion respiration.
When a fresh water plant transferred to the salty water the rate of respiration increase due to salt respiration.
The cause of increase in the rate of respiration during absorption of minerals by roots is also salt respiration.
Respiratory quotient / Respiration ratio.
R.Q. is the ratio of the volume of CO2 released to the volume of oxygen taken in respiration and is written as
R.Q.= Volume of CO2 evolved = CO2
Volume of O2 absorbed O2
Value of R.Q. varies with substrate. Thus the measurement of R.Q. gives some idea of the nature of substrate
being respired in a particular tissue.
When carbohydrates are completely oxidised the value of R.Q. is unity (=one). If fats and proteins are the
substrate the value of R.Q. is less than unity (0.5 to 0.9) and when organic acids are substrate the value is more
than unity (1.3 to 4.0).
In succulent like Opuntia, Bryophyllum where there is incomplete oxidation of carbohydrates no CO2 is
produced, hence the value of R.Q. is zero.
R.Q. is usually measured by Ganong's respirometer.
(1) When carbohydrates are the respiratory substrate (=germinating wheat, oat, barley, paddy grains or
green leaves kept in dark or tubers, rhizomes, etc.)
C6 H12O6 + 6O2 → 6CO2 + H 2O ; CO2 = 6 =1 (Unity)
Glucose O2 6
(2) When fats are the respiratory substrate (=germinating castor, mustard, linseed, til seeds)-for fatty
substances R.Q. is generally less than one .
Chapter 20 376
Respiration in Plants
(i) C18 H 36O2 + 26O2 → 18CO2 + 18H 2O ; CO2 18 = 0.7 (Less than unity)
Stearic acid O2 = 26
(ii) 2C51H98 O6 + 145O2 → 102CO2 + 98H 2O ; CO2 = 102 = 0.7 (Less than unity)
Tripalmitin O2 145
(3) When protein are the respiratory substrate (=germinating gram, pea, bean, mung seeds)-
value of R.Q. is less than unity (0.5-0.9).
(4) When organic acids are the respiratory substrate
(i) C4 H6O5 + 3O2 → 4CO2 + 3H2O ; CO2 = 4 = 1.33 (More than unity)
Malic acid O2 3
(ii) 2(COOH)2 + O2 → 4CO2 + 2H 2O ; CO2 = 4 = 4 (More than unity)
Oxalic acid O2 1
Organic acid R.Q.
Succinic acid 1.14
Taurtric acid 1.6
Acetic acid 1
(5) When there is incomplete oxidation of carbohydrates (In the respiration of succulents i.e. :
Bryophyllum, Opuntia)
2C6 H12O6 + 3O2 → 3C4 H6O5 + 3H 2O ; CO2 = 0 = 0 (Zero)
O2 3
(6) Respiration in the absence of O2 (in anaerobic respiration)
C6 H12O6 Zymase → 2C2 H5OH + 2CO2 ; CO2 = 2 =∞ (Infinite)
O2 0
Factors affecting rate of respiration
Many external and internal factors affecting the rate of respiration are as follows :
(1) External factors Relative rates of respiration 45°
40°
(i) Temperature : Temperature is the most important factor for 35°
respiration. Most of the plants respire normally between 0oC to 30oC. With every
100C rise of temperature from 0oC to 30oC the rate of respiration increases 2 to 30°
2.5 times (i.e., temperature coefficient (Q10o) is = 2 to 2.5), following Vant 25°
Hoff’s Law. Maximum rate of respiration takes place at 30oC, there is an initial
rise, soon followed by a decline. Higher the temperature above this limit, more 20°
is the initial rise but more is the decline and earlier is the decline in the rate of 10°
respiration. Probably this is due to denaturation of enzymes at high
temperature. 0° 5
01 2 3 4
Time in hours
Fig : Interrelationship between
respiratory rates of germinating
pea seeds, temperature and time
Chapter 20 377
Respiration in Plants
Below 0oC the rate of respiration is greatly reduced although in some plants respiration takes place even at-
20oC. Dormant seeds kept at –50oC survive.
(ii) Supply of oxidisable food : Increase in soluble food content Relative CO2 release
readily available for utilization as respiratory substrate, generally leads to
an increase in the rate of respiration upto a certain point when some
other factor becomes limiting.
(iii) Oxygen concentration of the atmosphere : Respiration is 5 10 15 20 25
aerobic or anaerobic depending upon the presence or absence of Oxygen %
oxygen. Air has 20.8% oxygen which is more than enough keeping in
view the requirements of plants. Due to this if the amount of oxygen in Fig : Effect of oxygen concentration on the rate
the environment of plants is increased or reduced upto quite low values of respiration. On the right of arrow – Rate of
the rate of respiration is not effected. On decreasing the amount of respiration increases with increases in oxygen
oxygen to 1.9% in the environment aerobic respiration become concentration. On the left of arrow -Rate of
negligible (extinction point of aerobic respiration) but anaerobic
respiration takes place. anaerobic respiration increases again with
decrease in oxygen concentration and ethyl
alcohol produced and CO2 is released
Oxygen poisoning : The significant fall in respiration rate was observed in many tissues in pure O2, even at
N.T.P. This inhibiting effect was also observed in green peas when they are exposed to pure oxygen exerting a
pressure of 5 atm- the respiration rate fall rapidly. The oxygen poisoning effect was reversible, if the exposure to
high oxygen pressure was not too prolonged.
(iv) Water : With increase in the amount of water the rate of respiration CO2 released per 100 grams dty matter per 24 hours 12
increases. In dry seeds, which have 8-12% of water the rate of respiration is 10
very low but as the seeds imbibe water the respiration increases. The life of
seeds decreases with increase of water. This increase is slow at first but very 8
rapid later. This is very clearly seen in the tissues of many xerophytes. As the
water contents of such plants is increased, often there is no great immediate 6
effect upon the rate of respiration. Minor variation in water content of well-
hydrated plant cell do not appear to have very great influence upon the rate 4
of respiration.
2
Figure shows that in wheat grains rate of respiration increases with
increase of water to 16-17%. The rate of respiration of seeds increase with 12 13 14 15 16 17
increase of water because water causes hydrolysis, and activity of respiratory Percentage of moisture
enzymes is increased. Also oxygen enters the seed through the medium of
water. Fig : Effect of moisture on the rate of
respiration of germinating wheat grains
(v) Light : Respiration takes place in night also which shows that light is not essential for respiration. But light
effects the rate of respiration indirectly by increasing the rate of 60
photosynthesis due to which concentration of respiratory substrates
is increased. More the respiratory substrate more is the rate of CO2 Production ml 40
respiration.
In case of blue green algae (Anabaena) respiration rate was 20
found to depend upon light and the effect was also influenced by
O2-concentration.
Chapter 20 378 0 20 40 60 80
CO2 Concentration %
Fig : Effect of CO2 concentration on the rate
of respiration of germinating mustard seeds
Respiration in Plants
(vi) Carbon dioxide (CO2) : If the amount of CO2 in the air is more than the usual rate of respiration is
decreased. Germination of seeds is reduced and rate of growth falls down. Heath, (1950) has shown that the
stomata are closed at higher cone. of CO2, due to which oxygen does not penetrate the leaf and rate of respiration
is lowered.
This fact is made use of in storage of fruits. Air containing 10% CO2 (in atmosphere it is only 0.03%) retards
respiratory break down and therefore reduces sugar consumption and thus prolongs the life of the fruit.
(vii) Inorganic salts : The chlorides of alkali cations of Na and K, as also the divalent cations of Li, and Ca
and Mg, generally increase the rate of respiration as measured by the amount of CO2 evolved. Monovalent
chlorides of K and Na increases the rate of respiration, while divalent chlorides of Li, Ca and Mg causes less
increase in respiration.
(viii) Injury and effects of mechanical stimulation : Wounding or injury almost invariably results in an
increase in the rate of respiration . Whenever a plant tissue is wounded the sugar content of the wounded portion is
suddenly increased. This increase in the sugar content is responsible for the observed temporary increase in the
respiration rate.
A purely mechanical stimulation of respiration has been demonstrated in leaves of a number of species by
Audus (1939,40,46). A gentle rubbing, touching, handling and bending of the leaf blade was sufficient to induce a
marked rise in the respiration rate (20 to 183%) which persisted for several days. If the treatment was repeated at
intervals, the stimulus gradually lost its effect in increasing the rate of respiration.
(ix) Effect of various chemical substances : Certain enzymatic inhibitors like cyanides, azides, carbon
monoxide, iodoacetate, malonate etc. reduce the rate of respiration even if they are present in very low
concentration.
However, various chemical substances such as chloroform, ether, acetone, morphine, etc., brings about an
increase in respiratory activity. Ripening fruits produce ethylene and this is accompanied by an increase in
respiration rate. Other volatile products responsible for the flavour (aroma) e.g., methyl, ethyl, amyl, esters of
formic, acetic, caproic and caprylic acid also associated with increased respiration rate, reach a maximum during
ripening of fruits.
(x) Pollutants : High concentration of gaseous air pollutants like SO2 , NOX and O3 inhibit respiration by
damaging cell membrane. These gaseous pollutant causes increase in pH which in turn affects the electron transport
system thus inhibiting respiration.
Heavy metal pollutant like lead (Pb) and cadmium(Cd) inhibit respiration by inactivating respiratory enzymes.
(2) Internal Factors
(i) Protoplasm : The rate of respiration depends on quality and quantity of protoplasm. The meristematic
cells (dividing cells of root and shoot apex) have more protoplasm than mature cells. Hence, the meristematic cells
have higher rate of respiration than the mature cells. Respiration rate high at growing regions like floral and
vegetative buds, germinating seedlings, young leaves, stem and root apices.
(ii) Respiratory substrate : With the increase of in the amount of respiratory substrate, the rate of
respiration increases.
Experiments of respiration
Chapter 20 379
Respiration in Plants
(1) Demonstration of aerobic respiration in plant tissues : Experimentally, aerobic respiration is proved
by respiroscope. As a result of this experiment water column arises (also mercury) due to absorption of evolved CO2
in respiration by KOH.
KOH stick
Cotton
Seeds
Fig : Respiroscope for
demonstration of aerobic
respiration in plants
(2) Demonstration of anaerobic respiration : In this experiment anaerobic condition created by filling the
test tube with mercury. After completion of experiment the level of mercury goes down, because CO2 evolved by
the respiration push the mercury level down. When KOH is introduced mercury level will again rise up to top.
CO2 gas
Germinating
seeds
Mercury
Trough
Fig : Demonstration of the evolution of CO2 in
anaerobic respiration
(3) To prove that CO2 is evolved in To aspirator Block cloth Air
aerobic respiration : In following cover
apparatus, when suction pump is started it Lime Lime ‘U’ Tube
pulls air through absorbed by KOH of U- water Potted water
tube. CO2 of air free of CO2 enters bottles C. plant Caustic
If the calcium hydroxide water does not potash
Bell jar Glass plate
turns milky it is an indication that all CO2
has been absorbed by KOH. The results (A) (B) (C) (D)
shows that lime water in bottle A turns milky
proved that liberation of CO2 takes place in Fig : Apparatus to demonstrate that CO2 is evolved in aerobic
aerobic respiration respiration
Chapter 20 380
Respiration in Plants
(4) Demonstration of liberation of heat energy during respiration : In this experiment bottle A filled
with boiled seed and bottle B filled with germinating seed. After 24 hours temperature of both thermometer noted.
Observation shows rise in temperature in bottle B because these seeds are respiring.
Thermometer
Germinating
seeds
Dry seeds
(A) (B)
Fig : Demonstration of evolution of
heat in respiration (A) Dry seeds
(B) Germinating seeds
(5) Measurement of rate of respiration by Ganong's respirometer : The apparatus consists of three
parts :
(i) A bulb for the respiring material, with stopper. Stopper
(ii) A graduated manometer tube. Lateral hole
(iii) A levelling or reservoir tube connected with manometer Bulb/Reservior
tube by a stout rubber tubing. Seeds Graduated manometer
2 ml of respiring material are put into the bigger bulb of the tube (KOH solution)
respirometer and 10% KOH in placed in the manometer tube.
Experiment start with the turning the glass stopper at the top.
Respiration now takes place in a closed space and the absorption of
CO2 liberated shown by rise in KOH solution in the graduated tube. Fig : Ganong’s respirometer
[For the measurement of R.Q. saturated solution of NaCl is
first place in manometer tube. (Pure water not used, as it absorbed CO2)].
Different respiratory substrates (carbohydrate, fat seed) are taken. In the graduated tube will remain more or
less and unaltered showing that the volumes of CO2 evolved and oxygen absorbed are the same and R.Q. =1. If
solid KOH pellets than added to salt solution in the tube, the accumulated CO2 is absorbed and can be measured
from the reading in the tube.
(6) Demonstration of fermentation : For this experiment glucose, baker's yeast and water are taken in
Kuhne's tube. As a result the level of solution falls in the upright arm and the solution gives alcoholic smell, proves
that alcoholic fermentation of glucose takes place.
Carbon dioxide
Chapter 20 381 Glucose and yeast
Respiration in Plants
Important Tips
• Glyoxylate cycle is called adaptation of Kreb's cycle.
• Effect of cyanide poisoning can be minimised by immediate supply of ATP.
• Cut fruit and vegetable : They often turn brownish due to oxidation of tannins or orthodihydroxyphenols to
orthoquinones/polyphenols by means of phenolases and Laccase(e.g., Apple, Potato, Cauliflower, Cabbage, Banana, Peach). This can
be prevented by sprinkling ascorbic acid (vit.C), vacuum packing, sugar syrup, steam boiling water or potassium metabisulphite.
• In prokaryotes aerobic cell respiration of glucose always produces 38 ATP molecules, as NADH2 molecules formed during glycolysis
are not enter the mitochondria.
• The main place of metabolism is cytoplasm, maximum reaction like glycolysis, fat oxidation into acetyl CoA, protein oxidation into α -
ketoglutaric acid, ED pathway and pentose phosphate pathway occurs in cytoplasm. Only Krebs cycle and ETC occurs in mitochondria.
• Pentose phosphate pathway called connective link between photosynthesis and fat synthesis.
• The low temperature and high CO2 concentration used in cold storage of fruits and tubers increases the rate of respiration.
• The potato growing in hilly areas are bigger in size because in hilly areas temperature is low. Respiration decreases on low temperature
therefore in potato complete oxidation of carbohydrate not takes place and carbohydrate/ starch in potato tuber accumulates and
increases the size.
• Animals cells respire anaerobically during straneous condition forms lactic acid, and fungi respire anaerobically so the requirement for
respiration of fungi considered similar to the animal cells.
• The R.Q. at compensation point = CO2 = Zero ( CO2 and O2 equal at compensation point).
O2
• Temperature affects germinating seeds because hydration makes enzyme more sensitive to temperature.
• Glucose before converting glycogen in muscles and liver converted into glucose 6-phosphate needed ATP. Glycogen also before
utilization converted into glucose –6-phosphate process called glycogenolysis.
• Thiamine pyrophosphate is the active form of vitamin B1 (Thiamine) work as coenzyme of pyruvate carboxylase dehydrogenase.
• Climateric fruits : Those fruits which show a high rate of respiration during their ripening e.g., Apple, Banana. In these fruits rise of
respiration called climatric rise.
• NADH produces during the glycolysis are utilized in formation of ethyl alcohol from acetaldehyde during anaerobic respiration. The net
gain of ATP is 2 ATP in anaerobic respiration which comes from the process of glycolysis.
• Aldolase and triose phosphate isomerase enzyme are common for EMP and C3 pathway.
• In white muscles fibers lactic acid accumulated more faster than red muscle fiber because red muscle fiber have more myoglobin (O2
storing pigment) as compared to white muscles fiber so white muscle get fatigued soon.
Chapter 20 382
Respiration in Plants
Chapter 20 383
Plant Growth and Development Part 1
Growth.
Growth in plants, as in any organism, consists of an irreversible increase in size, which is commonly
accompanied by increase in solid or dry weight and in the amount of protoplasm growth is essential character of
life. In growth anabolic processes dominate over the catabolic processes and therefore growth is the final product of
successful metabolism.
A correct definition of growth is difficult. In common parlance, the ‘growth’ may be applied to several things
and situations. However, growth can be defined as a vital process which brings about permanent change in any
plant or its part with respect to its size, form, weight and volume. Whole series of changes during life span of a plant
or organism is termed as development. Growth is generally a quantitative matter and is concerned with increasing
amount of organism. Development, on the other hand, is qualitative change referring to the changes in nature of
growth made by the organism. Growth is measurable whereas the development is most commonly assessed by
qualitative observation. During growth and development, there is formation of proteins and carbohydrates, thus
increasing the protoplasm formation.
(1) Regions of growth : In unicellular plants there is overall growth and not confined to any specific region
but in multicellular plants growth is restricted to specific regions having meristematic cells. On the basis of their
position in the plant body (higher plants) meristematic cells. On the basis of their position in the plant body (higher
plants) meristems are divided into three main categories.
(i) Apical meristems, (ii) Intercalary meristems, (iii) Lateral meristems
(i) Apical meristems : These meristems are found at shoot and root apex. As a result of activity of these
meristems plant increases in length. In angiosperms and gymnosperms there is a group of meristematic cells but in
bryophytes and pteridophytes there is a single tetrahedral cell found at the shoot apex.
(ii) Intercalary meristems : These meristems are found above the nodes. As a result of the activity of these
meristems increase in length takes place. e.g., Bambusa.
(iii) Lateral meristems : These meristems are made up of cells which divide in radial direction only. They
form laterally placed new cells towards the centre and
periphery. Cork cambium (phellogen) and vascular
cambium are the examples of lateral meristems. Increase
in girth of shoots and roots take place because of the Cell plate formation Nuclear division Meristematic cell
activity of this cambium. Cell Division (isodiametric)
(2) Phases of growth : Growth is not a very simple Primary cell wall Thin layer of Large vacuole
process. Before completion of this process a meristematic (stretching) cytoplasm
Nucleus
cell has to pass through three phases.
(i) Cell division Cell elongation
(ii) Cell enlargement Secondary wall patterns
(iii) Cell maturation (differentiation)
(i) Cell division (Formative phase) : A cell is Cell differentiation
metabolically highly active at the time of cell division. Its Fig : Showing cell formation, cell elongation and cell
cellular mass increases and replication of genetic material
differentiation
Chapter 21 384
Plant Growth and Development Part 1
(nucleic acids) takes place. Growth as a result of divisions is based on mitotic cell division. In the stage of mitosis
each chromosome is split lengthwise into two homologous chromatids which pass equally into daughter cells. As a
result of division each cell is only half the size of parent cell. These cells then proceed to enlarge.
(ii) Cell enlargement : Cell division is followed by cell enlargement. The cell increases in size due to
vacuolation (by absorption of water). A big central vacuole appears which pushes the cytoplasm to be limited to a
thin boundary layer against the cell wall. The new cell wall materials is synthesized to cope with the enlargement.
The cell enlargement has been explained in two different ways. According to the first view, the tergour of the cell
increases. As a result, certain gaps or lacunae appear in the cell wall. The new wall material is deposited in the
lacunae between particles of the old wall (intus-ucception) or below the lacunae (apposition). The other view
considers that as a result of growth of the cell wall the volume of the cell increases.
(iii) Cell maturation (Differentiation) : Cell differentiation following cell division and cell enlargement
leads to the development of specialized mature tissue cell , e.g., some cells are differentiated into xylem tracheids
and trachea and some others into sieve tubes and companion cells.
(3) Growth curve : The rate of growth varies in different species and different organs. In certain species of
plants such as Cacti, the rate of growth is exceedingly slow. In many plants, the growth rate is phenomally rapid
e.g., the young leaf sheath of banana grows for a time at the rate of almost three
inches per hour. Growth begins slowly, then enters a period of rapid enlargement, Growth Steady
following which it gradually decreases till no further enlargement occurs. The state
mathematical curve which represents this variation in growth rate is some what
flattened S-shaped curve or sigmoid curve. Time in which growth takes place has Log
been called grand period of growth. This term was coined by Sachs. The analysis phase
of growth curve shows that it can be differentiated into three phases : Lag
phase
(i) Lag phase : It represents initial stages of growth. The rate of growth is very
slow in lag phase. More time is needed for little growth in this phase. Time
Fig : A typical S-shaped grand
(ii) Log phase (Exponential phase) : The growth rate becomes
maximum and more rapid. Physiological activites of cells are at their period of growth curve
maximum. The log phase is also referred to as grand period of
growth.
(iii) Final steady state (Stationary phase) or Adult phase :
When the nutrients become limiting, growth slows down, so
physiological activities of cells also slows down. This phase is indicated
by the maturity of growth system. The rate of growth can be measured
by an increase in size or area of an organ of plant like leaf, flower, fruit
etc. The rate of growth is called efficiency index.
In many plants another phase is also evident in their growth Fig : Horizontal microscope for measurement of
curve. This is called linear phase or phase of maximum growth rate. growth
Sachs called it as grand phase.
(4) Measurement of growth : Growth in plants can be
measured in terms of (i) increase in length, e.g., stem, root (ii) increase
Chapter 21 385
Plant Growth and Development Part 1
in volume, e.g., fruit, (iii) increase in area, e.g., leaves (iv) increase in diameter, e.g., tree trunk. (v) increase in fresh
or dry weight. The following methods are designed to measure growth in length.
The following methods are designed to measure growth in length :
(i) Direct method : It is the simplest method of measuring growth and involves measurement of growth
between two marked points directly by a scale at regular intervals. This is not much used as in this case, growth over
short periods cannot be measured.
(ii) Horizontal microscope (Travelling microscope) : In this method, tip of a growing plant is marked
with the help of Indian Ink and horizontal microscope is focussed at the point. After a day or two, marked point is
obserbed by microscope. It is little bit raised. Distance between the two readings shows the actual growth of a plant.
It can be used for measuring growth of plants in the field.
(iii) Auxanometer : Several kinds of auxanometers have been devised to measure the growth in length of a
plant. Two of them are given below :
(a) Arch auxanometer : It conists of a vertical stand with a pulley. Attached to the pulley is a pointer which
moves on an arc scale. A silken thread is passed over the pulley, one end of which is tied to the plant apex and the
other carries a weight enough to keep the thread stretched. As growth occurs the pulley moves, causing the pointer
to move on the scale. Growth can be calculated on the basis of the distance moved by the pointer on the arc and
the length of the pointer as follows.
Growth of plant in length = Distance travelled by pointer × Radius of pulley
Length of pointer meausred from the centre of the pulley
Pulley Arc scale
Thread
Pointer
Weight
Potted Stand
plant
Fig : Arc auxanometer (arc indicator) for Smaller pulley Larger pulley
measurement of growth
Rotating Thread
(b) Pfeffer’s auxanometer (Automatic cylindrical drum Stand
auxanometer) : It is composed of two pulleys (large and Weight
small), revolving cylinder covered with a smoked paper, stand covered by
and 3 weights. The two pulleys magnify the growth. smoke paper
Magnification = Radius of larger pulley Pointer
Radius of smaller pulley
One end of thread is tied to the tip of stem and other Fig : Pfeffer’s auxanometer
end is tied to a small weight. The thread is passed over the
Chapter 21 386
Plant Growth and Development Part 1
smaller pulley. One more thread having small weight on its both sides passes over the larger pulley. One side of this
thread bears pointer towards the revolving cylinder, which is in close contact with the smoked paper.
Revolving of cylinder takes place by the start of clock work. If growth occurs, two pulleys move by the
downward movement of weight which is attached to the tip of stem. There is ring like and stair case like marking
developed on the smoked paper. However, if no growth occurs, a horizontal line is formed on the smoked paper.
(iv) Bose’s crescograph : The crescograph invented by Sir Jagdish Chandra Bose is a more delicate
instrument and gives magnification upto 10,000 times. The rate of growth of root can be measured by the use of a
root auxanometer.
(5) Factors influencing rate of growth : Growth is affected by the factor which affect the activity of
protoplasm. It is affected by a large number of factors both environmental and physiological. Physiologogical factors
such as absorption of water, minerals, photosynthesis, respiration etc, and environmental factors including climatic
and edaphic both. The effect on these factors on one region of plant are also transmitted to other region of the
plant.
Since growth is a resultant of many metabolic processes, it is affected by many external and internal factors,
which are as follows,
(i) External factors
(a) Light : Light affects variously e.g., light intensity, quality and periodicity.
• Intensity of light : In general, light retards growth in plants. High light intensities induce dwarfing of the
plant. Plants at hill tops are short whereas those of a valley are quite tall. Very weak light induces the rate of overall
growth and also photosynthesis. Development of chlorophyll is dependent on light and in its absence etiolin
compounds in formed which gives yellow colour to the plant. The phenomenon is called etiolation. Similarly high
light intensity affecting indirectly increases the rate of water lose and reduces the rate of water growth.
• Quality of light : The different colours (different wavelengths) affect the growth of plant. In blue-violet
colour light internodal growth is pronounced while green colour light reduces the expansion of leaves as compared
to complete spectrum of visible light. The red colour light favours elongation but they resemble etiolated plants.
Infrared and ultraviolet are detrimental to growth. However, ultraviolet rays are necessary for the development of
anthocyanin pigments in the flowers. Blue and violet colours increase size of lamina of leaf.
• Duration of light : There is remarkable effect on durtion of light on the growth of vegetative as well as
reproductive structures. The induction and suppresion of flowering are dependent on duration. The phenomenon is
termed photoperiodism.
(b) Temperature : Temperature has pronounced effect on the growth of plant. The temperature cardinals for
growth vary according to temperature zones. The minimum, optimum and maximum temperatures are usually 5oC
(arctic), 20 – 30oC (temperate) and 35 – 40oC (tropical). The optimum temperature needed for the growth of a
plant is much dependent on the stages of development. Low temperatures during nights reduces the rate of
respiration and high temperature during days increases photosynthesis accumulated photosynthate also increases
growth the tomato plants do not grow well under uniform temperatures condition of day and night but they grow
well under low night temperature (nyctotemperature) and flucuating day temperature (phototemperature). This
response of plant to temperature variation is called thermoperiodicity. When plants are exposed to extremes of
temperature they get injured and the injuries are called descication, chilling and freezing.
Due to hot or cold spells of wind, when the transpiration exceeds absorption, the plant tissue gets injured and
the injury is called desiccation. If a plant of hot climate is exposed to low temperature it gets injured and the injury
is called chilling. During winter, in hill plants water is withdrawn from the cell into the intercellular space. As a
Chapter 21 387
Plant Growth and Development Part 1
result, the dehydrated protoplasm coagulates. There is inter and intracellular ice formation due to further lowering
of temperature and as a result the plant tissue is injured. This injury is called freezing. A plant develops high
osmotic concentration of the cell sap and a thick bark to withstand these injuries. Besides, it also shows formation of
seeds, spores, tubers etc. when the temperature goes down.
(c) Water : As water is an essential constituent of the living cell, a deficiency of water causes stunted growth.
Moreover unless the cells are in a turgid condition, they cannot divide and unless new cells are added up by the
activity of the meristems, growth cannot take place. water is also essential for photosynthesis not only as a raw
material, but also for the photosynthetic activity of the cells. Water is also essential for the translocation of mineral
salts and ready-made food to the growing regions of the plants. Without food supply growth cannot take place.
(d) Oxygen : In poorly aerated soil there is low concentration of oxygen and a high concentration of CO2 .
Under such conditions plants usually show stunted growth. Normal growth of most plants occurs only when
abundant oxygen is present since O2 is important for respiration. It has been reported that oxygen plays some
important role during GI stage of cell division.
(e) Mineral salt : Absence of essential mineral salts results in abnormal growth. For example, the absence of
nitrogen prevents protein-synthesis, while the absence of iron prevents chlorophyll formation and thus leads to pale
and sickly growth of plants, known as chlorotic condition.
(f) Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticites etc have
detrimental effect on plant growth. Some plants are very sensitive to certain pollutants. Citrus and Gladiolus are
very sensitive to fluorides. Poor growth of tobacco is observed in regions where ozone concentration is high. White
pine cannot survive under high O3 concentration. Cotton plants are, similarly very sensitive to ethylene.
(g) Carbon dioxide : CO2 is essential for photosynthesis and hence nutrition. Due to change in
photosynthetic rate, with the increase or decrease in CO2 concentration, the plant growth is also affected.
(ii) Internal factors : Amongst internal factor i.e., age, health, hereditory factors, growth regulator, nutritional
relations, etc. growth regulators are very important. Some of the internal factors are :
(a) Nutrition : It provides raw material for growth and differentiation as well as source of energy. C/N
(carbohydrate/protein) ratio determines the type of growth. High C/N ratio stimulates wall thickening. Less
protoplasm is formed. Low C/N ratio favours more protoplasm producing thin walled soft cells. According to law of
mass growth, the initial rate of growth depends upon the size of germinating structure (seed, tubes, rhizome, bulb,
etc.)
(b) Growth regulators : These are manufactured by living protoplasm and are important internal growth
regulators which are essential for growth and development. These growth regulators include several phytohormones
and some synthetic substances.
Growth hormones and Growth regulators.
The term hormone used by first Starling (1906). He called it stimulatory substance. The growth and
development in plants is controlled by a special class of chemical substances called hormones. These chemicals are
synthesized in one part of the plant body and translocated to another where they act in a specific manner. They
regulate growth, differentiation and development by promoting or inhibiting the same. They are needed in small
quantities at very low concentrations as compared to enzyme. They are rarely effective at the site of their synthesis.
Chapter 21 388
Plant Growth and Development Part 1
Thus, growth hormones also called phytohormones term given by Thimann (1948), it can be defined as ‘the
organic substances which are synthesized in minute quantities in one part of the plant body and transported to
another part where they influence specific physiological processes’. Sometimes the term growth regulators is misled
with phytohormones. The term phytohormones as the definition indicates, is implied to those chemical substances
which are synthesized by plants and thus, they are naturally occurring. On the other hand, there are several
manufactured chemicals which often resemble the hormones in physiological action and even molecular structure.
Thus the synthetic substances which resemble with hormones in their physiological action are termed as growth
regulators.
Phytohormones can have a promoting or inhibiting effect on a process. A particular hormone may promote
certain processes, inhibit some others and not effect many others. In general, developmental processes are
controlled by more than one growth regulator. They may act synergistically i.e., in a cooperative and beneficial
manner (e.g., morphogenesis by auxins and cytokinins) or antagonistically i.e., in opposite manner (e.g., seed
germination is promoted by gibberallin and is inhibited by abscisic acid). A group of plant hormones including
auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth.
(1) Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring
structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots
and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth
regulators, auxins were the first to be discovered.
(i) Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants.
The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described
the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending
movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-
Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919)
removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928)
from coleoptile tip of Avena . Went also developed Avena curvature test for bioassay of auxin. Kogl and Haagen.
Smit (1931) introduced the term auxin.
(ii) Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins.
(a) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as
phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and
fungi.
The first naturally occuring auxin was isolated by Kogl and Haagen-Smit CH2COOH
(1913) from human urine. It was identified as auxin-a (auxentriolic acid,
C18H32O5). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, N
auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted |
from germinating corn seeds), and heteroauxin from human urine. Heteroauxin H
Indole acetic acid (IAA)
(C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-
acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid (4-chloro-IAA) etc., are some other
natural auxins.
Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf
primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are
synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in
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Plant Growth and Development Part 1
stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and
not through the vascular tissues. Auxins help in the elongation of both roots and shoots. However, the optimum
concentration for the two is quite different.
It is 10 ppm for stem and 0.0001 ppm for the root. Its translocation rate is 1–1.6 cm/hr. (In roots 0.1 to 0.2
cm/hr). Higher concentration of auxins show inhibitory effect on growth.
Natural auxins are of two types : free and bond auxins. The auxins which can easily be extracted are called
free auxins, whereas auxins which are hard to extract and need the use of organic solvents are termed as bound
auxins. The free form of auxin is active, while the bound auxin is inactive in growth. A dynamic equilibrium exists
between these two forms.
(b) Synthetic auxins : These are synthetic compounds which cause various physiological responses
common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the
weedicide, 2, 4, 5-T (2, 4, 5-trichlorophenoxy acetic acid), IBA (indole 3-butyric acid), NAA (naphthalene acetic
acid, PAA (Phenyl acetic acid), IPA (Indole 3-propionic acid). IBA is both natural and synthetic auxin. Certain
compounds inhibit action of auxin and compete with auxins for active sites are called antiauxins. e.g., PCIB (p-
chlorophenoxy isobutyric acid), TIBA (2, 3, 5-tri iodobenzoic acid). TIBA is used in picking cotton bolls.
COO COO
CH2 CH2 CH2CH2CH2COOH CH2COOH
O O
Cl Cl
Cl N Naphthalene acetic acid (NAA)
|
Cl H
Indole Butyric acid (IBA)
2, 4, 5-trichlorophenoxy-acetic
Cl acid (2, 4, 5-T)
2, 4-dichlorophenoxy-acetic
acid (2, 4-D)
(iii) Bioassay of Auxins : Testing of biological activity (growth) of a substance (auxin) by employing living
material is called bioassay. Auxin bioassay is also quantitative test as it measures amount of effect in response to a
particular concentration of auxin.
(a) Avena coleoptile curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated
the effect of auxins on plant Start of experiment Plain agar block
growth by performing some Cut tip of control
experiments with the oat (Avena coleoptile
sativa) coleoptile. Agar block Agar block has In to which Agar with IAA
auxin
diffused Plumule
• When the tips of the Auxin diffused into Accelerated Auxin IAA
coleoptiles were removed, no agar block growth from
growth took place. agar block Accelerated
Dicapitated growth
• When the freshly cut coleoptile
Seedling
coleoptiles were placed on agar
blocks for a few hours (during this (A) (B) (C) (D) (E)
period auxin diffused into the Fig : Oat coleoptile experiment
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Plant Growth and Development Part 1
agar block) and then the agar blocks were placed on the cut ends of the coleoptile, growth occurred.
• When the agar block with the diffused substance was placed laterally on the cut tip of the coleoptile, only
that side of the coleoptile elongated resulting in a curvature.
(b) Split pea stem curvature test : This test was also discovered by Went, 1934. Dark germinated seeds of
pea are decapitated. About half an inch part of stem between 2nd and 3rd node is removed and split longitudinally.
It is then floated on the test solution contained in a beaker. At first, negative curvature occurs due to water uptake.
Then positive curvature occurs which is proportional to the log of the concentration of auxin.
These experiments indicated that some substance is synthesised in the coleoptile tip is translocated downward.
He called this substance auxin.
(c) Root growth inhibition test (Cress root inhibition test) : Sterilized seeds of cress are germinated
over moist filter paper. Root lengths are measured. 50% of seedlings are placed in test solution while the rest are
allowed to grow over the moist filter paper. Lengths of roots are measured after 48 hours. Seedlings placed in test
solution show very little root growth while the roots of controlled seedlings show normal growth. The degree of root
growth inhibition is proportional to auxin concentration.
(iv) Functions of auxins : Auxins control several kinds of plant growth processes. These are as follows :
(a) Cell elongation : Auxins promote elongations and growth of stems and roots and enlargement of many
fruits by stimulating elongation of cells in all directions.
The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of microfibrils, synthesis of
more wall materials, increased membrane permeability and respiration.
Sugars Starch Sugars
Increased Amylase Salts
Respiration IAA
Increased Increased Greater
Synthesis membrane osmotic
permeability pressure
Loosening of cell Water
wall microfibrils
More cell wall Endosmosis
materials
Cell elongation
Fig. Action of IAA in cell elongation.
(b) Apical dominance : In many plants, the apical bud grows and the lower axillary buds are suppressed.
Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth
of lateral buds. This phenomenon is known as apical dominance.
This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is
checked by applying synthetic auxin (NAA).
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(c) Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced
when the auxin content falls below a minimum. Addicot and Lynch (1951) put forward auxin gradient theory
about abscission :
• No abscission if auxin content is high on the organ side.
• Abscission layer begins formation when auxin content becomes same on stem and organ sides.
• Abscission is favoured when auxin content is low on the organ side.
Premature drop of fruits such as apple, pear and citrus can be prevented to a great extent by spraying the trees
with a dilute solution of IAA, NAA or some other auxin.
(d) Weed control : Weeds are undesirable in a field with a crop. Weeds cause competition for water,
mineral, light and space. This causes poor yield. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but
2, 4-D does not affect mature monocotyledonous plants.
(e) Root differentiation : Many new plants are usually propogated by stem cutting e.g., Rose, Bougainvillea.
If we dip the lower cut end of a cutting in dilute solution of auxins (specially IBA gives very good results) very soon
large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
(f) Parthenocarpy : It is the process of formation of fruits without fertilization. Such fruits are called as
parthenocarpic fruits and are without seeds. Parthenocarpy can be induced by application of IAA in a paste form to
the stigma of a flower or by spraying the flowers with a dilute solution of IAA. Banana, oranges and grapes are now-
a-days grown parthenocarpically on commercial scale.
(g) Control of lodging : In some plants when the crop is ripe and there is heavy rain accompanied by strong
winds, the plants bends as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute
solution of any auxin is sprayed upon young plants the possibility of bending of plants is reduced as the stem
becomes stronger by the application of auxins.
(h) Flowering : In pineapple, NAA promotes flowering. In lettuce, auxins help in delaying the flowering. In
cotton plants, the use of auxins increases the cotton seeds production.
(i) Differentiation of vascular tissues : Auxins induce the differentiation of xylem and phloem in intact
plants and also in callus produced in vitro during tissue culture experiments.
(j) Sex expression : The spray of auxins increases the number of female flowers in cucurbits. In maize
application of NAA during the period of inflorescence differentiation can induce formation of hermaphrodite or
female flowers in a male inflorescence.
Thus auxins cause femaleness in plants.
(k) Healing : Healing of injury is effected through auxin induced division in the cells around the injured area.
The chemical was formerly named traumatic acid or traumatin.
(l) Nodule formation : In legumes, IAA is known to stimulate nodule formation.
(m) Respiration : According to French and Beevers (1953) the auxin may HO O OH
increase the rate of respiration indirectly through increased supply of ADP by CO CH2
rapidly utilizing the ATP in the expanding cells.
CH3 COOH
(2) Gibberellins : Gibberellins are weakly acidic hormones having gibbane Gibberellic acid
ring structure which cause cell elongation of intact plants in general and
increased internodal length of genetically dwarfed plants (i.e., corn, pea) in
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Plant Growth and Development Part 1
particular.
(i) Discovery : Gibberellins were first isolated from the fungus Gibberella fujikuroi (Fusarium moniliforme)
the causal organism of Bakanae disease or foolish seedling disease of rice plants in Japan by Kurosawa in 1926.
The characteristic symptoms of this disease are abnormal growth of stem and leaves, thin plants with long
internodes, early flowering or death before flowering and fruiting.
In 1939, Yabuta and Sumiki and coworkers working in Tokyo isolated an active substance from the fungus
and called it Gibberellin A. This gibberellin preparation was probably a mixture of several gibberellins. The first
gibberellin to be obtained was Gibberellin A-3. Cross et al. (1961) explained the detailed structure of gibberellic
acid. Now 60 gibberellins have been identified from different groups of plants (algae, fungi, mosses, ferns,
gymnosperms and angiosperms).
Many of them occur naturally in plants. Gibberella Fujikuroi has as many as 15 gibberellins. A single plant also
possesses a number of gibberellins. All the different types of gibberellins, known so far, have gibbane skeleton and
are acidic in nature. Therefore, these are termed as GA1 (C19H24O6 ) , GA2 (C19 H 26O6 ), GA3 (C19H22O6 ) , GA4
(C19H24O5) and so on. Of these gibberellic acid or gibberellin A3(GA3) is the commonest. Gibberellins are
synthesised in plants in leaves of buds, developing embryos, root tips, young apical leaves, shoot tips and seeds.
Gibberellins are transported readily in the plant, apparently moving passively in the stream either in xylem or
phloem. Their transport in non-polar. Anti-gibberellins like malic hydrazide, phosphon D, Alar and chorocholine
cheoride (CCC) or cycocel are also called antiretardants (stimulates flowering and inhibits the growth of nodes).
Commercial production of GA is still carried out by culturing this fungus in large vats.
(ii) Mechanism of action : Gibberellins are closely related with steroids. Gibberellins exhibit ecdysome like
effects. Ecdysome is a moulting hormone. The steroids have very specific effect in depressing genes and thus
activating specific genes. Another significant gibberellin treatment is production of enzymes like amylase and
protease. It is also considered that the effect of gibberellin is indirect.
Gibberellin
Promotes
Proteases Protein Tryptophan Auxin
Fig. Role of gibberellin in synthesis of auxin.
According to this view gibberellins show its physiological effects by altering the auxin status of the tissue.
(iii) Bioassay of gibberellin : Gibberellin bioassay is performed through dwarf maize/pea test and cereal
endosperm test.
(a) Dwarf pea bioassay : Seeds of dwarf pea are allowed to germinate till the just emergence of plumule.
GA solution is applied to some seedlings others are kept as control. After 5 days, epicotyl length is measured.
Increase in length of epicotyl over control seedlings is proportional to GA concentration.
(b) Barley endosperm bioassay : Endosperms are detached from embryos, sterilized and allow to remain in
1ml of test solution for 1-2 days. There is build up of reducing sugars which is proportional to GA concentrations.
Reducing sugars do not occur in edoperms kept as control.
(iv) Functions of gibberellin
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(a) Stem elongation : The gibberellins induce elongation of the internodes. The cell growth is promoted by
the increase in the hydrolysis of polysaccharides. It also increases the elasticity of cell wall. The elongation of stem
results due to rapid cell division and cell elongation induced by gibberellins.
(b) Leaf expansion : In many plants leaves become broader and elongated when treated with gibberellic
acid. This leads to increase in photosynthetic area which finally increases the height of the plant. Interestingly,
gibberellins show no effect on roots.
(c) Reversal of dwarfism : One of the most striking effects of gibberellins is the elongation of genetic dwarf
(mutant) varieties of plants like corn and pea. It is believed that dwarfism in the mutant variety of plant is due to
blocking of the capacity for normal gibberellin production (deficiency of gibberellin). When gibberellin is applied to
single gene dwarf mutants e.g., Pisum sativam, Vicia faba and Phaseolus multiflorus, they grow to their nomal
heights. It is further interesting to note that application of gibberellins to normal plants fail to show any remarkable
effects.
(d) Bolting and Flowering : Gibberellins induce stem elongation in ‘rosette plants’ e.g., cabbage, henbane,
etc. Such plants show retarded internodal growth and profuse leaf development. In these plants just prior to the
reproductive phase, the internodes elongate enormously causing a marked increase in stem height. This is called
bolting.
Bolting needs long days or cold nights. It has been further noticed that if cabbage head is kept under warm
nights, it remains vegetative. The exogenous application of gibberellins induced bolting in first year itself in plants
like cabbage (normally bolting occurs next year due to effect of endogenous gibberellins).
(e) Enzyme formation : One of the most dramatic effects of GA is its induction of hydrolytic enzymes in the
aleurone layer of endosperm of germinating barley seeds and cereal grains. GA stimulates the production of
digestive enzymes like proteases, α-amylases, lipases which help to mobilise stored nutrients. GA treatment
stimulates a substantial synthesis of new mRNA. Thus GA acts to uncover or depress specific genes, which then
cause the synthesis of these enzymes. It is assumed that GA acts on the DNA of the nucleus.
(f) Breaking of dormancy : Gibberellins overcome the natural dormancy of buds, tubers, seeds, etc. and
allow then to grow. In this function gibberellins act antagonistically to abscisic acid (ABA).
(g) Parthenocarpy : Gibberellins have been considered to be more effective than auxins for inducing
parthenocarpy in fruits like apple, tomato and pear. GA application has also resulted in the production of large fruits
and bunch length in seedless grapes.
(h) Sex expression : Gibberellins control sex expression in certain plants. In general, gibberellin promote the
formation of male flowers either in place of female flowers in monoecious plants such as cucurbits or in genetically
female plants like Cannabis, Cucumis.
(i) Substitution for vernalization : Vernalization is the low temperature requirement of certain plant (i.e.,
biennials) to induce flowering. The low temperature requirement of biennials for flowering can be replaced by
gibberellins.
(j) Malt yield : There is increased malt production when gibberellins are provided to germinating barley
grains (due to greater production of α-amylase).
(k) Delayed ripening : Ripening of citrus fruits can be delayed with the help of gibberellins. It is useful in safe
and prolonged storage of fruits.
(l) Seed germination : Gibberellins induce germination of positively photo-blastic seeds of lettuce and
tobacco in complete darkness.
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(3) Cytokinins (Phytokinins) : Cytokinins are plant growth hormones which are basic in nature, either
aminopurine or phenyl urea derivatives that promote cell division (cytokinesis) either alone or in conjugation with
auxin.
(i) Discovery : The first cytokinin was discovered by Miller, Skoog and Strong (1955) during callus tissue
culture of Nicotiana tobaccum (tobacco).
It was synthetic product formed by autoclaving Herring sperm (fish sperm) DNA. This synthetic product was
identified as 6-furfuryl amino-purine and named as kinetin. He found that normal cell division induced by adding
yeast extract.
Various terms such as kinetenoid (Burstran, 1961), phytokinin (Dendolph et al. 1963) phytocytomine (Pilet
1965) have been used for kinetin like substances but the term cytokinin proposed by Letham (1963) has been
widely accepted. Letham et al. (1964) discovered first natural, cytokinin in unripe maize grain (Zea mays). It was
named as zeatin (6 hydroxy 3 methyl trans 2-butenyl amino purine).
About 18 cytokinins have been discovered, e.g., dihydrozeatin, IPA (Isopentenyl adenine), benzyl adenine.
The most widely occurring cytokinin in plant is IPA. It has been isolated from Pseudomonas tumefaciens. Many are
found as constituents of tRNAs. Cytokinins are synthesized in roots as well as endosperm of seeds. Coconut millk
and Apple fruit extract are rich in cytokinins. Cytokinins in coconut milk called coconut milk factor.
Kinetin (6 furfuryl amino purine) is a derivative of the nitrogen base adenine. Plant physiologists use the term
cytokinins to designate group of substances that stimulate cell division in plants. NH CHH2 O
Cytokinins are prdouced in actively growing tissues such as embryos, developing fruits N N
and roots. Kinetin is the derivative of purine base adenine, which bears furfuryl group
at 9 position which migrated to 6 position of the adenine ring during autoclaving of
DNA. According to Fox (1969) cytokinins are substances composed of one hydrophilic N
group of high specificity (adenine) and one lipophilic group without specificity. N
Kinetin (6-furfuryl aminopurine)
Cytokinin is transported to different parts of the plant through xylem elements. According to Osborne and
Black (1964), the movement of cytokinin is polar and basipetal.
(ii) Mechanism of action : Most known cytokinins have an adenine nucleus with purine ring intact with N6
substituents of moderate size. Cytokinins never act alone. In conjugation with auxins, they stimulate cell division
even in permanent cells. It was noticed by Skoog and Miller that callus cultures grew slowly on basal medium, but
growth could be promoted by adding hormones like IAA and cytokinins. No response occurred with auxin or
cytokinin alone. When both the hormones are present in equal amount, cells divide rapidly but fail to differentiate.
However, when quantity of cytokinins is more than auxins, shoot bud appears from callus. With more concentration
of auxins, roots develop fast. The similarity in structure of most cytokinins to adenine, a constituent of DNA and
RNA suggests that basic effect of cytokinin might be at the level of protein synthesis.
(iii) Bioassay of cytokinins : Bioassay is done through retention of chlorophyll by leaf discs, gains of weight
of a tissue in culture, excised radish cotyledon expansion, etc.,
(a) Tobacco pith culture : Tobacco pith culture is divided into two weighted lots one supplied with
cytokinin and the other without it. After 3-5 weeks, increase of fresh weight of treated tissue over control is noted. It
is a measure of stimulation of cell division and hence cytokinin activity.
(b) Retardation of leaf senescence : Leaves are cut into equal sized discs with the help of a cutter. They
are devided into two lots. One lot is provided with cytokinin. After 48-72 hours, leaf discs are compared for
chlorophyll contents. Cytokinin retards chlorophyll degradation.
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(c) Excised radish cotyledon expansion : Excised radish cotyledons are measured and placed in test
solution as well as ordinary water (as control). Enlargement of cotyledons indicates cytokinin activity.
(d) Root inhibition test : Kiraly and his coworkers (1966) used root inhibition test for cytokinin bioassay.
They found, that amount of root inhibition of actively growing seedlings is related to cytokinin activity.
(iv) Functions of cytokinins
(a) Cell division : Cytokinins are essential for cytokinesis and thus promote cell division. In presence of
auxin, cytokinins stimulate cell division even in non-meristematic tissues. In tissue cultures, cell division of callus
(undifferentiated mass of parenchyma tissue) is enhanced when both auxin and cytokinin are present. But no
response occurs with auxin or cytokinin alone.
(b) Cell enlargement and Differentiation : Under some conditions cytokinins enhance the expansion of
leaf cells in leaf discs and cotyledons. These cells considered to be mature and under normal conditions do not
expand. Cytokinins play a vital role in morphogenesis and differentiation in plants. It is now known that kinetin-
auxin interaction control the morphogenetic differentiation of shoot and root meristems.
(c) Delay in senescence : Cytokinin delay the senescence (ageing) of leaves and other organs by controlling
protein synthesis and mobilization of resources (Disappearance of chlorophyll). It is called Richmond Lang effect. It
was reported by Richmond and Lang (1957) while working on detached leaves of Xanthium.
(d) Counteraction of apical dominance : Auxins and cytokinins act antagonistically in the control of
apical dominance. Auxins are responsible for stimulating growth of apical bud. On the other hand, cytokinins
promote the growth of lateral buds. Thus exogenous application of cytokinin has been found to counteract the usual
dominance of apical buds.
(e) Breaking of dormancy : Cytokinins breaks seeds dormancy of various types and thus help in their
germination. They also induce germination of positively photoplastic seed like lettuce and tobacco even in darkness.
(f) Accumulation and Translocation of solutes : Cytokinins induce accumulation of salts inside the cells.
They also help solute translocation in phloem.
(g) Sex expression : Cytokinins promote formation of female flowers in some plants.
(h) Enzyme activity : Cytokinins stimulate the activity of enzymes especially those concerned with
photosynthesis.
(i) Parthenocarpy : Development of parthenocarpic fruits through cytokinin treatment has been reported by
Crane (1965).
(j) Pomalin : A combination of cytokinin (6-benzladenine) and gibberellin (GA4, GA7) called pomalin is
particularly effective in increasing apple size.
(k) Initiation of interfasicular cambium : Cytokinins induce the formation of interfasicular cambium in
plants e.g., Pinus radiata.
(l) Nucleic acid metabolism : Guttman (1957) found a quick increase in the amount of RNA in the nuclei
of onion root after kinetin treatment.
(m) Protein synthesis : Osborne (1962) demonstrated the increased rate of protein synthesis on kinetin
treatment.
(n) Flowering : Gibberellins also play an important role in the initiation of flowering. Lang (1960)
demonstrated that added gibberellin could substitute for the proper environmental conditions in Hyoscyamus niger
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Plant Growth and Development Part 1
which requires long day treatment for flowering. Such effects of gibberellin are common among vernalised and long
day plants.
Gibberellin is also known to play essential role in germination of cereal seeds.
(4) Ethylene : Ethylene is a gaseous hormone which stimulates transverse growth but retards the longitudinal
one.
(i) Discovery : The effect of ethylene had been known since long. Kerosene lamps and hay have been used
to fruit merchants to hasten colour development (ripening) in fruits. These effects are due to ethylene. Neljubow
(1901) observed that ethylene gas alters the tropic responses of roots. Denny (1924) reported that ethylene induces
ripening of fruits. Crocker et al. (1935) identified ethylene as natural plant hormone.
Ethylene is produced in plants from the amino acid methionine. It is synthesized in almost all plant parts-roots,
leaves, flowers, fruits, seeds. It is more synthesized in nodal regions. Maximum synthesis of ethylene occurs during
climacteric ripening of fruits. High concentration of auxin induce ethylene formation. When a fruit ripens its
respiration rate gradually decreases but it is reversed by a sharp increase called climactric. Some of the inhibitory
effects earlier attributed to auxin are known to be caused by ethylene.
The commercial product for providing ethylene is ethaphon (2-chloroethyl phosphoric acid). Ethaphon is a
liquid from which ethylene gas is released, hence this substance is used for artificial ripening of fruits.
(ii) Bioassay of ethylene : It is done on the principle of triple response which includes three characteristic
effects of ehtylene on etiolated seedlings of pea-viz.
• Swelling of nodes,
• Inhibition of elongation of internodes of stem,
• Induction of horizontal growth of stem against gravity.
(a) Triple pea test : Pratt and Biale (1944) developed this method for bioassay of ethylene which base on
the physiological effect of ethylene to cause
• Subapical thickening of stem,
• Reduction in the rate of elongation and
• Horizontal nutation (transverse geotropism) of stem in etiolated pea seedlings. In presence of ethylene,
epicotyls show increase growth in thickness and reduced rate of longitudinal and horizontal growth.
(b) Pea stem swelling test : Cherry (1973) used pea seedlings to measure ethylene concentration by
marked increase of stem swelling expressed as a ratio of weight to length. In one ppm of ethylene the ratio is about
4.0.
(iii) Functions of ethylene
(a) Fruit growth and Ripening : Ethylene promotes fruit growth and its ripening. The harmone is used in
the artificial ripening of climacteric fruits (e.g., Apple, Banana, Mango).
(b) Transverse growth : Ethylene inhibits longitudinal growth but stimulates transverse growth so that stem
looks swollen.
(c) Epinasty (leaf bending) : Epinasty represents more growth on upper surface of leaf than on lower
surface. Epinasty is said to be controlled by ethylene in many plants.
(d) Abscission : Ethylene stimulates formation of abscission zone in leaves, flowers and fruits.
(e) Apical dominance : Ethylene inhibits the growth of lateral buds and thus cause apical dominance (in
pea). It is believed that auxin might be functioning partly through synthesis of ethylene in causing apical dominance.
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Plant Growth and Development Part 1
(f) Root initiation : In low concentration, ethylene stimulates root initiation and growth of lateral roots and
root hair.
(g) Flowering : Ethylene stimulates flowering in pineapple and related plants though in other cases, the
hormone causes fading of flowers. Fading flowers of Vanda are known to release ethylene. Sleep disease (inrolling
of petals in blossomed flowers) in due to ethylene.
(h) Sex expression : Ethylene application increases the number of female flowers and fruits in cucumber
plants.
(i) Dormancy : It breaks dormancy of different plant organs but not of lateral buds.
(5) Abscisic acid (ABA) : Abscisic acid is a mildly acidic growth hormone, which functions as a general
growth inhibitor by counteracting other hormones (auxin, gibberellins, cytokinins) or reactions mediated by them.
(i) Discovery : The hormone was first isolated by Addicott et al. (1963) from cotton balls. They named it as
abscisin II. Simultaneously, Wareing and Cornforth isolated a substance that can induce bud dormancy. They
named the substance as dormin. Later, both these substances were found to be the same and were named as
abscisic acid. It is produced in many parts of the plants but more abundantly inside the chloroplasts of green cells.
The synthesis of abscisic acid is stimulated by drought, water logging and other adverse environmental conditions.
Therefore, it is also called stress hormone. The hormone is formed from mevalonic acid or xanthophylls. Chemically
it is dextro-rotatory cis sesquiterpene. The hormone is transported to all parts of the plant through diffusion as well
as through conductive channels.
CH3 CH3
OH O
O COO– HO CHO
Abscisic acid (ABA) Xanthoxine
In some plant tissues (especially in young shoots) occurs a related compound called xanthoxine.
Whether xanthoxine is an intermediate of the ABA-biosynthesis or whether it is an independent product
remains unknown. The structure indicates that both ABA and xanthoxine are terpene derivatives. This was proven
when it could be shown that radioactively labelled mevalonic acid is integrated into ABA though it does not
elucidate which intermediates are produced. Two alternative biosyntheses have been discussed :
(a) ABA is a degradation product of xanthophyll (especially of violaxanthin).
(b) ABA is produced from a C15 precursor using a separate pathway and is thus independent from the
carotenoid/xanthophyll metabolism.
The first idea seemed initially more plausible since the structures of xanthophylls and ABA correspond to a
large degree. In vitro occurs conversion only upon exposure to strong light and with an extremely low yield, though.
(ii) Bioassay of abscisic acid
(a) Rice seedling growth inhibition test : Mohanty, Anjaneyulu and Sridhar (1979) used rice growth
inhibition method to measure ABA like activity. The length of second leaf sheath after six days of growth is
measured.
(b) Inhibition of α-amylase synthesis in barley endosperm test : ABA inhibits the synthesis of α-
amylase in the aleurone layers which is triggered by gibberellins. Goldschmidt and Monselise (1968) developed the
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Plant Growth and Development Part 1
bioassay method to estimate ABA activity by determining the extent of inhibition of α-amylase synthesis induced by
treating barley seed endosperm with GA.
(iii) Functions of abscisic acid
(a) Control : It keeps growth under check by counter acting the effect of growth promoting hormones, i.e.,
auxins, cytokinins and gibberellins. As growth is primarily controlled by gibberellins, abscisic acid is popularly called
antigibberellic hormone. It will inhibit seed germination, growth of excised embryos, growth of Duckweed and other
plants.
(b) Dormancy : Abscisic acid acts as growth inhibitor and induces dormancy of buds towards the approach of
winter. Dormancy of seeds is mainly caused by abscisic acid. Because of its action in inducing dormancy abscisic
acid (ABA) is also called dormin. The buds as well as seeds sprout only when abscisic acid is overcome by
gibberellins.
(c) Abscission : ABA promotes the abscission of leaves, flowers and fruits in plants.
(d) Senescence : Abscisic acid stimulates senescence of leaves by causing destruction of chlorophyll (an
effect opposite to that of cytokinins) and inhibition of protein and RNA synthesis. The effect, however, can be
reversed by application of cytokinins in Lemna.
(e) Antitranspirant : Abscisic acid can be used as antitranspirant. Application of minute quantity of ABA to
leaves reduces transpiration to a great extent through partial closure of stomata. It thus conserves water and reduces
the requirement of irrigation.
(f) Hardiness : Abscisic acid promotes cold hardiness and inhibits growth of pathogens.
(g) Flowering : ABA delays flowering in long day plants. However, in some short day plants (e.g., strawberry,
black current) it promotes flowering.
(h) Rooting : Abscisic acid can be used to promote rooting in many stem cuttings.
(6) Wound hormone or Traumatic acid or Necrohormone : Haberlandt (1913) reported that injured
plants cells release a chemical substance (wound hormone), which stimulate the adjacent cells to divide rapidly in
order to heal up the wound. English et al. (1939) finally isolated and crystallized this wound hormone and named
it as Traumatic acid. Although traumatic acid has been found to be very active in inducing meristematic activity in
uninjured green bean pods, but it is not effective in most of the plant tissues including tobacco pith tissues.
(7) Morphactins : Morphactins are synthetic growth regulators which act in variety of ways on the natural
regulation mechanisms of plants. The important ones are phenoxyalkancarboxylic acid (synthetic auxin), substituted
benzoic acids, Malic acid hydrazide, Fluorene-9 carboxylic acids and their derivatives, Chlorflurenol, Chloroflurun,
Flurenol, Methylbenzilate, Dichlorflurenol, etc. Morphactins have fundamental action on morphogenesis of plants
and this characteristic designation (morphactins) is derived from morphologically active substances.
HO C=O Cl HO C=O
O – CH3 O – CH2 – CH2 – CH2– OCH3
Morphactins IT 3456 and IT 3233
Chapter 21 399
Plant Growth and Development Part 1
The actions of these substances are systematic and after their uptake they are transported and distributed not
polarly (as seen by IAA) but basi- and acropetally. Generally these are growth inhibitors. These contain ‘fluorene
ring’ in their structure.
(i) Functions
(a) Seed germination : In general, morphactins inhibit germination seeds particularly the emergence of the
radicle from the seed shell. This property can be counteracted with GA3 and almost completely by cytokinins. The
germination of fern spores is also delayed by morephactins.
(b) Growth seedling : Morphactins inhibit the growth of seedling affecting the shoot and often also root.
With this property they show a similarity with cytokinin. The inhibitory effect of seedling shoot growth can be partly
counteracted with GA3 but not the inhibition of root growth.
(c) Stem elongation : They have inhibitory effect on the stem elongation. Increased concentration produces
dwarfing in the plants. The inhibitory effect of morphactins is not only observed in stem elongation but also on the
new growing shoot organs.
(d) Polarity of cell division : Denffer and others (1969) observed in the dividing cells of the root tips of
Allium that treatment of morphactin (CFI) results in random orientation of the mitotic spindle and plane of cell
division, i.e., they exercise depolarisation during cell division.
(e) Histogenesis and Morphogenesis : Morphactins induce anomalies espicially in new-formed organs in
case of Begonia. Formation of cornets (fusion of leaf with stem) and ochria (fusion of calyx with other floral parts).
The single flowers and the number and differentiation of floral organs were reduced.
(f) Apical dominance and branching : Morphactins treatment with grasses and cereals increased tillering
and also increased number of lateral buds and stimulated extension growth of lateral shoots.
(g) Prolonged bud dormany : The emergence of buds from the storage organs of various perennial species
is delayed when plants are treated with morphactins as reported in Solanum tuberosum and Malus sylvestris.
(h) Root growth and Root branching : Lateral buds are inhibited or retarde and primary roots are
promoted at low concentration but inhibited at higher concentration of morphactins. In general, the action of
morphactins on the longitudinal growth of the roots system may be considered as the reverse of their action on the
shoot system.
(i) Realisation of flowering : Morphactins have also been found effective on flowering stimulation,
sequence of flowering, position and number of flowers, formation of flowers, inflorescence and parthenocarpy, etc.
(j) Depot effect : First morphactine are accumulated in plants and after sometime show their effect.
(8) Jasmonic acid (Jasmonates) : According to Parthier (1991), jasmonic acid and its methyl esters are
ubiquitous in plants. They have hormone properties, help regulating plant growth, development and they seem to
participate in leaf senescence and in the defense mechanism against fungi.
O 8 11 O O O– Glucose
COOH
546 7 9 10 12
3 COOH Jasmonic acid
1 COOH
2
Just like all other plant hormones jasmonates have both activating and inhibiting effects. Synergistic and
antagonistic effects on other hormones have also been observed. Jasmonate derivatives induces the accumulation
of proteins so-called jasmonate-induced-proteins that were found in all plant species tested. Their accumulation can
Chapter 21 400
Plant Growth and Development Part 1
also be caused by desiccation or ABA effects. Jasmonate-induced-proteins are of varying molecular weights, and
molecules of different size classes have immunologically been shown to be related. The major portion of these
proteins is not glycosylated, has no proteolytic activity and is metabolically stable. Labelling with immunogold and
electron microscopy showed that some of them are located within the nucleus, while others were detected in the
vacuole. None have ever been found in mitochondria. Their synthesis can be inhibited by cycloheximid, but not by
chloramphenicol. Chloramphenicol affects mitochondrial proteins. Jasmonate-induced-proteins are lacking in roots,
in bleached leaves and in leaves of chlorophyll-deficient Hordeum vulgare mutants. They exist in etiolated leaves,
though. Jasmonates do not only regulate the transcription of these proteins, they do also influence the rate of
translation of different groups of mRNA. They do, for example, decrease the production rate of several essential
housekeeping proteins.
Just like ABA jasmonates inhibit a premature germination of the oil-containing seeds of Brassica and Linum.
After germination they induce the synthesis of the seed storage proteins Napin and Cruciferin as well as that of
several more elaiosome-associated proteins.
(9) Calines (Formative hormones) : Certain other natural growth hormones in plants called as calines or
formative hormone which are throught to be essential for the effect of auxin an root, stem and leaf growth they are :
(i) Rhizocaline or Root forming hormone : It is produced by the leaves and translocated in a polar
manner down the stem.
(ii) Caulocaline or Stem forming hormone : It is produced by the roots and is transported upward in the
stem.
(iii) Phyllocaline or Self forming hormone : It is produced probably by the cotyledons. It stimulates
mesophyll development in the leaves and is synthesized only in the presence of light.
None of these caline has yet been isolated.
(10) Vitamins
• The term vitamin has been derived from vital amine. They are heterogenous group of organic compounds
which are needed in very small quantities (as accessory food factors) for different metabolic processes.
• They are essential for normal growth and development and maintenance of health as well as vigour.
• Vitamins are synthesised by plants and microbes, though they are required by all types of organisms.
• Most of the vitamins functions as coenzymes and prosthetic groups of various enzymes connected with
protein, fat and carbohydrate metabolism.
• Vitamin K is component of electron transport chains. Some vitamins are essential in maintaining cell
membranes and acting as antioxidants. Therefore, vitamins are important growth regulators.
• Vitamins are of two types : Fat soluble- it includes vitamins A, D, E and K, water soluble – vitamin B
complex and vitamin C.
Important Tips
• The double sigmoid growth curve occurs in some fruits e.g., Grapes, plum.
• Measurement of growth in young root by making it at 1mm intervals with Indian Ink was first done by Strasburger.
• Inflexion point is the point at which growth begins to decline (beginning of decelerating phase) after the exponential increase.
• 20000 tons of Avena tips give 1g of IAA.
• The development of shoot and root is determined by cytokinin and auxin ratio.
• IBA is the most potent root initiator.
• Mixture of 2, 4-D and 2, 4, 5-T (dioxin) is given the name ‘Agent orange’ which was used by USA in Vietnam war for defoliation
of forests (i.e., in chemical warfare).
Chapter 21 401
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