CHAPTER 5 RESPONSE IN PLANTS

CHAPTER 5
RESPONSE IN

PLANTS

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5.1 TYPES OF RESPONSES

TYPES OF PLANT RESPONSE

TROPISMS RESPONSES NASTIC RESPONSE
 Responses of certain parts of a plant such as  Include photonasty, seismonasty,

roots and shoots that respond towards or nyctinasty, thermonasty and
away from a stimulus thigmonasty
 There are several types of tropism in plants:
- Thigmotropisms
- Geotropisms
- Hydrotropism
- Phototropism
- chemotropism

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CHARACTERISTICS OF TROPISM

The response is permanent Positive tropism:
and influenced by plant Growth towards direction of
hormones
stimulus

The response is slow Negative tropism:
and not apparent Growth away from

The response direction stimulus
is dependent on the

direction of the stimulus

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TYPES OF TROPISM

THIGMOTROPISM
Is the plant response to TOUCH

Tendrils exhibit
positive thigmotropism
by coiling or wrapping
around an object such
as a wooden stake for

support

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TYPES OF TROPISM

GEOTROPISM
Is the plant response to GRAVITY

roots show positive Shoots show negative
geotropism geotropism

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TYPES OF TROPISM

HYDROTROPISM
Is the plant response to WATER

Roots show positive Shoots show negative
hydrotropism hydrotropism

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TYPES OF TROPISM

PHOTOTROPISM
Is the plant response to LIGHT

Shoots show positive
phototropism because they

grow towards light

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TYPES OF TROPISM

CHEMOTROPISM
Is the plant response to CHEMICAL

Roots show negative chemotropism when Roots show positive
they grow away from chemical chemotropism when they
substances such as poison grow towards mineral salts

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CHARACTERISTICS OF
NASTIC RESPONSES

Not all nastic responses This response is quicker and
are growth responses more apparent compared to

tropism response

Some nastic responses
are intended for
survival

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TYPES OF NASTIC RESPONSES

PHOTONASTY
Is the plant response to LIGHT

Japanese roses close
at night

Japanese roses open when they receive
maximum sunlight

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TYPES OF NASTIC RESPONSES

SEISMONASTY
 Is the plant response that occurs

due to a mechanical stimulus such
as shock, touch, wind and raindrops.
 Seismonasty responses can be seen

on leaves, stigma and stamen

Mimosa plant

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TYPES OF NASTIC RESPONSES

NYCTINASTY Leucaena leucocephala
 Is a circadian rhythm that responds (River tamarind) during night

towards the onset of darkness – closes its leaves
 Normally the leaves of legume plants

such as the river tamarind closes its
leaves at night and unfurls them
during daytime

Leucaena leucocephala
(River tamarind) during day –

unfurls leaves

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TYPES OF NASTIC RESPONSES

THERMONASTY
 Is a plant response towards changes

in the surrounding temperatures
 Example : Tulips open when there is

an increase in temperature and
close when the temperature drops

Tulip close
Tulip opens

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TYPES OF NASTIC RESPONSES

THIGMONASTY
 Is a plant response towards vibration
 This response can be seen in carnivorous

plants such as the Venus flytrap
 The exerted stimulus causes it to clamp its

leaves shut and trap the insect inside it

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5.2 PHYTOHORMONE

 Phytohormones OR plant hormones are chemical substances that stimulate and coordinate
responses in plants, at low concentration.

 Phytohormones are synthesized in a certain plant organ and transported to target organ
through the phloem

AUXIN
F1 : Functional in phototropism and geotropism
F2 : Stimulates development of the apical dominance in shoots and

roots
F3 : Stimulates growth and elongation in root and shoot cells
F4 : Stimulates development of adventitious roots at the end of stem

cuttings
F5 : Stimulates cell division in cambium during secondary growth
F6 : Inhibits abscission of fruits and young leaves
F7 : Inhibits growth of lateral buds

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GIBBERELLIN
F1 : stimulates growth and elongation in the stem cell
F2 : Stimulates development of leaves, flowers and fruits
F3 : Stimulates development and germination of seeds
F4 : causes growth of flowers in dwarf plants
F5 : inhibits development of roots

CYTOKININ
F1 : stimulates division and elongation of root and stem cells

when auxin is present
F2 : stimulates seed germination
F3 : inhibits development of apical dominance
F4 : delays leaf senescence
F5 : stimulates growth of lateral buds

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ABSCISIC ACID
F1 : inhibits growth of plants
F2: stimulates abscission of mature fruits, leaves and flowers
F3 : induces seed dormancy
F4 : induces stomatal closing during drought season
F5 : inhibits growth of buds and seed germination

ETHYLENE
F1 : stimulate ripening in fruits
F2 : stimulates senescence process in plants
F3 : stimulates abscission of leaves and fruits

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Effects of Auxin on Growth Response

 Plants respond to stimulus in the form
of growth

 This growth response is controlled by
the auxin

 Auxin has been synthesized at the PREPARED BY CIKGU HUSRITA MRSM TRANSKRIAN
coleoptile tips will stimulate cell
elongation

 Auxin distribution influences the
direction of the shoot growth

 Uniform distribution of auxin causes the
coleoptile to grow upwards

 The area which receives more auxin will
grow faster and cause coleoptiles to
curve to the side that receives less or
none of the auxin

Effects of Auxin on Growth Response

 Tropism is closely related to auxin
distribution

 External stimuli such as light and gravity
influences the auxin distribution in plants

 Auxin produces different effects to the
cells in the shoots and the cells in the
roots

 High auxin concentration stimulates cell
elongation in the shoots but also inhibits
cell elongation in the roots

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The Role of Auxins in Plant Responses

The Role of Auxin in Phototropism
 The response direction of shoot tips
depends on the direction of light stimulus
 The auxin concentration is higher on the

shaded side
 The cells in the shaded side elongate

more that the cells in the bright side
 As a result, the shoots bend towards the

light
 Shoots show positive phototropism

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The Role of Auxins in Plant Responses

The Role of Auxin in Geotropism

 Auxin also controls plant respond  In shoots:  In roots:
to gravity i. High auxin concentrations cause High auxin concentrations at the
cells at the bottom to grow faster bottom of roots inhibit root cell
 Seeds germinate horizontally in and also allow the shoot to bend elongation and cause roots to bend
the soil
upwards downward
 Due to the pull of gravity, auxin
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tips and roots

 Roots grow downwards following
the pull of gravity and exhibiting
positive geotropism.

 Shoots grow upwards against the
pull of gravity, exhibiting negative
geotropism

5.3 APPLICATION OF PHYTOHORMONES IN AGRICULTURE

 Phytohormones are important chemicals that are directly involved in the development of
flowers, fruits, stems and roots

 Exists naturally or be synthesized in the laboratory
 There are phytohormones that can be extracted for multiple purposes in agriculture

AUXIN GIBBERELLIN
• Encourages growth in crops • Treats mutated dwarf plants to
• Promotes root growth (asexual reproduction) on
grow to normal heights
woody plant stems for horticultural crops • Promotes rapid elongation of
• Produces fruit without seeds via parthenocarpy
• Used as weed killer flower stems
• Induces dormancy in potatoes during storage or • Used to produce larger grapes
• Promotes germination of seeds
distribution
• Promotes growth of low-lying and lush plants such as salad, oats and tobacco
during low temperatures and
low light intensity

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CYTOKININ ETHYLENE
• Tissue culture technique – promote division and • Used commercially to promote maturation of

differentiation of cells fruit quickly and evenly
• Tissue culture technique – used together with • Promotes simultaneous flowering in plants in

auxin to form plant organs such as roots and the field
stems
• Used to delay leaf senescence in newly cut
flowers

ABSCISIC ACID
• Inhibits germination and growth

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