Chapter 24
Plant Response to Stimuli
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3.5.2 Responses in the Flowering Plant
Objectives –
What you will need to know from this section
1. Describe the organs used by plants to respond to stimuli
2. Explain the term: growth regulation.
3. Define the following terms: tropisms, phototropism,
geotropism, thigmotropism, hydrotropism, chemotropism.
4. Name examples of phototropism & geotropism.
5. Define the term: growth regulator.
6. Outline the transport of regulators via the vascular system.
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7. Explain the term: Combined Effect
8. Explain the term: Growth Promoter.
9. Explain the term: Growth Inhibitor
10.Name 4 methods of anatomical/chemical
adaptations that protect plants.
11.State 2 examples of the use of plant regulators.
Mandatory Activity:
Effects of I.A.A. Growth Regulators on Plants
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Responses in the Flowering Plant
• The ability to detect change and to respond
is called sensitivity. Response is a form of
defence that allows organisms to
survive.
• Plant adapt to new situations by modifying
their growth, by means of chemicals called
growth regulators [hormones].
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• A hormone is a chemical produced in one
part of an organism, transported to other
tissues where it has its effect.
• External factors that regulate the growth of
plants are:
1. light intensity,
2. day length,
3. gravity,
4. temperature
5. and others.
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Plant growth regulators
[hormones]
Plant growth regulators [hormones] are
chemicals that interact with one another to
control a particular development or response.
Plants only grow at their tips,
in small regions of active cell
division called meristems—
the tips of shoots, roots
and side buds.
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Plant Growth Regulators
1. Chemicals that control the growth of plants
2. They are produced in the meristems
3. They are transported in the xylem and
phloem
4. Even a small amount of growth regulator can
have great effect on growth
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LEARNING CHECK
1. The ability to detect change and to respond
is called?
2. Plants adapt to new situations by?
3. What is a growth regulator?
4. List the properties of plant growth
regulators.
5. What is a meristem?
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Tropisms
• A tropism is a plant’s response to a stimulus
e.g. sunlight, gravity.
• Phototropism is a growth response of a stem
towards light, so that it can receive the
maximum amount of light for photosynthesis
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Geotropism is the growth of a plant in response
to gravity—it allows the root to get the water and
nutrients it needs from the soil, and raises the
leaves up into the light for photosynthesis.
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Thigmotropism is the growth of a plant in
response to contact, e.g. ivy wraps around
objects which help support it.
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Hydrotropism is the response of roots to
water – they will grow towards it.
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Chemotropism is the response to chemicals,
e.g. pollen tubes growing down the carpel in
response to chemicals released by the ovule.
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LEARNING CHECK
1. What is meant by the term “tropism”.
2. What is a stimulus.
3. Name 5 different tropisms and the stimulus
involved in each case.
4. List the advantage to the plant of each
tropism.
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Plant growth regulators
• Plant growth regulators interact with one
another to control a particular development or
response.
• Some regulators promote growth, i.e. speed
up growth e.g. auxins, gibberellins, cytokinins.
• Some regulators inhibit growth, i.e. slow down
growth e.g. abscisic acid and ethene.
New roots developing
Growth shut down for
winter
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Auxins
• Growth promoters i.e. promote cell enlargement
and growth,
• are involved in phototropism and apical
dominance
PHOTOTROPISM
APICAL DOMINANCE
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Apical dominance
• Auxins are responsible for
apical dominance, where the
main bud inhibits the growth
of buds lower down stem.
• This photograph shows side
buds sprouting when the
main stem is cut off [pruned].
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Phototropism
When light shines on plants it causes auxin to
move down the shaded part of the stem, causing
the shoot to bend and grow towards the sunlight phototropism
Quicker growth
here due to more
hormones
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Gibberellins
1. cause stem lengthening,
2. mobilise the stored food in germinating seeds,
3. break dormancy in buds and seeds in Spring.
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Cytokinins
Cytokinins stimulate cell division (mitosis) and
trigger leaf growth in spring.
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Abscisic Acid
Abscisic acid triggers bud and seed
dormancy in autumn and inhibits cell growth.
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Ethene
Ethene promotes ripening of fruit and the fall
of leaves, flowers and fruits.
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Uses of plant regulators
1. Artificial auxins are used in rooting powders to
stimulate root formation on stem cuttings.
2. A synthetic auxin [2,4-D] is used as a
weedkiller.
3. Fruit is transported green and unripe, and can
then be quickly ripened by spraying it with
ethene.
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LEARNING CHECK
1. What is a plant growth regulator?
2. Explain the terms “promote” and “inhibit”.
3. Name 5 growth regulators and a function
of each.
4. Give 3 examples of how we make use of
plants growth regulators [natural or
artificial]
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Adaptations for Protection in Plants
Plants can adapt themselves for protection in
two ways:
1. Structural or anatomical adaptations
2. Chemical adaptations
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Plant Defences
Some adaptations that plants use to protect
themselves include:
1. Anatomical:
Spines, thorns or stinging hairs to deter animals
from eating them, e.g. cacti, nettles.
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2. Chemical:
Toxins that cause illness or death, e.g. Oak
produces tannins in their leaves to protect them
against caterpillars.
The leaves and acorns of
the oak tree are poisonous
to cattle, horses, sheep,
and goats in large amounts
due to the toxin tannic acid,
and cause kidney damage
and gastroenteritis.
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LEARNING CHECK
1. Why is defence important to plants?
2. List four methods used by plants to defend
themselves.
3. Explain the term “structural adaptation”.
4. What is a toxin?
5. What advantage is it to a plant that it grows
throughout its life [from meristems]?
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AUXINS -- HIGHER LEVEL
Auxins affect virtually every aspect of plant
development, and how they respond to environmental
stimuli. Auxins influence includes
1. phototropism,
2. geotropism,
3. cell enlargement and growth,
4. apical dominance,
5. root growth,
6. fruit development,
7. vascular development,
8. and senescence [aging].
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AUXINS
1. Growth – the zone of elongation grows,
extending the shoot & root.
2. Apical dominance – auxin produced in the
shoot tip inhibits the growth of buds lower
down stem.
3. Tropisms - involved in phototropism —stem
bends towards light due to cell enlargement
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IAA (Indoleacetic acid) is an auxin that is made
in the meristems of shoots, buds and roots, and
in the tips of coleoptiles [of grasses].
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The coleoptile is the
protective sheath around
the leaves and shoot of
grass seedlings, such as
corn or oats.
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Auxin
Animations of auxin
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LEARNING CHECK
1.
2.
3.
4.
5.
6.
7.
What is an auxin?
List 4 effects of auxins in a plant.
List the properties of auxins.
What does elongation mean?
What is IAA?
What is a meristem?
What is a coleoptile?
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Phototropism & Auxins
If a plant receives light from one side only:
•the light causes auxin to travel down by active
transport in the phloem tissue
•from the stem / meristem
•down the ‘dark’ side of the stem (or coleoptile).
Quicker growth
here due to more
auxin
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Phototropism & Auxins
• The extra auxin loosens the cellulose fibres in
plant cell walls &
• this allows the cells to elongate (get longer)
faster than cells on the ‘bright’ side.
• causing the stem to bend towards the light
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