Plant Response to Stimuli

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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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• Explain the term Combined Effect
• Explain the term Growth Promoter.
• Explain the term Growth Inhibitor
• Name 4 methods of anatomical/chemical adaptations
  that protect plants.
• State 2 examples of the use of plant regulators.
• Mandatory ActivityEffects 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
• light intensity,
• day length,
• gravity,
• temperature
• 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

• The ability to detect change and to respond is
  called?
• Plants adapt to new situations by?
• What is a growth regulator?
• List the properties of plant growth regulators.
• What is a meristem?

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Tropisms

• A tropism is a plants 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 gravityit 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

• What is meant by the term tropism.
• What is a stimulus.
• Name 5 different tropisms and the stimulus
  involved in each case.
• 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.

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Auxins

• Growth promoters i.e. promote cell enlargement
  and growth,
• are involved in phototropism and apical dominance

APICAL DOMINANCE
PHOTOTROPISM
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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
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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

• What is a plant growth regulator?
• Explain the terms promote and inhibit.
• Name 5 growth regulators and a function of each.
• 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
• Structural or anatomical adaptations
• 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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Structural Adaptations

• Bark/epidermis prevents entry of microbes and
  reduces loss of water
• Thick cuticle on stem or leaves in plants that
  live in dry places
• Thorns prevent plants from being eaten by
  herbivores
• Stinging cell in epidermis prevent plants been
  eaten
• Guard cells change shape when they lose water
  which causes stomata to close and this reduces
  water loss

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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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Chemical Adaptations

1. Production of Tannins to make the plant
   indigestible
2. Production of toxic chemicals to prevent growth
   of insect larvae
3. Production of nicotine in legumes that damage
   nerves and muscles
4. Production of heat shock proteins to protect
   enzymes when temperatures are high.

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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
• phototropism,
• geotropism,
• cell enlargement and growth,
• apical dominance,
• root growth,
• fruit development,
• vascular development,
• and senescence aging.

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• AUXINS
• Growth the zone of elongation grows, extending
  the shoot root.
• Apical dominance auxin produced in the shoot
  tip inhibits the growth of buds lower down stem.
• 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

• 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).

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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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