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Life Processes in Plants
Plant responses to the Environment
1
J Gerber and J Goliath
Plant hormones
◊ General functions of the following plant hormones:
 Auxins
 Gibberellins
 Abscisic acid
 Investigate geotropism and phototropism by controlling variables
◊ Role of auxins in geotropism through the regulation
of differential growth
◊ Role of auxins in phototropism through the regulation
of differential growth
◊ Uses of plant hormones in agriculture:
 Auxins as herbicides
 Auxins to control apical dominance
 Auxins and gibberellins to increase/stimulate fruit development
2
J Gerber and J Goliath
Plant defense mechanisms
◊ The role of the following as plant defense mechanisms:
 Chemicals (protection) against infection from pathogenic micro-
organisms and from being eaten by herbivores)
 Thorns (protection from being eaten by animals/to reduce water
loss
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J Gerber and J Goliath
Activity 1- Tropism
Instructions
 Explanation of activity on phototropism and
geotropism.
 In pairs/groups discuss the information and
answer the questions that follow.
 After 15 minutes each pair/group give
feedback.
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J Gerber and J Goliath
Plant hormones
 Performs a specific function, which controls further growth
and development of the plant.
 Stimulation – a process is turned on or activated.
 Inhibition – a process is turned off or stopped.
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J Gerber and J Goliath
Auxins
 Group of plant hormones that are mainly produced in the
apical meristem of the stems and roots.
 Indole acetic acid is the most common natural auxin.
 Auxins move down in the phloem to cells lower down in the
plant.
 Small amounts of auxins are produced in the growing points
of roots.
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J Gerber and J Goliath
Functions of auxins
 Stimulate cell division.
 Stimulate cell elongation.
 Promotes apical dominance (agriculture).
 Develops adventitious roots.
 Cause tropic movement in stems and roots.
 Stimulates the development of fruit.
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J Gerber and J Goliath
Functions of gibberellins
 Stimulate stem elongation.
 Stimulate root growth.
 Promote flowering.
 Promote the growth of lateral buds.
 Stimulate the germination of seeds.
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J Gerber and J Goliath
Functions of abscisic acid
 Inhibits cell division and growth in stems and roots.
 Prevents bud and seed growth in winter.
 Promotes the abscission of leaves and frui.t
 Controls the stomata opening and closing.
 Speeds up fruit ripening.
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J Gerber and J Goliath
Role of auxins in phototropism
 Phototropism is the growth
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movement of a plant or part of a
plant in response to unilateral light
stimulus.
A high auxin concentration builds up
on the side of the stem furthest of the
light source.
Cell division and elongation are
stimulated and grow faster.
The stem curves in the direction of
the light and positively phototropic.
Negative phototropism occurs in
roots – grow away from a light
source.
J Gerber and J Goliath
Role of auxins in geotropism
 Young seedling is placed horizontally
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– roots grow downwards and stem
upwards.
Auxins accumulate on the lower side
of the root.
Higher auxin concentration on the
lower side of the root inhibits cell
division and elongation.
Cells on the upper side of the root
grow faster – lower auxin
concentration.
Root curves downwards – positively
geotropism.
Stem curves upwards – negatively
geotropism.
J Gerber and J Goliath
Uses of plant hormones in agriculture
 Root formation in cuttings.
 Production of seedless fruit.
 Ripening of fruit.
 Increase in fruit size.
 Stimulation of germination of seeds.
 Weed control.
 Apical dominance.
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J Gerber and J Goliath
Auxins as herbicides
 Synthetic herbicides (weed killers) contain high
concentrations of auxins.
 Accelerate the metabolism of broad-leaved dicotyledonous
weeds and stimulate growth.
 Weeds grow so fast that water absorption is insufficient.
 Plants weaken and die.
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J Gerber and J Goliath
Apical dominance
 Apical dominance occurs when the apical
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bud of the main shoot grows more strongly
than lateral bud below.
Removing the apical bud from the top of
the plant, auxin is also removed.
More lateral grow below the apical bud.
Pruning promotes the grow of lateral
branches.
The value of pruning – plant grows bushier
and fruit formation can be controlled.
J Gerber and J Goliath
Plant defense mechanisms- thorns
 Sharp thorns protect the
plants against herbivores.
 Less damage to plants by
herbivores and plants
continue to grow.
 Reduce water loss.
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J Gerber and J Goliath
Plant defense mechanisms- chemicals
 Many plants produce
chemicals and play a role in
the defence against
herbivores and pathogenicorganisms.
 Chemicals are poisonous or
influence the digestibility
of plant material.
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J Gerber and J Goliath
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