Plant responses to the Environment Life Processes in Plants J Gerber and J Goliath 1.
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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
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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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Activity 1- Tropism
InstructionsExplanation 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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Plant hormonesPerforms 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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AuxinsGroup 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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Functions of auxinsStimulate 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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Functions of gibberellinsStimulate stem elongation.Stimulate root growth.Promote flowering.Promote the growth of lateral buds.Stimulate the germination of seeds.
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Functions of abscisic acidInhibits cell division and growth in stems
and roots.Prevents bud and seed growth in winter.Promotes the abscission of leaves and frui.tControls the stomata opening and closing.Speeds up fruit ripening.
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Role of auxins in phototropism Phototropism is the growth
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.
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Role of auxins in geotropism Young seedling is placed
horizontally – 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.
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Uses of plant hormones in agricultureRoot 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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Auxins as herbicidesSynthetic 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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Apical dominance Apical dominance occurs
when the apical 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.
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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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