Chapter 14: Plant Tropism & Hormonal Control
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Transcript of Chapter 14: Plant Tropism & Hormonal Control
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Chapter 14: Plant Tropism
& Hormonal Control
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How do plants respond to their environment?
Plants can’t move or see!Plants respond to stimuli
Physical factors ?
Chemical factors
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Chapter 14 goals:Know the effects of the 5 main groups of plant hormones:
1. Auxins2. Gibberellins3. Cytokinins4. Inhibitors (Abscisic Acid)5. Ethylene
Understand:PhototropismGeotropismThigmotropism
Relationship between photoperiod and flowering
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What is Tropism? Tropism = Directed Growth
+ve toward source–ve away from sourceEg. phototropism
Plants use hormones to direct growth in response to stimuli1. Auxins2. Gibberellins3. Cytokinins4. Inhibitors (Abscisic Acid)5. Ethylene
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AuxinsProduced continually in the tip of a shoot
(meristem)Cause localised growthAuxin inhibits lateral bud growthLight effects membrane permeability to auxinDiffuses through cell layers (not vascular
tissue)Moves away from light & softens cell walls
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Gibberellins Produced in flowers, fruit, seeds, growing
buds and elongating stemsCause wholesale growth
Cytokinins Stimulates cell division and differentiation Cytokinin : Auxin Ratio Cytokinin = stems and leaves develop
Cytokinin = roots develop
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Abscisic AcidGrowth inhibitionProduced in cholorolopastsControls:
leaf drop (deciduos plants)ripe fruit drop bud and seed dormancyVernalisation (flowering after cold)
Protects plants against extreme conditions:Salinity, Temperature & Water levelsControl stomata closure
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EthyleneReleased by ripening fruitTriggered by auxin and abscisic acidStimulates further ripening
Increases rate of respirationBreak-down of starch into sugar
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PhototropismGrowth towards lightLight effects membrane permeability to auxinDiffuses through cell layers (not vascular tissue)Moves away from light & softens cell walls
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Geotropism Growth in response to gravityStems grow against gravityRoots grow with gravityAmyloplasts involved in direction of root
growth
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Apical dominance – one main stem Auxin inhibits lateral bud growthAuxin = taller plants without side branchesBushfire, commercial benefit
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Photoperiod and Flowering Most plants flower irrespective of photoperiod
Neutral plantsOthers depend on exposure to darkness
A pigment in leaves detects exposure to ‘red light’
‘Short-day’ (Long-night) Flower in cooler months
‘Long-day’ (Short-night) Flower in warmer months