Chapter 14: Plant Tropism

& Hormonal Control

How do plants respond to their environment?

Plants can’t move or see!Plants respond to stimuli

Physical factors ?

Chemical factors

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

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

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

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

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

EthyleneReleased by ripening fruitTriggered by auxin and abscisic acidStimulates further ripening

Increases rate of respirationBreak-down of starch into sugar

PhototropismGrowth towards lightLight effects membrane permeability to auxinDiffuses through cell layers (not vascular tissue)Moves away from light & softens cell walls

Geotropism Growth in response to gravityStems grow against gravityRoots grow with gravityAmyloplasts involved in direction of root

growth

Apical dominance – one main stem Auxin inhibits lateral bud growthAuxin = taller plants without side branchesBushfire, commercial benefit

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