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. Auxins 2. Gibberellins 3. Cytokinins 4. Inhibitors (Abscisic Acid) 5. Ethylene Understand: Phototropism Geotropism Thigmotropism Relationship between photoperiod and flowering

What is Tropism? Tropism = Directed Growth +ve toward source –ve away from source Eg. phototropism Plants use hormones to direct growth in response to stimuli 1. Auxins 2. Gibberellins 3. Cytokinins 4. Inhibitors (Abscisic Acid) 5. Ethylene

Auxins Produced continually in the tip of a shoot (meristem) Cause localised growth Auxin inhibits lateral bud growth Light effects membrane permeability to auxin Diffuses through cell layers (not vascular tissue) Moves away from light & softens cell walls

Gibberellins Produced in flowers, fruit, seeds, growing buds and elongating stems Cause wholesale growth Cytokinins Stimulates cell division and differentiation Cytokinin : Auxin Ratio Cytokinin = stems and leaves develop Cytokinin = roots develop

Abscisic Acid Growth inhibition Produced in cholorolopasts Controls: leaf drop (deciduos plants) ripe fruit drop bud and seed dormancy Vernalisation (flowering after cold) Protects plants against extreme conditions: Salinity, Temperature & Water levels Control stomata closure

Ethylene Released by ripening fruit Triggered by auxin and abscisic acid Stimulates further ripening Increases rate of respiration Break-down of starch into sugar

Phototropism Growth towards light Light effects membrane permeability to auxin Diffuses through cell layers (not vascular tissue) Moves away from light & softens cell walls

Geotropism Growth in response to gravity Stems grow against gravity Roots grow with gravity Amyloplasts involved in direction of root growth

Apical dominance – one main stem Auxin inhibits lateral bud growth Auxin = taller plants without side branches Bushfire, commercial benefit

Photoperiod and Flowering Most plants flower irrespective of photoperiod Neutral plants Others 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