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Chapter 14: Plant Tropism & Hormonal Control

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: Auxins Gibberellins

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Chapter 14: Plant Tropism & Hormonal Control

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  1. Chapter 14: Plant Tropism & Hormonal Control

  2. How do plants respond to their environment? • Plants can’t move or see! • Plants respond to stimuli • Physical factors • ? • Chemical factors

  3. Chapter 14 goals: • Know the effects of the 5 main groups of plant hormones: • Auxins • Gibberellins • Cytokinins • Inhibitors (Abscisic Acid) • Ethylene • Understand: • Phototropism • Geotropism • Thigmotropism • Relationship between photoperiod and flowering

  4. What is Tropism? • Tropism = Directed Growth • +vetoward source • –veaway from source • Eg. phototropism • Plants use hormones to direct growth in response to stimuli • Auxins • Gibberellins • Cytokinins • Inhibitors (Abscisic Acid) • Ethylene

  5. 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

  6. 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

  7. 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

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

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

  10. Geotropism • Growth in response to gravity • Stems grow against gravity • Roots grow with gravity • Amyloplastsinvolved in direction of root growth

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

  12. 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

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