Module 5.5 part 7

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  • Module 5.5 controlling plant growth & commercial use of plant hormones
    • Effects of Plant hormones
      • Cytokinins
        • promotes cell division & cell expansion/delay leaf senescence/overcome apical dominance
      • Abscisic acid
        • inhibit seed germination & growth/stomatal closure during water stress
      • Auxins
        • promote cell elongation/inhibit growth of lateral shoots (apical dominance)/inhibit leaf abscission
      • Gibberellins
        • promote seed germination & elongation of stems
      • Ethene
        • promotes fruit ripening
    • Role of auxin's in apical dominance
      • 1. breaking off the shoots tip (apex) results in growth of lateral branches
      • 2. applying auxin paste to the cut end restores apical dominance
      • 3. applying a ring of auxin transport inhibitor below the apex removes apical dominance
      • ... auxin level in lateral buds increases when the apex is removed
    • lateral evidence
      • abscisic acid  (ABA) inhibits bud growth, lateral buds start to grow when ABA levels drop
      • Auxin and ABA are synergistic (promote each others effects)
      • cytokinins can override apical dominance if applied directly
      • shot apex is normallly the sink for cytokinins from the root, if it is removed they are diverted to the lateral buds
    • Do gibberellins cause stem elongation?
      • tall peas
        • dominant homozygous alleles
      • dwarf peas
        • recessive homozygous alleles
    • how does gibberellin work?
      • stimulating cell elongation
        • loosening cell walls
      • stimulating cell division
        • stimulating a cell cycle protein
      • causes growth in the internodes
    • Commercial
      • Auxins
        • weed killer
        • high concentration causes rapid growth
          • tissues become sistorted and the damage allows pathogens to entre
        • Synthetic auxin's (higher conc. than natural)
          • useful in fields of cereals/lawns
            • grasses are less sensitive
          • weed killers selectivly destroy broad-leaved weeds
        • stimulates root growth in cuttings
        • commercially available as rooting powders
        • inhibits fruit abscission
          • e.g. in orchards fruit drops and rots
      • Gibberellins
        • treats unpollinated flowers
          • causes fruit to develop e.g. seedless grapes
        • fruits formed without fertilisation are called parthenocarpic fruits
      • Ethene
        • timulates fruit ripening
          • useful in fruits that are liable to damage during transport
            • e.g.tomatoes
        • fruits picked when unripe and hard
        • transported
        • artificially ripened in transit  or at the destination using ethene

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