Plant Responses

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  • Plant Responses
    • What do plants respond to?
      • Water
      • Touch
      • Chemicals
      • Light
      • Chemicals
    • Types of Plant Response
      • Taxis
        • Movement of the entire organism, e.g. single celled photosynthesising Protoctista
      • Nastism
        • Non-growth movement e.g. the closing of a Venus Fly Trap or a Daisy closing its petals at night
      • Tropism
        • A tropic response is a response in the direction of, and determined by, a directional stimulus
    • What controls plant responses?
      • Plant Growth Regulators
        • Transport
          • Diffusion
          • Active Transport
          • Mass flow in the xylem or phloem
        • Features of PGRs
          • Effective at very small concentrations
          • The same conc. can have different effects on different tissues
          • Different conc. can have different effects on the same tissue
        • Hormones
          • Similarities with PGRs?
            • Both have complementary protein receptors on the cell surface membrane of their target tissues
              • This ensures specificity in the action of PGRs and Hormones
          • Differences from PGRs
            • Hormones are transported in the blood
            • PGRs are not excreted from exocrine glands whereas hormones are
            • Hormones can target organs, PGRs target tissues and cells
    • Auxin
      • Apical Dominance
        • Auxin diffuses down from the apical bud
          • It acts as a repressor on the development of the lateral buds
            • The further away from the apical bud, the lower the concentration of Auxin and the less repression occurs
              • When the source of Auxin is removed cell division begins in the lateral buds
      • Phototropism
        • Negative
        • Positive
          • When light shines on a plant a protein called phototropin causes Auxin to collect on the side of the plant furthest from the sun
            • When light shines directly above a plant equal amounts of Auxin collect on each side
            • Auxin binds to receptors on the plasma cell membrane
              • This causes the active transport of H+ ions into the cell wall
                • This provides the optimum pH for enzymes called expansins to loosen hydrogen bonds in the cell wall
                  • Water can diffuse into the cell causing it to expand
                    • The side furthest from the light source grows at a faster rate causing the plant to bend towards the source
            • Light is detected by the tip of the plant
      • Stimulates cell elongation and cell division
    • Gibberellins
      • Causes cell elongation and division in the internode region
      • Dwarf plants contain two recessive alleles so don't produce the active protein necessary for GA production
        • Treating dwarf plants with GA causes them to grow
        • Treating normal varieties with GA will have no effect as the plants already produce GA
      • Gibberellins are responsible for stem elongation
    • Abscission
      • The organised shedding of part of a plant
      • Cytokinins prevent the leaves of a deciduous tree from senescing
        • When cytokinin production drops senescence begins and leaves are shed
      • There is an abscission zone at the base of the petriole where cells can easily break away along the middle lamellae
        • Below this there is a protective layer. Cells contain waterproof Suberin to prevent water from escaping and pathogens from entering
    • Uses of Plant Growth Regulators
      • Auxin
        • Herbicide
        • Promotes flowering in commercial flower production
        • Promotes root growth
      • Gibberellins
        • Delays fruit senescence prolonging their picking season
        • Increases sugar cane yield by lengthening the cane
      • Ethene
        • Causes ripening in e.g. tomatoes, bananas etc.
        • Causes fruit to drop

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