Plant Responses

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  • Plant Responses
    • Types of plant responses
      • avoiding herbivory
        • some plants (e.g. oak) produce tannins which are toxic to micro-organisms and  herbivores
          • can also act as a chemical signal to tell other plants to produce tannins
        • herbivory is the consumption of plants
        • some plant produce alkaloids which are bitter and therefore deter herbivory
        • plants like the sensitive mimosa plant have leaves which fold up in response to touch, scaring away the insects
      • abiotic stress
        • shortage of water
          • reduce water loss through closing stomata, coordinated by hormones especially abscisic acid
        • tropisms - directional growth movement
          • phototropism - response to light
            • shoots display positive phototropism
          • geotropism - response to gravity
            • shoots show negative geotropism
            • roots show positive geotropism
          • chemotropism - response to certain chemicals
            • pollen tube shows positive chemotropism as it grows towards ovum during reproduction
          • thigmotropism - response to touch
            • certain plants, like ivy, grow so that their stem (or branch) winds around a support
    • Plant Hormones
      • seed germination
        • gibberellin is activated when seed takes up water
          • gibberellin travels to aleurone layer
          • gibberellin  stimulates the production of amylase which hydrolyses starch to glucose
            • glucose provides a substrate for respiration and photo-synthesis and protein synthesis in the embryo so it can start to grow
      • leaf loss (abscission) in deciduous plants
        • region of cells at end of leaf called abscission zone
        • auxin produced in then leaf inhibits abscission by making the cells insensitive to ethylene
          • when auxin concentration drops, the cells in abscission zone become sensitive to ethylene, stimulating production of celluiase enzyme in the abscission zone
            • cellulose digests the cell walls in the abscission zone, making the leaf drop
        • cytokinins enter the leaf and stop the leaf from senescing (aging and turning brown)
          • when concentration of cytokinins drops the leaf ages and the production of auxin in the leaf
            • when auxin concentration drops, the cells in abscission zone become sensitive to ethylene, stimulating production of celluiase enzyme in the abscission zone
              • cellulose digests the cell walls in the abscission zone, making the leaf drop
      • stomatal closure
        • guard cells possess receptors for abscisic acid and control the opening and closing of stomata
          • when abscisic acid binds to the receptors a number of chemical pathways inside the cell are activated, leading to an increase in pH and the transfer of calcium ions from the vacuole to the cytoplasm
            • calcium stimulates loss of charged ions from the cell, increasing the water potential so that water leaves the cell via osmosis
              • cell becomes more flaccid and therefore stomata close
    • Experimental Evidence
      • auxin and apical dominance
        • auxin is produced at the tip of the shoot and inhibits the growth of lateral buds
          • removal of apical bud allows lateral buds to grow
          • if auxin or synthetic auxin is placed on a cut tip of a shoot, lateral bud growth is inhibited
        • more recent evidence shows that this link may not be direct because auxin stimulates the production of abscisic acid which inhibits lateral bud growth
      • gibberellins and stem elongation
        • growing seedlings with certain fungi (from which gibberellins can be isolated) increases seedling growth via greater elongation of the stem
        • applying a cream containing gibberellins to fungi causes stem elongation
      • gibberellins and seed dormancy
        • early germination can be caused by application of gibberellins to a seed
        • inhibiting gibberellins can prevent early germination (abscisic acid inhibits seed germination)
    • Commercial uses of plant hormones
      • gibberellins
        • delay fruit senescence (aging) and drop to make harvesting easier
        • improve fruit shape and size
        • activate enzymes stored in barley to produce malt for brewing
        • speed up seed production in breeding programmes
      • ethene
        • spped up fruit ripening and promote fruit drop
        • promote growth of lateral branches
      • auxins
        • promote root growth in cuttings
        • produce seedless fruit
        • selective weedkiller
      • cytokinins
        • delay leaf senescence to avoid discolouring of vegetables
        • promote bud and shoot growth during tissue culture
        • promote growth of lateral buds

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