Plant Responses

• Abiotic Stress


  Non-living environmental factor that could harm a plant e.g. mineral deficiency, drought, depleted oxygen supply, pollution
• Plant response to abiotic stress and herbivory:

  • may produce antifreeze enzymes
  • may contain bitter-tasting tannins
  • bitter-tasting nitrogen compounds (alkaloids)
  • cell-signalling pheromones that trigger defensive responses in other organisms
• Mimosa pudica


  seismonasty (touch sensitivity) causes leaves to fold
• Phototropism


  response to light
• geotropism


  response to gravity
• hydrotropism


  response to water
• thermotropism


  response to temperature
• thigmotropism


  response to touching a surface or object
• how is leaf abscission controlled

  1. as leaf ages - cytokinin and auxin levels lower and ethene levels increase
  2. triggers production of cellulase enzymes
  3. cell walls break down in abscission layer
  4. leaves break from branch
  5. below abscission layer a Suberin layer forms to prevent entry of pathogens
• gibberellins

  stimulates:

  • germination
  • elongation at cell internodes
  • fruit growth
  • rapid growth/flowering
• auxins

  • involved in tropic responses (IAA)
  • control cell elongation
  • suppress lateral buds to maintain apical dominance
  • promote root growth e.g. in rooting powders
• roots show positive geotropism


  1. gravity causes IAA to accumulate on lower side of the root
  2. IAA inhibits the elongation of root cells
  3. cells on the upper side of the root elongate faster, so the root tips bend downwards
• shoots show positive phototropism


  1. IAA diffuses to the shaded side of the root tip
  2. IAA causes the shaded side to elongate
  3. shoot bends towards light

  NOTE: in roots, IAA INHIBITS growth
• stomata closure

  1. ABA binds to receptors on guard cell membranes
  2. specific ion channels open
  3. calcium ions enter cytosol from the vacuole
  4. increased Ca2+ conc.
  5. other ion channels open
  6. ions leave guard cell, increasing water potential in the cell
  7. water LEAVES the cell by osmosis
  8. cells are now flaccid
• ABA - abscisic acid
• OPENING stomata

  guard cells fill with water - plump and turgid
• CLOSING stomata


  guard cells lose water - flaccid
• apical dominance

  • the growth of side shoots does not take place
  • maintained by the action of auxin, abscisic acid and cytokinins
• experimental evidence that gibberellins control stem elongation and germination


  • stem elongation - tall plants have higher gibberellin conc. than dwarf plants
  • germination - mutant seeds with non-functional gibberellin gene do not germinate unless gibberellin is applied externally, inhibitors of gibberellin production prevent germination
• maintaining apical dominance

  • auxins stimulate growth of the apical bud
  • auxins inhibit growth of lateral buds

  when the apex is removed

  1. auxin levels drop, causing abscisic acid levels to drop
  2. cytokinins (initially concentrated near auxin reserves in bud) diffuse evenly to promote bud growth in the other parts of plant = lateral buds
• commercial use of AUXINS

  • rooting powder
  • growing seedless fruit
  • herbicides
  • low concs prevent leaf and fruit growth
  • high concs promote fruit drops
• commercial use of CYTOKININS

  • prevent yellowing of lettuce leaves
  • promotes shoot growth
• commercial use of GIBBERELLINS

  • delay senescence in citrus
  • elongation of apples and grape stalks
  • brewing beer for malt production
  • increase sugar cane yield
  • speed up seed formation in conifers
  • prevent lodging
• commercial use of ETHENE

  • speeds up ripening
  • promotes lateral growth
  • promotes fruit drop
• Tropism


  Directional growth response in plants