plant responses

• tropisims - directional growth response
• tropisms - directional growth responses controlled by plant hormones
• nastic responses - non directional growth responses not controlled by hormones
• responses to herbivory

  • physical defences
  • chemical defences - alkaloids, tannins, terpenoids
  • pheromones - chemicals produced by one to affect another
  • thigmonasty - response to touch or vibration
• responses to abiotic stress

  • leaf fall in autumn
  • antifreeze production - genes switched on in low temps causing plants to produce antifreeze proteins
  • stomata closure - during water stress
• directional plant responses

  • geotropism - roots show positive, shoots show negative
  • phototropism - shoots grow positive
  • chemotropism - response to chemicals
  • thigmotropism - response to touch
• auxin - promotes cell elongation, inhibits growth of side shoots, inhibits leaf abscission
• cytokinins - promotes cell division
• gibberellins - causes stem elongation, promotes seed germination
• abscisic acid - maintains seed and bud dormancy, stimulates stomatal closure, stimulates antifreeze production
• ethene - promotes fruit ripening, promotes leaf abscission
• Auxins

  • made in meristem tissue near the tip of roots and shoots
  • cause plant cells to elongate
• how does auxin work

  1. binds to receptors in the cell surface membrane promoting active transport of h ions into cell walls
  2. the fall in pH means the weakening of the cellulose bonds and the activation of expansins which break the cellulose bonds
  3. water is taken in by osmosis elongating the cell permanently
  4. as the cell matures auxins are destroyed and the cellulose cell wall becomes rigid
• auxins and phototropism

  cells in the shoots contain phototropins which become phosphorylated under blue light. this causes auxin produced in the shoot tip to be transported to the shaded side of the plant. cells in the shaded side elongate and cause the shoot to bend towards the light
• roots are positively geotrophic
• apical dominance

  the apical bud contains apical meristem, this shows dominance over the lateral buds. meaning the plant grows straight up without wasting energy expanding laterally. if the tip of the plant is cut the side branches will grow
• gibberellins promote growth between stem internodes, they affect gene expression and cause cell division and elongation.
• auxins and gibberellins are antagonistic as gibberellins stimulate side shoot growth and auxin inhibits growth
• auxin and gibberellins are synergistic as they can cause plants to grow very tall
• gibberellins and germination

  1. when a seed absorbs water, the embryo releases gibberellin which moves by diffusion.
  2. this travels to the aleurone layer in the endosperm
  3. this switches on the genes for proteases and amylases
  4. starch is hydrolysed for respiration
• leaf abscission in deciduous plants

  in winter there is less light and therefore less photosynthesis occurs. water is lost through the leaves and frost damage is more likely so plants loose leaves
• leaf abscission process

  1. auxins are produced in young leaves which inhibits abscission
  2. reduced light levels reduces the auxin concentrations
  3. ethene is produced as the leaves age
  4. when the concentration of ethene is higher than auxin the abscission genes become switched on
  5. the abscission layer of cells in the stalk have thin walls that are weakened by enzymes breaking the cellulose bonds
  6. the vascular bundle is sealed
  7. the leaf is blown off
• abscisic acid and stomatal closure

  aba binds to receptors on the cell surface membrane of the guard cells

  calcium ions enter and potassium ions leave

  the water potential increases

  water leaves by osmosis

  guard cell becomes flaccid and the stomata close
• commercial uses of auxin

  • rooting powder
  • parthenocarpy (seedless fruits)
  • selective weedkillers
• commercial uses of gibberellins

  • speeding up malting by causing breakdown of stored starch
  • longer sugar canes
  • inducing seed germination in young plants
  • synthesis inhibitors
• commercial uses of cytokines

  • stop lettuce yellowing
  • promote shoot growth in tissue culture
• commercial uses of ethene

  • fruit dropping at same time
  • speeds up ripening
  • promotes female flowers