plant responses

• chemical responses to herbivory

  1. tannins -> reduce nutritional value and prevents digestion of proteins - it can also be toxic
  2. alkaloids -> give a bitter taste that can be toxic
  3. pheromones -> released by nearby plants to alert other plants to release tannins
• tropism

  growth response to a directional stimulus

  1. positive tropism -> towards the stimuli
  2. negative tropism -> away from the stimuli
• phototropism

  light
• hydrotropism

  water
• geotropism

  gravity

  roots experience positive geotropism

  shoots experience negative geotropism
• chemotropism

  chemicals
• thigmotropism 


  contact
• auxin

  cell elongation
• gibberellins

  seed germination and stem growth
• abscisic acid

  inhibit seed germination

  stomata closure
• ethene

  fruit ripening
• leaf abscission

  cytokinin decreases causing the leaves to age and brown

  auxin levels decrease causing cells in the abscission layer to become more sensitive to ethene

  ethene production increases

  increases cellulase production which digests cells in the abscission layer causing them to fall

  a layer of suberin then forms to prevent pathogens from entering
• cytokinin

  produced by young leaves to make it a suitable sink for phloem transport
• germination

  hormone: gibberellins

  diffuses from the embryo to the aleurone

  stimulates production of amylase which hydrolyses the starch food stores into glucose

  glucose is then respired aerobically to provide energy in the form of ATP for growth
• stomatal closure

  hormone: abscisic acid

  binds to the receptors on guard cells changing the ph of the cytoplasm

  both positive and negative ions leave by facilitated diffusion causing the water potential to increase

  water leaves the guard cells by osmosis down its water potential gradient

  the guard cells loose turgidity and the stomata closes
• cell elongation

  hormone: auxin

  H+ ions are transported into the cell walls which lowers the ph providing optimal conditions for expansins

  these enzymes loosen the cellulose so it can accommodate more water allowing cells to elongate
• shoot illuminated on all sides

  auxin is distributed evenly

  move down the shoot tip thus causing elongation of cells across the zone of elongation
• shoot illuminated on one side

  auxin moves towards the shaded side

  causing elongation of cells on that side

  results in the shoot bending towards the light source
• auxin experimental evidence

  auxin produced in the apex

  inhibits lateral bud growth

  presents apical dominance
• experiment of auxin

  1. auxin paste applied to cut shoot -> no lateral bud growth
  2. ring of auxin transporter inhibitor placed below the apical tip -> lateral bud growth occurred
• stem elongation experiment background

  tall pea plant had a higher GA concentration than the dwarf pea plant

  this was due to the different le / Le alleles

  Le allele coded for an enzyme that synthesised GA1 from GA20

  le allele did not code for the enzyme so no GA1 was produced
• stem elongation experiment process

  dipped a homozygous le dwarf pea plant into a root stock containing the enzyme that synthesises GA1 from GA20

  the dwarf pea plants grew using their own GA20
• commercial use of auxin

  rooting powder

  seedless fruit growth
• commercial uses of gibberellins

  used to delay senescence in citrus fruits

  elongation of apples in combination with cytokinins

  for elongation of grape stalks
• commercial use of cytokinin

  prevent yellowing of lettuce leaves

  to promote shoot growth
• commercial use of ethene

  speeds up ripening

  promotes lateral growth and promotes fruit drop