Hormones

• known role of auxin

  control cell elongation, prevent abscission, maintain apical dominance, tropisms, stimulate ethene release, fruit ripening
• known role of gibberellin

  stem elongation, mobilisation of food stores during germination, stimulate pollen tube growth
• known role of ethene

  fruit ripening, promotes abcsission
• known role of ABA (abscisic acid)

  maintains dormancy, stimulates cold protective responses, stimulates stomatal closure
• seed germination

  embryo activated when seed absorbs water, and gibberellins produced

  gibberellins thought to control genes for amylases and proteases

  ABA may act antagonistically to gibberellins
• auxins (IAA or indoleacetic acid)

  made in tips of roots and shoot, and meristems

  can move up and down the plant through transport tissues

  effect of auxin controlled by concentration and interaction with other hormones
• effect of auxin on apical shoot

  auxins affect plasticity of cell wall; auxins bind to receptors on membrane, which reduce internal pH to 5, optimum for enzymes that stretch/make cell wall flexible

  auxin destroyed as cells mature, enzymes become inactive
• apical dominance

  high concentration of auxin suppresses lateral growth

  main shoot stimulated by auxin production in tip, so quick growth
• root growth

  low concentration of auxin promotes root growth
• gibberellins

  involved in germination

  affect length of internodes

  little to no gibberellins produce plants with short stems - helps to reduce waste, and makes them less vulnerable to weathering