Plant hormones

• A plant's response to stimuli such as light and gravity are called tropisms
• Tropisms can be positive - the plant grows towards the stimulus

  Tropisms can be negative - the plant grows away from the stimulus
• A plants response to light is called phototropism

  A plants response to gravity is called gravitropism
• Shoots are positively phototropic

  Roots are negatively phototropic
• Roots are positively gravitropic

  Shoots are negatively gravitropic
• Auxins are a plant hormone which control growth at the shoot tip and root tip
• Auxins are produced at the tip and diffuse backwards
• In the shoot, auxin promotes growth by cell elongation

  In the root, auxin inhibits growth by cell elongation (prevent)
• Why do shoots grow towards light?

  • Auxin produced at shoot tip + diffuses backwards
  • Auxin accumulates on shady side
  • Auxin promotes growth by cell elongation on shady side
  • This causes the shoot to grow towards the light - positively phototropic
• Why do roots grow away from light?

  • Auxin produced at root tip + diffuses backwards
  • Auxin accumulates on shady lower surface of root
  • Auxin inhibits growth by cell elongation on lower surface of root, causing root to grow away from the light
  • So they are negatively phototropic
• Why do roots grow downwards?

  • Auxin produced at root tip + diffuses backwards
  • Gravity causes auxin to accumulate on the lower surface of root
  • Auxin inhibits growth by cell elongation at lower surface of root causing it to grow downwards
  • So they are positively gravitropic
• Why do shoots grow in opposite directions to gravity?

  • Auxin produced at shoot tip and diffuses backwards
  • Gravity causes auxin to accumulate on lower surface of shoot
  • Auxin promotes growth by cell elongation in lower surface of shoot, causing it to grow upwards in opposite directions to gravity
  • So it is negatively gravitropic
• Selective herbicides (weed killers)

  • Disrupt normal growth of broad-leaved weeds, which kills them
  • They leave narrow leaves plants, such as grasses, untouched
• Root cuttings

  • Auxin allows cutting to produce roots rapidly and start growing into a new plant
• Seedless fruit

  • Fruits containing seeds will grow after the plant has been pollinated
  • Parthenocarpic fruit (without seeds) an be produced n some plants if auxin and gibberellin are applied to the unpollinated flowers
• Controlling dormancy

  • Gibberellin is a hormone that breaks dormancy in seeds, causing them to germinate (grow roots + seeds)
  • So, growers can make seeds germinate at the time of year they normally wouldn't, they can make sure all seeds in a batch germinate at the same time
• Gibberellin is a plant hormone that:

  • Breaks seed dormancy (causes germination)
  • Produces seedless fruit
  • Growth of shoots
  • Fruit growth
• Ethene is a plant hormone that:

  • Causes fruit to ripen
  • Causes leaves to fall off the plant