Plant hormones

• Tropism is the response of a plant to a specific stimulus
• Phototropism is the response of a plant to light
• Gravitropism is the response of a plant to gravity
• Auxin


  A plant hormone which controls the growth of a plants shoots and roots
• How do plants show positive phototropism

  • One side of the shoot is in the light, causing the auxin to move to the shaded side - not much auxin is found on the light side
  • At the shaded side the cells elongate
  • The shoot finds the light
• Shoots are

  Positively phototropic and negatively gravitropic
• Roots are

  Negatively phototropic and positively gravitropic
• How do plant roots show positive gravitropism

  • Auxin produced by the root tip diffuses to lower side of the root tip
  • The root curves and bends in the direction of gravity (downwards)
  • As a result, more water and nutrients are available to the plant roots
• Uses of auxins

  • Promote cell division - In meristems and enlargement of daughter cells
  • Tissue culture - Used to produce clones of plants
  • Selective weed killers
• Gibberellins are hormones that initiate seed germination
• Uses of gibberellins

  Seed germination

  Flowering

  Fruit growth
• Ethene is a type of plant hormone involved in fruit ripening
• Bananas produce ethene
• How does phototropism help a plant to survive

  Plant receives more light

  More photosynthesis

  The plant produces more glucose