Plant Hormones

• Tropism


  The response of a plant to a specific stimulus
• Phototropism


  The response of a plant's shoot or root to light
• Gravitropism


  The response of a plant's shoot or root to gravity
• Auxin


  A hormone which controls the growth of a plant's shoots and roots. When auxin is unevenly distributed in a growing plant, the rate of growth will also be unequal.
• Positive phototropism in plant shoots

  1. One side of the shoot is in the light, causing auxin to move to the shaded side

  2. At the shaded side, the cells are stimulated to grow - the shoot bends towards the light

  3. As a result, photosynthesis can occur at a faster rate
• Negative gravitropism in plant shoots

  1. In a horizontal shoot, auxin accumulates in the lower side due to gravity

  2. Cells on the lower side of shoot grow more, causing the shoot to bend away from the direction of gravity

  3. As a result, more light is usually available for photosynthesis
• Positive gravitropism in plant roots

  1. In a horizontal root, auxin accumulates in the lower side, causing cells to grow less

  2. The root bends in the direction of gravity

  3. As a result, more water and nutrients are available to the plant roots
• Gibberellins


  A type of plant hormone involved in seed germination
• Ethene


  A type of plant hormone involved in cell division and ripening
• Uses of auxins


  • Weedkillers - auxin causes cells to grow at a rapid rate, causing plant death
  • Rooting powder - auxin causes new plant to grow very quickly
  • Tissue culture - auxin promotes growth of roots and shoots
• Uses of ethene


  • Control of food ripening in the food industry - allows fruit to be ripened just before they are sold
• Uses of gibberellins


  • Termination of seed dormancy
  • Promotion of flowering
  • Increase of fruit size