6.4 - Plant Hormones

• The response of plants to light is called phototropism
• The response of plants to gravity is called geotropism
• Auxin is a plant hormone that controls responses to light and gravity
• Auxin causes shoot cells to elongate (grow).
• Auxin causes root cells to have inhibited growth. This causes the lower side to grow slower than the upper side.
• Plant shoots grow towards the light as more auxin gathers on the shaded side. These cells elongate (grow) more, bending the shoot.
• The stages of plant shoots:

  1. Auxin is produced at the very tip of the shoot
  2. In shoots, auxin triggers cell growth
  3. Light causes auxin to concentrate on the darker side of the shoot tip
  4. Auxin now spreads down the shoot
  5. Cells on the darker side grow faster than cells on the lighter side
  6. This causes the shoot to grow towards the light
• Auxin is produced in the root but gravity causes the auxin to concentrate on the lower side.
• Giberellins are important in starting the germination of seeds.
• The chemical ethene controls cell division and the ripening of fruits.
• Agriculture is the growing of food crops
• Horticulture is the growing of plants in gardens
• Auxins are used as:

  • weedkillers in gardens
  • rooting powder
  • promoting growth in tissue culture
• Gibberellins are used as:

  • End seed dormancy (force seed germination earlier than normal)
  • Encourage plants to flower
  • Make fruit larger
• Ethene is used in the food industry to control ripening of fruit during storage and transport.