Plant hormones

• Tropism is the response of a plant to a specific stimulus
• Phototropism is the response of a plant’s shoot or root to light
• Gravitropism is the response of a plant’s shoot or root to gravity
• Auxin is a hormone which controls the growth of a plant’s shoots and roots
• Uneven distribution of auxin in a growing plant leads to unequal growth rates
• Plant shoots show positive phototropism by:

  • One side of the shoot is in the light, causing auxin to move to the shaded side
  • Cells on the shaded side are stimulated to grow, bending the shoot towards the light
  • This allows photosynthesis to occur at a faster rate
• Plant shoots show negative gravitropism by:

  • Auxin accumulates in the lower side of a horizontal shoot due to gravity
  • Cells on the lower side grow more, causing the shoot to bend away from gravity
  • This usually results in more light available for photosynthesis
• Plant roots show positive gravitropism by:

  • Auxin accumulates in the lower side of a horizontal root, causing cells to grow less
  • The root bends in the direction of gravity
  • This leads to more water and nutrients being available to the plant roots
• Gibberellins are a type of plant hormone involved in seed germination
• Ethene is a type of plant hormone involved in cell division and ripening
• Uses of auxins:

  • Weedkillers - auxin causes rapid cell growth leading to plant death
  • Rooting powder - auxin promotes quick growth of new plants
  • Tissue culture - auxin stimulates growth of roots and shoots
• Uses of ethene:

  • Control of food ripening in the food industry, allowing fruit to ripen just before sale
• Uses of gibberellins:

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