plant hormones

• plants produce hormones to coordinate growth and control growth and responses to light and gravity
• a movement in response to a stimulus is a tropism
• movement towards a stimulus is a positive tropism
• movement away from a stimulus is a negative tropism
• unequal distribution of auxin causes unequal growth
• shoots have a positive phototropic response, so they bend towards the light:

  • auxin moves to the shaded side of the shoot
  • this causes the cells further away from the sun to grow faster
  • this bends the plant towards the light
  • movement towards a stimulus is a positive tropism
• shoots have a negative geotropic response, as they grow away from gravity:

  • if a shoot is placed horizontally, auxin moves to the lower side due to gravity
  • this stimulates more growth on the lower side, as auxin causes cells to grow
  • this makes the shoot bend and grow away from the ground
  • this is beneficial as light levels are likely to be higher away from the ground
• roots have a positive geotropic response, as they grow downwards with gravity:

  • if a root is horizontal, auxin moves to the lower side of the root
  • in roots, auxin inhibits growth so this means cells on the upper side grow more
  • this makes the root bend and grow downwards
  • useful as there are more likely to be increased levels of water and nutrients lower down, and it provides more stability for the plant
• plant growth hormones are used in agriculture and horticulture
• auxins are used:

  • as weed killers
  • many weeds are broad leaved
  • weed-killers containing auxin cause the cells in weeds to grow too rapidly
  • and this causes the weed to die
• auxins are used:

  • as rooting powders
  • where cuttings can be taken of desirable plants
  • and the tips dipped in rooting powders
  • the cuttings can be placed in the soil, and the rooting powder will encourage the rapid development of roots
• auxins are used:

  • for promoting growth in tissue cultures
  • cells are scrapped of the parent plant, and placed in agar with nutrients
  • plant hormones including auxin are added
  • this encourages the cells to form roots and shoots
• ethene is used:

  • in the food industry to control the ripening of fruit during storage and transportation
  • fruit is picked when it is not ripe which means it is firm and doesn't get bruised or damaged in transportation, and ethene production during transportation is inhibited through chemicals
  • when it needs to be sold fruit is exposed to ethene gas and warmer temperatures
  • ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening
• gibberellins are used:

  • to end seed dormancy
  • as high concentration of gibberellins promotes seed germination
• gibberellins are used:

  • to promote flowering
  • as gibberellins allows plants to flower in any conditions whereas without it specific conditions (longer days and lower temperatures) would be required for flowering
• gibberellins are used:

  • to increase fruit size
  • the seeds in fruits produce gibberellins to increase fruit size
  • this means that seedless fruits and smaller
  • so they can be sprayed with gibberellins to increase their size
  • higher levels of gibberellins promote the development and growth of fruit