Plant Hormones

• Plant Hormones


  Make sure plants grow in the right direction
• Auxin


  A plant growth hormone
• Auxin controls plant growth

  1. Produced in tips and moves backwards

  2. Stimulates cell elongation in cells behind tips

  3. If tip removed, shoot may stop growing

  4. Extra auxin promotes shoot growth but inhibits root growth
• Shoot tip exposed to light

  More auxin accumulates on shaded side, making cells grow faster, causing shoot to bend towards light
• Shoot growing sideways


  Gravity produces unequal auxin distribution, with more on lower side, causing lower side to grow faster and shoot to bend upwards
• Root growing sideways


  More auxin on lower side inhibits growth, causing upper side to elongate faster and root to bend downwards
• Investigating plant growth responses

  1. Put cress seeds in Petri dishes, shine light from different directions, observe seedlings growing towards light

  2. Place seedlings with roots pointing different directions, store vertically in dark, observe roots growing downwards
• Controlling variables


  • Control number of seeds, type of seed, temperature, water, light intensity
• Auxins


  Useful for controlling plant growth, e.g. killing weeds, growing from cuttings, tissue culture
• Gibberellin


  Plant growth hormone that stimulates seed germination, stem growth, and flowering
• Ethene


  Gas produced by aging plants that stimulates fruit ripening
• Commercial uses of ethene

  1. Speed up ripening of fruit on plant or during transport

  2. Delay ripening while in storage by blocking ethene or reducing its production