5.4 Plant Hormones

• Auxins

  • Produced in the tips of the shoots and roots - they diffuse to the cells behind the tips and have the following effects:
  • Shoots - promote cell elongation
  • Roots - inhibit cell elongation
  • Unequal distributions of auxin cause unequal growth rates in plant roots and shoots
• Auxins in shoots with uneven light

  1. Auxin produced in the tip concentrates on the shaded side
  2. Cells on the shaded side elongate and grow faster than cells on the sunny side
  3. This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light
• Uses of auxins

  Selective weed killers

  • Contain auxins to only kill broad-leafed plants, so narrow-leafed plants are untouched

  Rooting powders

  • Can easily and cheaply produce lots of clones of a desirable plant
  • Cuttings of the plants are taken and the tips are dipped in auxins (rooting powders) to encourage rapid development of roots

  Promoting growth in tissue culture

  • Scrapings of cells can be taken from a desirable plant and used to produce clones in a petri dish, which are planted and allowed to grow in soil
• Use of ethene

  • Control ripening of fruit during storage and transport
  • Can be inhibited to delay ripening of fruits in storage:
  • Add chemicals to prevent production
  • Reacting ethene in the air around fruit with substances to remove it
• Use of gibberellins


  • End seed dormancy - germinate earlier than it normally would
  • Encourage plants to flower
  • Make fruits grow larger