Plant Growth Regulators

• Growth regulators are produced in small amounts in meristematic tissue and transported to another part of the plant where they cause an effect (often called plant hormones).
• They are transported mostly in the phloem by diffusion.
• They produce differing effects depending on their concentrations.
• Regulators interact in various ways:
  • Some work together to increase an effect.
  • Others interfere with each other, and the combination can have no effect.

Growth Promoters

• Growth promoters increase the rate of growth.
• Examples include auxins and gibberellins.
• One type of auxin is IAA (Indole Acetic Acid).
• Auxin is produced in meristematic tissue in shoot tips, root tips, young leaves, and developing seeds.
• Functions of auxin include:
  • Causes stem elongation.
  • Causes root growth.
  • Causes cells to differentiate.
  • Causes fruit to develop.
  • Inhibits side branching in stems (apical dominance).
  • Tissue culturing by stimulating root formation.
  • Causes phototropism (growth towards light).
  • Causes geotropism (growth response to gravity).

Apical Dominance

• Auxins produced in the apical meristem pass down the stem and inhibit the growth of side branches.
• This allows the plant to grow tall.
• If the apical tip is removed (e.g., trimming a hedge), side branches form, allowing the plant to grow wider (e.g. allowing the hedge to fill out and grow thicker).

Importance of IAA in Controlling Phototropism

• IAA is produced in shoot and root tips.
• If the stem is exposed to light from one side:
  • IAA diffuses down the shady side.
  • The higher concentration of IAA in shaded cells causes elongation on the shaded side, without elongation on the bright side.
  • This causes the stem to bend towards the light.

Growth Inhibitors

• Growth inhibitors decrease the rate of growth or stop it completely.

• They are made in stem nodes, ripe fruit and decaying leaves.

• Examples include ethene and abscisic acid.

• Ethene is a growth inhibitor that:

• Causes fruits to ripen.

• Stimulates leaves to fall in autumn.

• Causes aging in plants.

• Used commercially to ripen bananas.

• Stimulates more ethene production-one bad apple rots the barrel! i.e. damaged fruit releases more ethene, stimulating ethene production in nearby fruit/vegetables.

  

• Abscisic acid is a stress regulator- it causes plants to respond to harmful conditions.

• It is produced in leaves, stems and root caps.

• Abscisic acid:

  • Causes stomata to close in dry conditions (to conserve water).
  • Causes production of bud scales (to protect buds during the winter).
  • Causes dormancy in seeds (to survive unfavourable conditions).

Commercial Use of Plant Growth Regulators

• Rooting powder: Contains an artificially produced growth regulator (NAA) to stimulate root growth in cuttings.
• Gibberellins: Used to increase the size of fruit.
• Ethene: Used commercially to ripen bananas.
• Artificial weed killers (e.g., Roundup): Used to kill unwanted plants.