Plant Hormones

Overview

• Growth & Development

  • Growth: Increase in number of cells due to cell division, followed by an increase in cell size.
  • Development: Changing of shape leading to cell specialization and function change.

• Plant Hormones

  • Chemicals regulating plant growth, development, and cell differentiation.
  • Exist in small amounts and control various processes like growth, flowering, leaf shedding, fruit development, and ripening.
  • Produced by cells directly as plants lack glands.

Three Key Hormones

1. Auxins

   • Produced in the tips of shoots (apical bud/apical meristem) and root tips.
   • Move downward from shoot tips towards roots.
   • Sensitive to light (Phototropism) and gravity (Geotropism).

   Functions

   • Tropic Movements: Phototropism and Geotropism.
   • Apical Dominance: High auxin near apical bud inhibits lateral bud growth.
   • Promotes Growth: Encourages cell elongation and mitosis.
   • Phototropism: Auxins move away from light; causes stem to bend toward light due to cell elongation on the shaded side.
   • Geotropism: In roots, high auxin concentration inhibits growth, causing roots to grow downward (positive geotropism).
   • Apical Dominance: Removal of apical bud results in bushier plants due to decreased auxin concentration.

2. Gibberellins

   • Stimulate stem growth by lengthening internodes.
   • Promote seed germination and cell elongation.
   • Not involved with apical dominance or tropic movements.

3. Abscisic Acid (ABA)

   • Helps plants cope with adverse conditions.
   • Functions as a growth inhibitor:
     • Causes stomata to close.
     • Promotes abscission and dormancy in seeds and buds.
     • Dormancy prevents seed germination until conditions are optimal.

Weed Control with Hormones

• Selective Weedkillers:
  • Target specific plants using high concentrations of auxin.
  • Useful for eliminating broadleaf weeds without affecting narrow leaf plants like grain crops.

Plant Defence Mechanisms

• Plants employ both chemical and physical defenses against herbivores (e.g., caterpillars, birds, giraffes).

Chemical Defenses

• Digestive Enzyme Inhibitors: Tannins.
• Bitter Tasting Chemicals: Caffeine.
• Toxins: Pyrethrins.
• Stinging Hairs: Found in plants like stinging nettles.
• Essential Oils: Peppermint.
• Allergy-Causing Chemicals.
• Antibacterial Chemicals: E.g., garlic.

Physical Defenses

• Structures: Thorns, spines, etc.
  • Thorns may contain pathogenic bacteria & fungi and serve as a deterrent with warning coloration.