Summary of Plant Growth Substances and Hormones

Plant Growth Substances

• Definition:

  • Small amount organic compounds necessary for plant growth and development.
  • Also called hormones, phytohormones.

• Types of Plant Hormones:

  • Auxins
  • Cytokinins
  • Gibberellins
  • Abscisic Acid
  • Ethylene

Auxins

• Functions:

  • Promote cell elongation in shoots.
  • Inhibit lateral bud development (apical dominance).
  • Encourage root development in cuttings.
  • Affect apical dominance and tropic responses (phototropism and gravitropism).

• Key Effects:

  • Alters osmotic pressure and enzyme activity.
  • Stimulates fruit development and leaf/fruit abscission.

Cytokinins

• Roles:

  • Stimulate cell division and differentiation.
  • Delay aging in plants.

• Function vs. Auxins:

  • Promote lateral shoots in stems; inhibit root branching.

Gibberellins (GA)

• Functions:

  • Promote seed germination and fruit development.
  • Stimulates stem elongation; affects internodal distance.

• Application Issues:

  • May cause excessive stretching in low light.

Abscisic Acid (ABA)

• Functions:
  • Inhibits growth; induces seed dormancy.
  • Closes stomata during drought conditions.

Ethylene

• Characteristics:

  • Only gaseous plant hormone, diffuses throughout the plant.

• Functions:

  • Stimulates ripening in fruits, leaf/fruit fall, and cell wall softening.

Applications of Plant Growth Substances

• Auxins: Used for rooting cuttings, thinning fruits for larger size.
• Gibberellins: Enhance growth in hybrid corn, stimulate flower development, and increase grape berry size.
• Cytokinins: Extend vase life of cut flowers, assist in tissue culture.
• Ethylene: Used in commercial fruit ripening.
• Abscisic Acid: Synthetic growth inhibitors used for controlling height and stimulating flowering.

Allelopathy

• Definition: Release of chemicals by plants to inhibit growth of competitors.
• Mechanisms:
  • Volatilization, leaching, exudation of chemicals such as tannic acid and creosote.