Unit 4: Plant Growth Regulators

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Introduction to Plant Growth Regulators

Plant Growth Regulators (PGRs), often called phytohormones, are small, simple chemical molecules that regulate plant growth, development, and responses to the environment. These substances are produced in one part of the plant and typically translocated to another part to exert their physiological effects.

Auxins: Discovery and Physiological Roles

Auxins are primarily produced in the shoot apical meristems and young leaves.

• Discovery: Initial observations were made by Charles Darwin and his son Francis during their work on phototropism in canary grass coleoptiles.
• Chemical Nature: The most common natural auxin is Indole-3-acetic acid (IAA). Synthetic auxins include 2,4-D and NAA.
• Physiological Roles:
  • Promotes cell elongation and apical dominance.
  • Initiates root formation in stem cuttings.
  • Prevents premature fruit and leaf drop (abscission).
  • Induces parthenocarpy (seedless fruit production) in some plants.

Gibberellins: Nature and Function

Gibberellins are a group of acidic plant hormones.

• Discovery: Identified first in Japan in relation to the "bakanae" (foolish seedling) disease of rice caused by the fungus Gibberella fujikuroi.
• Chemical Nature: There are over 100 known gibberellins, with Gibberellic Acid (GA3) being the most studied.
• Physiological Roles:
  • Promotes stem elongation and increases the length of stalks in grapes.
  • Stimulates seed germination by breaking dormancy.
  • Promotes "bolting" (internode elongation) in rosette plants like cabbage.
  • Delays senescence in fruits.

Cytokinins: Promoting Cell Division

Cytokinins are hormones specifically associated with cytokinesis (cell division).

• Discovery: Discovered during attempts to culture tobacco pith tissue, where they were found to promote growth in the presence of auxin. Kinetin was the first cytokinin identified.
• Physiological Roles:
  • Promotes active cell division in roots, shoots, and developing fruits.
  • Helps overcome apical dominance by promoting lateral bud growth.
  • Delays leaf senescence by promoting nutrient mobilization.
  • Induces chloroplast development in leaves.

Abscisic Acid (ABA): The Stress Hormone

ABA acts primarily as a growth inhibitor and plays a critical role in plant responses to stress.

• Chemical Nature: A terpene-derived compound.
• Physiological Roles:
  • Induces stomatal closure during water stress to prevent transpiration.
  • Promotes seed and bud dormancy.
  • Increases plant tolerance to various kinds of stresses.
  • Promotes leaf abscission and senescence.

Ethylene: The Gaseous Hormone

Ethylene is unique among phytohormones because it is a simple gaseous molecule.

• Discovery: Identified as the substance released from ripe oranges that accelerated the ripening of stored bananas.
• Physiological Roles:
  • Promotes horizontal growth of seedlings and swelling of the axis.
  • Highly effective in fruit ripening (e.g., tomatoes, apples).
  • Promotes senescence and abscission of leaves and flowers.
  • Breaks seed and bud dormancy and initiates flowering in pineapples.

Applications of PGRs in Agriculture

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