Plant Hormones - Gibberellins

Gibberellin is a plant growth hormone found mainly in meristematic tissues like stem apex, root apex, buds, and seeds. It promotes internode elongation, breaks seed dormancy, and supports fruit development. Gibberellic acid (GA₃) is the most studied form of gibberellin.

• Discovery — First discovered by Kurosawa (1926) from Gibberella fujikuroi, causing Bakanae disease in rice. Crystalline form isolated by Yabuta & Sumiki (1938). Active form: GA₃ (Gibberellic acid).
• Synthesis & Transport — Synthesised in young leaves, seeds, roots, and stem tips from mevalonic acid. Transport is non-polar. Over 150 types are known.
• Stem Elongation & Bolting — Elongates internodes; converts dwarf plants to tall (e.g. maize, pea). Causes bolting in rosette plants (beet, cabbage).
• Seed Germination & Dormancy — Breaks seed and bud dormancy; stimulates amylase and protease for germination in cereals.
• Parthenocarpy & Fruit Growth — Produces seedless fruits (tomato, apple, pear); increases grape bunch length. Pomalin (GA₄ + GA₇ + cytokinin) = apple enlarger.
• Other Effects — Causes maleness in some plants; delays citrus ripening; inhibits root growth; delays senescence; prevents abscission.
• Vernalization Substitute — Overcomes the need for cold treatment and induces flowering in long-day plants.