Plant Growth Regulators
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Direct answer
Plant growth regulators are small, simple chemical molecules produced in one part of the plant that, at low concentrations, influence growth, differentiation and development in another. NCERT divides them broadly into growth promoters — auxins, gibberellins and cytokinins — and growth inhibitors or modifiers, principally abscisic acid (ABA) and the gaseous hormone ethylene. They are synthesised mostly in actively growing regions such as shoot apical meristems, young leaves, developing seeds and fruits.
What you must remember
- Auxins (IAA natural; IBA, NAA synthetic): promote cell elongation in shoot tips, cause apical dominance, initiate rooting in stem cuttings, and induce parthenocarpy; the synthetic auxin 2,4-D is used as a herbicide to kill dicotyledonous weeds.
- Gibberellins (over 100 known, GA3 the most studied): promote stem elongation in sugarcane, induce bolting and flowering in rosette plants like cabbage, delay leaf and fruit senescence, and speed malting in the brewing industry.
- Cytokinins (zeatin natural, kinetin synthetic): promote cell division, delay leaf senescence (the Richmond-Lang effect), help mobilise nutrients and overcome apical dominance when applied as the auxin-to-cytokinin ratio falls.
- Abscisic acid: the stress hormone — it closes stomata under water stress, promotes seed and bud dormancy and abscission, and generally inhibits growth.
- Ethylene: the only gaseous regulator; promotes fruit ripening, epinasty, senescence and abscission, breaks seed and bud dormancy, initiates flowering in pineapple, and is used commercially to ripen bananas and mangoes.
- Discovery anchors: auxin emerged from coleoptile experiments (Darwin to Went), gibberellin from the foolish seedling disease of rice caused by Gibberella fujikuroi, and cytokinin from autoclaved herring sperm DNA and coconut milk.
Common confusion
Auxin and gibberellin actions are the commonest mix-up: auxin causes apical dominance and rooting of cuttings, while gibberellins cause internode elongation and bolting. For senescence, cytokinins delay it while ABA and ethylene promote it — a favourite statement-comparison. Also remember that ethylene acts as both promoter and inhibitor depending on the process, so it cannot be boxed as purely one type.
Exam-focused takeaway
NEET-UG builds match-the-following questions on hormone versus application — 2,4-D as a dicot weedicide, GA3 for bolting and malting, ethylene for fruit ripening, ABA for stomatal closure — and assertion-reason items on apical dominance and the stress response. Statement-based questions test the Richmond-Lang effect and the foolish seedling disease. Learn each hormone with one natural and one synthetic name and two applications; the examples, not the definitions, earn the marks here.
Frequently asked questions
Which plant hormone is called the stress hormone and why?
Abscisic acid, because it accumulates under water stress and closes stomata, besides promoting dormancy and inhibiting growth.
What is bolting and which hormone induces it?
Bolting is the sudden elongation of the internodes of a rosette plant before flowering, as in cabbage; it is induced by gibberellins.
Which hormone delays leaf senescence?
Cytokinins delay leaf senescence, an effect known as the Richmond-Lang effect, and they also promote nutrient mobilisation.
What is the plant growth regulator 2,4-D used for?
It is a synthetic auxin used as a herbicide to eliminate dicotyledonous weeds from lawns and crop fields without harming grasses.
Which is the only gaseous plant growth regulator?
Ethylene, which promotes fruit ripening and is used commercially to ripen bananas and mangoes.
Which hormone is used in the brewing industry?
Gibberellic acid (GA3), which speeds the malting process by increasing the rate of starch hydrolysis in germinating barley.