Plant Growth and Development: Growth Regulators, Photoperiodism and Vernalization
Examiners link each hormone to one signature response and one bioassay, then test the day/night logic of flowering. Read the wording of photoperiod questions carefully - most marks are lost there.
Core rules
- Auxin (IAA): apical dominance, cell elongation, root initiation, parthenocarpy; inactivated by light; assay - Avena coleoptile curvature test.
- Gibberellins (GA₃): internode elongation, bolting, seed germination through α-amylase induction in barley aleurone; dwarf mutants respond by elongating.
- Cytokinins: cell division, lateral bud growth, delay of senescence (Richmond–Lang effect).
- Abscisic acid: the stress hormone - stomatal closure, seed dormancy, bud dormancy. Ethylene: fruit ripening, abscission, epinasty, triple response of seedlings.
- Photoperiodism: short-day plants flower when the night exceeds a critical dark period (Xanthium, chrysanthemum); long-day plants when the night is shorter than critical (spinach, radish); day-neutral plants are unaffected. The uninterrupted dark period is decisive and is perceived through phytochrome (Pr ⇌ Pfr).
- Vernalization: cold treatment of imbibed seeds or seedlings promotes flowering in biennials and winter cereals; the stimulus is perceived at the shoot apex and can be transmitted.
Worked example
A plant flowers only when the dark period exceeds 10 hours, and fails to flower when a 5-minute light flash is given mid-dark. Reasoning: the flash breaks the long night, so the plant is a short-day (long-night) plant, and the critical factor is uninterrupted darkness rather than the length of day.
Traps that cost marks
Assigning stomatal closure to gibberellin; mixing the ethylene triple response with auxin action; saying vernalization replaces photoperiod in all plants; quoting a chemically isolated florigen as established fact in the textbook sense.