Quick Summary
Plant growth is regulated by five major plant growth regulators (PGRs/phytohormones). Each hormone has multiple roles, and their interaction (often antagonistic) controls development. Photoperiodism and vernalisation link environmental signals to flowering. These concepts appear frequently in NEET.
Plant Growth — Phases
1. Cell division: Meristematic zone (mitosis) 2. Cell elongation: Elongation zone (vacuolation, cell wall loosening) 3. Cell differentiation/maturation: Cells acquire specific form and function
Growth curve: S-shaped (sigmoid)
- Lag phase → Log phase (exponential) → Stationary phase → Decline
Plant Growth Regulators (PGRs)
1. Auxins (IAA — Indole Acetic Acid)
Discovery: Charles Darwin (coleoptile bending), Fritz Went (isolated auxin)
Functions:
- Promotes cell elongation (primary effect)
- Apical dominance (suppresses lateral buds)
- Root initiation (adventitious roots)
- Fruit development (parthenocarpy)
- Prevents leaf/fruit abscission at low concentration
- High concentration inhibits root growth (roots more sensitive than shoots)
Synthetic auxins: 2,4-D (selective herbicide — kills broad-leaved weeds), NAA, IBA (rooting powders)
2. Gibberellins (GA₃)
Discovery: Kurosawa (bakanae disease of rice — foolish seedling disease)
Functions:
- Stem elongation (bolting in rosette plants)
- Seed germination (breaks dormancy, activates amylase)
- Fruit development (seedless grapes — parthenocarpy)
- Delays senescence
- Overcomes photoperiod requirements for flowering
3. Cytokinins
Discovery: Skoog & Miller (kinetin from herring sperm DNA)
Natural: Zeatin (found in maize endosperm)
Functions:
- Promotes cell division (cytokinesis)
- Delays leaf senescence (Richmond-Lang effect)
- Promotes lateral bud growth (overcomes apical dominance)
- Promotes nutrient mobilisation
4. Abscisic Acid (ABA) — "Stress Hormone"
Functions:
- Stomatal closure (during water stress)
- Seed dormancy (promotes)
- Inhibits germination
- Promotes leaf abscission
- Fruit ripening (minor role)
- Antagonises gibberellins
5. Ethylene
Nature: Gaseous hormone (only gaseous PGR)
Functions:
- Fruit ripening (promotes — used commercially to ripen bananas)
- Promotes senescence and abscission
- Promotes root hair development
- Triple response in seedlings (horizontal growth, swelling, inhibition)
Summary Table
| Hormone | Key Effect | Antagonist |
|---|---|---|
| Auxin | Cell elongation, apical dominance | Cytokinin |
| Gibberellin | Stem elongation, germination | ABA |
| Cytokinin | Cell division, anti-senescence | Auxin (for dominance) |
| ABA | Dormancy, stomatal closure | Gibberellin |
| Ethylene | Ripening, abscission | Auxin (at low conc.) |
Photoperiodism
Discovered by: Garner & Allard (1920) with Nicotiana tabacum cv. Maryland Mammoth and Glycine max Biloxi
Critical night length (NOT day length): Flowering is determined by the length of the dark period.
| Plant Type | Flowering condition | Example |
|---|---|---|
| Short-day plants (SDP) | Night > critical night length | Chrysanthemum, Xanthium, rice, tobacco |
| Long-day plants (LDP) | Night < critical night length | Wheat, barley, spinach, henbane |
| Day-neutral plants | Not affected by photoperiod | Tomato, cucumber, sunflower |
Phytochrome: Photoreceptor pigment that detects light. Pr (inactive) ↔ Pfr (active). Pfr inhibits SDP flowering, promotes LDP flowering.
Vernalisation
- Promotion of flowering by low temperature exposure
- Perceived at: shoot apical meristem
- Effect: not transmissible via grafting (not hormonal — epigenetic)
- Examples: Winter wheat, rye, carrots, beet, cabbage
Seed Dormancy
Causes: Impermeable seed coat, presence of inhibitors (ABA), immature embryo, after-ripening requirement.
Breaking dormancy: Scarification (mechanical/chemical treatment), cold stratification, gibberellin treatment, light.
Important Questions
One Mark
- Name the gaseous plant hormone.
- What is bolting? Which hormone is responsible?
- Name the scientist who discovered photoperiodism.
- What is the role of ABA in stomatal regulation?
Short Answer (3 marks)
- What is apical dominance? How do auxins and cytokinins interact to control it?
- Distinguish between short-day plants and long-day plants with one example each.
- What is vernalisation? Name two crops that require vernalisation.
Long Answer (5 marks)
- Describe the physiological effects of auxins on plant growth. Mention two commercial applications.
- Explain photoperiodism. What is the role of phytochrome in flowering?
Common Mistakes
- Photoperiodism is controlled by night length, not day length — "short-day plant" actually needs LONG nights.
- Ethylene is a gas at room temperature — not the same form as other liquid/solid hormones.
- ABA promotes dormancy and stress response — it is NOT only an abscission promoter.
- 2,4-D is a synthetic auxin used as herbicide, NOT a cytokinin or gibberellin.
Board Exam Tips
- PGR table (5 hormones, key functions, examples) is a standard 5-mark question.
- Photoperiodism (SDP, LDP, day-neutral) with examples is a reliable 3-mark question.
- "Explain apical dominance" with auxin-cytokinin interaction appears as a 3-mark question.
- NEET: ABA roles, ethylene effects, gibberellin applications, and photoperiodism are tested in 2–3 questions per paper.