Biology - Chapter 13: Plant Growth and Development
Plant Growth Regulators (Phytohormones) and Photoperiodism
1. Plant Growth Regulators
Plant growth regulators (phytohormones) are chemical substances that control growth and development in plants.
| Hormone | Main Function | Key Feature |
|---|---|---|
| Auxin | Apical dominance | Promotes root initiation and stem growth |
| Gibberellins | Stem elongation, bolting | Helps in seed germination |
| Cytokinins | Cell division | Delay senescence (aging) |
| Abscisic Acid (ABA) | Growth inhibitor | Stress hormone |
| Ethylene | Fruit ripening | Only gaseous hormone |
💡 Quick Concept:
Auxin = Growth
Gibberellin = Height
Cytokinin = Division
ABA = Inhibition
Ethylene = Ripening
2. Important Questions & Answers
Q1. Difference between arithmetic and geometric growth?
Answer:
- Arithmetic Growth: Only one daughter cell divides while the other becomes mature.
- Geometric Growth: Both daughter cells continue dividing → rapid growth (S-shaped curve).
Q2. What is apical dominance?
Answer: It is the suppression of lateral buds due to the presence of apical bud (controlled by auxin).
Removing the apical bud promotes bushy growth.
Q3. Why is ABA called stress hormone?
Answer: ABA helps plants survive stress conditions by closing stomata and inducing dormancy.
Q4. What is photoperiodism?
Answer: It is the response of plants to the duration of light and dark periods.
Plants are classified as short-day, long-day, and day-neutral plants.
Q5. What is vernalisation?
Answer: It is the induction of flowering by exposing plants to low temperature.
Q6. What is plasticity in plants?
Answer: The ability of plants to change their structure according to environmental conditions.
🔥 Exam Booster:
Auxin → Apical dominance
ABA → Stress
Ethylene → Fruit ripening
Photoperiodism → Flowering