Chapter 12 — Plant Growth and Development
Key Concepts Quick Recap
| Hormone | Chemical nature | Site of production | Key functions |
|---|
| Auxin (IAA) | Indole-3-acetic acid | Shoot apices, young leaves | Cell elongation, apical dominance, adventitious root formation, fruit development |
| Gibberellin (GA₃) | Diterpenoid | Young leaves, root tips, seeds | Stem elongation, seed germination, bolting, breaking dormancy |
| Cytokinin | Adenine derivative (zeatin) | Root tip, developing seeds | Cell division, delay senescence, nutrient mobilisation |
| ABA (Abscisic acid) | Sesquiterpene | Leaves, roots (stress) | Stomatal closure, dormancy, stress response — "stress hormone" |
| Ethylene | Gas (CH₂=CH₂) | Ripening fruits, nodes | Fruit ripening, abscission, triple response in seedlings |
I. Choose the Best Answer (MCQs)
1. Apical dominance is caused by: (a) Cytokinin (b) Auxin (c) Gibberellin (d) Ethylene
Answer(b) Auxin Auxin (IAA) produced at the shoot apex moves basipetally (downward) and inhibits growth of lateral buds — apical dominance. Removal of the apex (decapitation) removes IAA → lateral buds grow → bushier plant. Cytokinin promotes lateral bud growth (opposes auxin).
2. Which hormone is used to break seed dormancy and promote germination? (a) ABA (b) Ethylene (c) Gibberellin (d) Auxin
Answer(c) Gibberellin Gibberellins promote seed germination by: stimulating synthesis of hydrolytic enzymes (α-amylase) that break down starch in the endosperm to sugar → embryo gets food → germination. Also used to break dormancy of potato tubers and buds.
3. The plant hormone that promotes fruit ripening is: (a) Auxin (b) Cytokinin (c) Gibberellin (d) Ethylene
Answer(d) Ethylene Ethylene (a gas) promotes fruit ripening by activating enzymes that break down cell walls (cellulase, pectinase), converting starch to sugar, and softening the fruit. Used commercially to ripen bananas and tomatoes after transport.
4. Photoperiodism is related to: (a) Response to gravity (b) Response to light quality (c) Response to relative length of day and night (d) Response to temperature
Answer(c) Response to relative length of day and night Photoperiodism = response to the relative duration of light (photoperiod) and dark periods. The pigment phytochrome (Pr and Pfr forms) mediates this response. The critical period is actually the length of the DARK period, not the light period.
5. Long-day plants flower when: (a) Day length > critical period (night < critical period) (b) Day length < critical period (c) Temperature is high (d) They are exposed to cold treatment
Answer(a) Day length > critical period (night < critical period) Long-day plants (LDP): flower when the dark period is SHORTER than the critical dark period. Examples: Wheat, Barley, Spinach, Henbane. Short-day plants (SDP): flower when dark period is LONGER than critical. Examples: Chrysanthemum, Soybean, Tobacco.
6. Vernalisation refers to: (a) Growth due to light (b) Promotion of flowering by low temperature (c) Dormancy of seeds (d) Leaf movement due to gravity
Answer(b) Promotion of flowering by low temperature Vernalisation is the induction (or acceleration) of flowering by low temperature treatment. Biennial and winter annual plants require vernalisation. Example: Winter wheat (Triticum) needs 4-6 weeks at 0-5°C to flower in spring. Vernalin (or FLC suppression) is the hypothetical signal.
7. Weedkillers like 2,4-D act as: (a) Cytokinins (b) Synthetic auxins at toxic concentrations (c) ABA analogues (d) Gibberellin inhibitors
Answer(b) Synthetic auxins at toxic concentrations 2,4-D (2,4-dichlorophenoxyacetic acid) is a synthetic auxin. At high concentrations, it disrupts plant growth — causes abnormal cell elongation, vascular disruption, and death in broadleaf plants. It is selective — monocots (grass, cereals) are more resistant. Used as a herbicide in cereal farming.
II. Short Answer Questions
Q1. Explain the role of auxin in plant growth.
Answer
Auxin (Indole-3-acetic acid, IAA) is the first plant hormone discovered (Darwin and Boysen-Jensen; later isolated by Went, 1928).
Functions: 1. Cell elongation: Auxin loosens cell walls by activating H⁺ pump → wall acidification → cell wall loosening → water enters → cell elongates. 2. Phototropism: Light causes unequal auxin distribution — more auxin on shaded side → faster elongation on dark side → plant bends toward light. 3. Geotropism: Gravity causes auxin accumulation on lower side → roots bend down (negative phototropism), shoots bend up. 4. Apical dominance: IAA from apex inhibits lateral bud growth. 5. Rooting: Promotes adventitious root formation — used in vegetative propagation (IBA used in hormone rooting powders). 6. Fruit development: Prevents abscission, promotes fruit growth (parthenocarpy with high concentrations).
Q2. What is photoperiodism? Classify plants based on their photoperiodic response.
Answer
Photoperiodism is the response of plants to the relative length of the light and dark periods in a 24-hour cycle. The critical factor is the length of the DARK period (night).
Classification:
| Type | Flowering condition | Examples |
|---|
| Short-day plants (SDP) | Dark period > critical night length | Chrysanthemum, Soybean, Tobacco, Cocklebur (Xanthium) |
| Long-day plants (LDP) | Dark period < critical night length | Wheat, Spinach, Henbane, Radish (bolting) |
| Day-neutral plants (DNP) | Flower regardless of photoperiod | Tomato, Cucumber, Sunflower, Cotton |
Key experiment (Garner and Allard, 1920): Discovered photoperiodism studying Maryland Mammoth tobacco (short-day plant) — it flowered in winter regardless of other conditions.
Phytochrome:
- Pr (red light absorbing, inactive) ⇌ Pfr (far-red absorbing, active)
- Light converts Pr → Pfr; dark reverts Pfr → Pr slowly.
- Pfr/Pr ratio signals day/night length → controls flowering.
Q3. Describe the functions of ABA (Abscisic acid).
Answer
ABA is called the "stress hormone" or "dormin." Synthesised in leaves, stems, roots under stress.
Functions: 1. Stomatal closure: ABA triggers stomatal closure under drought — causes K⁺ to leave guard cells → turgor decreases → stomata close → reduces water loss. 2. Seed dormancy: Promotes and maintains seed dormancy; inhibits germination. 3. Bud dormancy: Induces bud dormancy in winter; counteracted by gibberellins in spring. 4. Leaf abscission: Promotes formation of abscission layer. 5. Stress response: Accumulates during drought, salinity, cold → signals adaptive responses. 6. Inhibition of growth: Generally inhibits seed germination and shoot growth — antagonises gibberellins.
Common Mistakes
- Confusing ABA (inhibitor, stress hormone) with ethylene (ripening, abscission) — both promote abscission but for different reasons.
- Saying LDP need long days — they actually need SHORT NIGHTS (dark period below critical length).
- Forgetting that phytochrome (not chlorophyll) mediates photoperiodism.
- Saying vernalisation uses light — it uses LOW TEMPERATURE.
Exam Checklist
- Five hormones: auxin, gibberellin, cytokinin, ABA, ethylene — one key function each.
- Auxin: cell elongation, apical dominance, phototropism, rooting.
- Gibberellin: stem elongation, seed germination (α-amylase), breaking dormancy.
- ABA: stomatal closure, dormancy — "stress hormone."
- Ethylene: fruit ripening, abscission.
- Photoperiodism: SDP (long dark), LDP (short dark), DNP (neutral); phytochrome Pr/Pfr.
- Vernalisation: flowering by cold temperature; winter annuals/biennials.
