Plant Growth and Development
'Growth is the irreversible increase in size. Development is the journey from seed to senescence.' — Plant Physiology
1. Chapter Overview
Plants grow DIFFERENTLY from animals — they grow CONTINUOUSLY throughout life, with growth LOCALISED at meristems. This chapter covers GROWTH phases (meristematic, elongation, maturation), GROWTH rates (arithmetic vs geometric), PLANT GROWTH REGULATORS (auxins, gibberellins, cytokinins, ethylene, abscisic acid), PHOTOPERIODISM (response to day length), and VERNALISATION (cold treatment for flowering).
2. Plant Growth
Characteristics of Plant Growth
- Plants are OPEN systems of growth (continue growing throughout life)
- Growth is LOCALISED at meristems (apical, lateral, intercalary)
- Growth is IRREVERSIBLE
Phases of Growth
| Phase | Description | Cell Activity |
|---|---|---|
| Meristematic (Formative) | At root/shoot apices | Active CELL DIVISION |
| Elongation | Just behind apex | Cells EXPAND (vacuolation) |
| Maturation | Further behind apex | DIFFERENTIATION (cells become specialised) |
Types of Growth
| Type | Description | Graph | Example |
|---|---|---|---|
| Arithmetic | Grows by CONSTANT amount per unit time | Linear | Root elongation in cm/day |
| Geometric (Exponential) | Grows by CONSTANT PERCENTAGE per unit time | J-shaped | Cell division early in embryo |
Growth Rate Measurement
- Absolute growth rate: Increase in size per UNIT TIME
- Relative growth rate: Growth per UNIT TIME per UNIT INITIAL SIZE
Growth Curve (Sigmoid / Grand Period of Growth)
Phases: Lag phase (slow) → Log phase (exponential) → Stationary phase (plateau)
3. Differentiation, Dedifferentiation, and Redifferentiation
- Differentiation: Cells BECOME specialised (e.g., meristem → xylem vessel)
- Dedifferentiation: DIFFERENTIATED cells REGAIN the ability to DIVIDE (e.g., interfascicular cambium formation)
- Redifferentiation: Dedifferentiated cells AGAIN become specialised (e.g., secondary xylem formation)
Plasticity
- Ability of plants to ALTER their growth form in RESPONSE to environment
- Example: Heterophylly in buttercup (different-shaped leaves in air vs water)
4. Plant Growth Regulators (Phytohormones)
Overview
| PGR | Type | Major Function | Site of Production |
|---|---|---|---|
| Auxins | Growth PROMOTER | Cell ELONGATION, apical dominance, root initiation | Shoot APICAL meristem |
| Gibberellins | Growth PROMOTER | Stem ELONGATION, seed germination | Young leaves, roots, EMBRYO |
| Cytokinins | Growth PROMOTER | Cell DIVISION, delay senescence | Root TIPS |
| Ethylene | Growth PROMOTER (fruits) & INHIBITOR (other) | Fruit RIPENING, abscission, senescence | TISSUES under stress |
| Abscisic acid (ABA) | Growth INHIBITOR | Stomatal CLOSURE, DORMANCY, stress tolerance | Chloroplasts, roots |
Auxins (IAA — Indole-3-acetic acid)
- NATURAL: IAA (indole-3-acetic acid)
- SYNTHETIC: NAA, 2,4-D (weedicide), IBA
- Functions:
- Cell ELONGATION (acid growth hypothesis — auxin activates H⁺ pump → cell wall loosens)
- APICAL DOMINANCE (auxin from shoot apex SUPPRESSES lateral bud growth)
- Root INITIATION (rooting hormone)
- Parthenocarpy (fruit development WITHOUT fertilisation — seedless fruits)
- TROPISMS (phototropism — asymmetric auxin distribution)
Gibberellins (GA₃ — Gibberellic acid)
- Over 100 types known
- Functions:
- Stem ELONGATION (internode lengthening — bolting in cabbage)
- Seed GERMINATION (activates hydrolytic enzymes in barley)
- Parthenocarpic fruits (grapes)
- OVERCOME dwarfism (mutants that lack GA)
Cytokinins
- Natural: Zeatin, Kinetin (synthetic)
- Functions:
- Cell DIVISION (cytokinesis)
- DELAY senescence (Richmond-Lang effect — leaves stay green longer)
- Promote SHOOT formation in tissue culture
- Counter APICAL dominance (promote LATERAL bud growth)
Ethylene (C₂H₄) — Gaseous Hormone
- Functions:
- Fruit RIPENING (climacteric fruits — apple, banana)
- Epinasty (leaf curling due to asymmetric growth)
- Abscission (leaf/fruit drop)
- SenESCENCE
- PROMOTES root hair formation
- BREAKS seed dormancy in some species
Abscisic Acid (ABA) — Stress Hormone
- Functions:
- Stomatal CLOSURE (during water stress)
- Seed DORMANCY (prevents premature germination)
- DESICCATION tolerance
- ANTAGONISTIC to gibberellins
- Senescence and abscission
5. Photoperiodism
- Definition: Response of plants to RELATIVE LENGTHS of day and night (photoperiod)
- Discovered by GARNER and ALLARD (1920) — tobacco variety 'Maryland Mammoth'
Types of Plants
| Type | Flower when | Examples |
|---|---|---|
| Short Day Plants (SDP) | Day length < Critical period | Xanthium, Soybean, Chrysanthemum |
| Long Day Plants (LDP) | Day length > Critical period | Spinach, Wheat, Radish |
| Day Neutral Plants (DNP) | Not affected by day length | Tomato, Cotton, Sunflower |
Phytochrome — The Light Receptor
- Pigment that DETECTS light (red/far-red)
- Pr (inactive, 660 nm) ↔ Pfr (active, 730 nm)
- Pfr triggers FLOWERING in LDP; Inhibits flowering in SDP
6. Vernalisation
- Definition: INDUCTION of flowering by EXPOSURE to PROLONGED COLD (typically winter)
- Plants are VERNALISED as SEEDS or young seedlings
- Effect is TRANSMITTED through cell division (heritable for that generation)
- Examples: Winter wheat, Cabbage, Sugar beet, Carrot
Application
- Farmers plant winter varieties in AUTUMN → seeds VERNAISE over winter → flower in SPRING
- Spring varieties do NOT require cold — planted in spring → harvest in summer
7. Seed Dormancy
| Cause of Dormancy | Mechanism | Breaking Method |
|---|---|---|
| Hard seed coat | Water/O₂ impermeable | Scarification (mechanical/acid) |
| Immature embryo | Embryo needs DEVELOPMENT | After-ripening (dry storage) |
| Chemical inhibitors | ABA, phenolic compounds | Leaching with water |
| Light requirement | Needs specific light | Exposure to appropriate light |
8. Common Mistakes
- Auxin promotes cell ELONGATION, not division: Cytokinin promotes cell division
- Ethylene is a GASEOUS hormone: It diffuses easily — ripening one apple near others can ripen the lot
- ABA is NOT the 'abscission hormone': Ethylene is more directly involved in abscission. ABA is the STRESS hormone
- Photoperiodism responds to the LENGTH OF DARKNESS, not just light: The DARK period is critical — interrupting the dark period with a brief light pulse can alter flowering response
- Gibberellins cause BOLTING (stem elongation) in cabbage and beet: This is not just 'growth' — it's a developmental switch to flowering
9. CBSE Exam Focus
- Phases of plant growth and growth rates (3-mark)
- Plant growth regulators — auxin, GA, cytokinin functions (5-mark)
- Ethylene and ABA — functions and applications (3/5-mark)
- Photoperiodism — SDP, LDP, DNP (5-mark)
- Vernalisation — definition, examples (3-mark)
- Seed dormancy and germination (3-mark)
10. Self-Test (5+ Q&A)
Q1: Differentiate between auxin and cytokinin. A: Auxin: Cell ELONGATION, APICAL dominance, root initiation. Cytokinin: Cell DIVISION, DELAYS senescence, promotes LATERAL bud growth. They often work ANTAGONISTICALLY.
Q2: What is apical dominance? How is it controlled? A: The PHENOMENON where the MAIN central stem GROWS more strongly than SIDE stems. AUXIN from the shoot apex SUPPRESSES lateral buds. Removing the apical bud → lateral buds grow.
Q3: What are short day plants and long day plants? Give examples. A: SDP: Flower when day length is SHORTER than a critical period (Xanthium, Chrysanthemum). LDP: Flower when day length is LONGER than a critical period (Wheat, Spinach).
Q4: Why is ethylene used in fruit ripening? A: Ethylene is a NATURAL plant hormone that triggers the RIPENING process (conversion of starch to sugar, softening, colour change). Commercially, fruits like BANANAS are ripened in ethylene chambers.
Q5: What is vernalisation and how does it help plants? A: Exposure of seeds/seedlings to PROLONGED COLD to INDUCE flowering. It PREVENTS premature flowering and ENSURES that flowering occurs in favourable SPRING conditions after winter.
11. Conclusion
Plant growth and development are REGULATED by an intricate BALANCE of phytohormones. Auxins, gibberellins, and cytokinins PROMOTE growth; ethylene promotes ripening and senescence; ABA INHIBITS growth and stress responses. Environmental factors — DAY LENGTH (photoperiodism) and COLD (vernalisation) — DICTATE when plants flower, ensuring REPRODUCTIVE success in favourable seasons. Understanding these processes is CRITICAL for AGRICULTURE — manipulating growth regulators (for larger fruits, delayed ripening) and environment (greenhouse, artificial light) to MAXIMISE crop production.
