HORMA - 301
Growth And Development of Horticultural Crops
Growth And Development of Horticultural Crops
UNIT IV: Developmental Physiology and Biochemistry
📚 Contents
1. Dormancy
Definition: Dormancy is a temporary suspension of visible growth, even under favorable environmental conditions. It is a survival adaptation regulating seasonal growth, flowering, and fruiting.
Types of Dormancy
- A. Endodormancy (True dormancy): Controlled internally by physiological factors (bud/seed). High ABA levels. Common in temperate fruit trees.
- B. Ecodormancy: Caused by unfavorable environmental conditions. Growth resumes when the environment improves.
- C. Paradormancy: Caused by inhibitory signals from other plant parts (e.g., apical dominance).
Biochemistry During Dormancy
- High Abscisic Acid (ABA): maintains dormancy
- Low Gibberellins & Cytokinins: suppress growth
- Low respiration rate
- Accumulation of protective proteins (dehydrins)
- Increased sugars and starch for cold tolerance
- Enzymatic activity declines (amylase, invertase, protease)
2. Bud Break
Definition: The resumption of growth in dormant buds when physiological blocks are removed.
Physiological & Biochemical Changes
- Decrease in ABA
- Increase in GA and Cytokinins
- Enhanced respiration
- Cell division resumes (reactivation of meristems)
- Enzymes reactivate (amylase → converts starch to sugars → energy)
Environmental Requirements
- Chilling requirement in temperate fruit crops
- Warm temperature after chilling → rapid bud break
- Light promotes bud break in some species
3. Juvenility
Definition: The early developmental phase during which a plant cannot reproduce, even under inductive conditions.
Characteristics
- No flowering
- High vegetative vigor
- Thorns, juvenile leaf shape (in citrus, apple)
- High rooting ability
- Different hormonal balance (high auxin, low GA ratio)
Physiological Basis
- Gene expression patterns differ from the adult phase
- Hormonal shifts: High auxin/GA maintains juvenility; Cytokinins increase during transition.
- Reduced sensitivity to flowering signals.
4. Vegetative to Reproductive Interphase
Definition: The transitional stage where the shoot apical meristem shifts from producing leaves (vegetative) to producing flowers (reproductive).
Physiological Events
- Activation of floral meristem identity genes (e.g., LFY, AP1)
- Increase in cytokinin at the meristem
- Change in sugar transport: more assimilates directed to the apex
- Decrease in nitrogen-rich compounds; increase in carbohydrates
Environmental Influences
Photoperiod, Vernalisation, Temperature, Light quality.
5. Flowering
Definition: The physiological process where the vegetative meristem transforms into a floral meristem capable of forming reproductive organs.
Mechanisms Regulating Flowering
| A. Photoperiodism | Critical night length regulates flowering. Florigen (FT protein) moves to the meristem. |
| B. Vernalisation | Low-temperature requirement. Prepares meristem for flowering induction. |
| C. Autonomous Pathway | Independent of environmental cues, age-driven. |
| D. Hormonal Control | GA promotes (LD plants), ABA delays, and Auxin influences development. |
6. Pollination
Definition: Transfer of pollen from anthers to the stigma.
Types
- Self-pollination
- Cross-pollination
Physiology
- Pollen viability is influenced by temperature & humidity
- Stigma receptivity involves the secretion of proteins, sugars
- Pollen hydration & germination on stigma
- Pollen tube growth guided by Ca²⁺ gradients
7. Fertilization
Definition: Fusion of male (sperm) and female (egg) gametes.
[Image of double fertilization in plants]Process
Pollen tube enters embryo sac → One sperm fertilizes egg (zygote) → Other fuses with polar nuclei (endosperm/double fertilization).
Biochemical Events
- Ca²⁺ bursts
- Activation of zygotic genes
- Synthesis of proteins for embryo development
- Hormonal increase (auxin, cytokinin)
8. Fruit Set
Definition: The transition from flower to young fruit when the ovary begins to grow after fertilization.
Requirements
- Successful pollination and fertilization
- Adequate carbohydrate supply
- Hormonal balance: ↑ Auxin & GA promote set, ↓ ABA improves retention
Failures
Poor pollination, Low temperature, Carbohydrate shortage, Nutrient deficiencies (B, Ca).
9. Fruit Drop
Types
- A. Pre-bloom drop: Failure of pollination
- B. June drop: Physiological drop due to competition
- C. Pre-harvest drop: Hormonal imbalance
Control
Auxin sprays (NAA), Proper irrigation, Balanced nutrition, and GA.
10. Fruit Growth
Fruit growth has three phases (Sigmoid Curve):
| Phase 1: Cell Division | Phase 2: Cell Enlargement | Phase 3: Maturation |
|---|---|---|
| Soon after fruit set. High auxin/cytokinin. High metabolic activity. | Dominant phase. Water accumulation. GA/BR regulate enlargement. | Dry matter accumulation. Chlorophyll breakdown. Seed maturation. |
11. Fruit Ripening
Definition: Final stage of fruit development involving physiological and biochemical changes, making fruits edible.
Two Types
| Climacteric Fruits | Show rise in respiration & ethylene (Banana, Mango, Apple). |
| Non-climacteric Fruits | No respiratory rise (Citrus, Grapes, Strawberry). |
Biochemical Changes
- Starch → sugars (amylase, invertase)
- Chlorophyll degradation → carotenoids, anthocyanins
- Pectin breakdown → softening (pectinase)
- Increase in ethylene in climacteric fruits
12. Seed Development
Stages
- A. Embryo Formation: Zygote → embryo; shoot/root primordia.
- B. Endosperm Development: Provides nutrition; accumulates starch/proteins.
- C. Reserve Deposition: Protein bodies, Lipid accumulation.
- D. Seed Maturation: Desiccation tolerance, ABA increases (dormancy), Seed coat hardens.
📚 REFERENCES
Buchanan B, Gruissem W & Jones R. 2002. Biochemistry & Molecular Biology of Plants. Wiley.
Epstein E. 1972. Mineral Nutrition of Plants. Wiley.
Fosket D.E. 1994. Plant Growth and Development. Academic Press.
Leopold A.C. & Kriedemann P.E. 1985. Plant Growth and Development. McGraw-Hill.
Salisbury F.B. & Ross C.W. 1992. Plant Physiology. 4th Ed. Wadsworth Publ.
Peter K.V. 2008. Basics of Horticulture. New India Publishing Agency.
Roberts J.A., Downs S.J., Parker P. 2002. In Plants (I. Ridge, Ed.). Oxford Univ. Press.