Unit 2 | Growth And Development of Horticultural Crops | MSc Horticulture - 2nd Year 3rd Semester

HORMA - 301

Growth And Development of Horticultural Crops
UNIT II

📚 Contents

1. Annual, Semi-perennial and Perennial Horticultural Crops
2. Environmental Impact on Growth and Development
3. Effect of Light
4. Photosynthesis and Growth
5. Photoperiodism
6. Vernalisation
7. Effect of Temperature on Growth and Development
8. Heat Units (Growing Degree Days – GDD)
9. Thermoperiodism
10. QUICK EXAM REVISION

1. Annual, Semi-perennial and Perennial Horticultural Crops

A. Annual Crops

Definition: Plants completing their entire life cycle (seed → seed) within a single growing season.

Characteristics

• High metabolic rate, short life span
• High responsiveness to environmental changes
• Rapid vegetative growth followed by quick reproductive transition
• Weak perennial structures (no extensive wood or storage organs)

Examples

• Vegetables: radish, spinach, beans
• Flowers: marigold, petunia
• Fruit crops (grown as annuals): tomato, capsicum, strawberry

B. Semi-perennial Crops

Definition: Plants living 2–4 years, but economically productive for only limited seasons.

Characteristics

• Productivity declines after a few cycles
• Often replaced after disease, pest buildup, or yield drop
• Require periodic replanting
• Medium vegetative structure

Examples

• Banana
• Papaya
• Pineapple

C. Perennial Crops

Definition: Plants that live for many years and produce repeatedly.

Characteristics

• Distinct vegetative → reproductive cycles
• Long juvenile period in fruit crops
• Strong wood formation
• Highly adapted to seasonal environmental cues

Examples

• Mango, citrus, apple
• Litchi, guava
• Perennial ornamentals (rose, jasmine)

2. Environmental Impact on Growth and Development

Plant growth is strongly influenced by environmental factors. Genetics determines the potential, but environment determines expression.

Major Environmental Factors Affecting Growth:
Light (intensity, quality, duration), Temperature (day & night), Water availability, Soil nutrients, CO₂ concentration, Humidity & wind, Seasonal changes.

How Environment Influences Growth and Development

1. Cell Division & Enlargement: Optimal temperature and hydration increase cell division. Low temperature or drought slows mitosis and inhibits expansion.
2. Photosynthesis & Respiration: Light, temperature, CO₂ determine carbon gain. Imbalance → reduced biomass.
3. Hormonal Balance: Environmental cues regulate auxin, cytokinin, ABA, ethylene—all controlling: Shoot/root growth, Bud break, Flowering, Senescence.
4. Phenology (timing of growth stages): Environment determines: When plants flower, Fruit maturity period, Onset of dormancy, Tuber and bulb formation.
5. Morphogenesis: Light direction, temperature gradients, and photoperiod shape: Leaf size, Internode length, Root-to-shoot ratio.
6. Stress Effects:
   Heat → protein denaturation, sunburn
   Cold → chilling injury, delayed flowering
   Excess water → reduced aeration, root decay
   Drought → reduced expansion, stomatal closure

👉 Environmental factors continuously interact and influence plant performance, not in isolation.

3. Effect of Light

Light affects plants in three primary ways:

A. Light Intensity

Positive Effects

• Higher photosynthesis → increased biomass
• Improved carbohydrate accumulation
• Compact, sturdy growth
• Better flowering in sun-loving crops (rose, tomato)

Negative Effects

• Very low intensity → etiolation, pale leaves, weak stems
• Excessive intensity → leaf scorching, photo-inhibition
• Shade conditions reduce yield in many fruit crops

B. Light Quality (Wavelength Effect)

Light Type	Effect
Red light (~660 nm)	Flower induction, stem elongation
Blue light (~450 nm)	Chlorophyll synthesis, stomatal opening, compact growth
Far-red (~730 nm)	Shade-avoidance: tall, weak stems
UV radiation	Can reduce leaf size, cause damage.

UV-A promotes protective responses like boosting anthocyanin (red/purple) and other UV-absorbing pigments for photoprotection. The phytochrome system mediates many of these responses.

C. Light Duration (Photoperiod)

• Positive: Proper day length induces flowering at the right stage. Regulates dormancy, tuberization (potato), bulbing (onion).
• Negative: Incorrect photoperiod causes delayed flowering. Premature bolting in vegetables (cabbage, radish). Flowering inhibition in SD or LD plants under wrong daylength.

4. Photosynthesis and Growth

Photosynthesis produces sugars + energy, directly controlling biomass accumulation.

Factors Affecting Photosynthesis

1. Light: Intensity → increases photosynthesis to saturation point. Quality → blue/red most effective. Duration → longer light increases total daily carbon gain.
2. CO₂ Concentration: Higher CO₂ increases photosynthesis up to optimum. Greenhouse enrichment (800–1000 ppm) significantly increases yield.
3. Temperature: Influences enzyme activity (Rubisco). C₃ plants optimal around 20–30°C; C₄ around 30–40°C. Low temp → reduced photosynthesis & chlorosis. High temp → increased respiration > photosynthesis (net loss).
4. Water Availability: Water stress → stomatal closure → ↓ CO₂ uptake → ↓ photosynthesis. Leads to reduced leaf expansion.
5. Leaf Characteristics: Leaf age: young → low, mature → high photosynthesis. Leaf Area Index (LAI): moderate LAI maximizes light capture. Chlorophyll content: directly proportional to photosynthetic capacity.
6. Nutrient Supply: 
• Nitrogen: essential for chlorophyll and Rubisco. 
• Magnesium: central element of chlorophyll. 
• Iron: needed for electron transport. 
• Deficiency → reduced photosynthesis.

5. Photoperiodism

Definition: Photoperiodism is the response of plants to the relative length of day and night, especially for flowering.

Types of plants

Type	Requirement	Examples
Short day plants	Night longer than critical period	Chrysanthemum
Long day plants	Night shorter than critical period	Spinach
Day neutral	No photoperiod response	Tomato

Mechanism

• Controlled by phytochrome
• Night length is physiologically critical

📌 Leaves perceive photoperiodic stimulus.

6. Vernalisation

Definition: Vernalisation is the induction of flowering by exposure to low temperature for a specific duration.

Characteristics

• Promotes early flowering
• Acts on meristematic tissues
• Can be reversed by devernalisation (high temperature)

Examples

• Biennial vegetables: carrot, beetroot
• Temperate cereals and crops

📌 Physiological basis includes gene activation related to flowering.

7. Effect of Temperature on Growth and Development

Temperature affects:

A. Enzyme Activity & Metabolism

• Each enzyme has an optimum temperature
• Low temperature → slowed respiration, protein inactivity
• High temperature → enzyme denaturation

B. Growth Rate

• Temperature regulates cell division, elongation, and differentiation
• Cool temperatures → compact growth
• Warm temperatures → rapid stem elongation
• Extreme temperatures → growth arrest

C. Photosynthesis vs Respiration

• Moderate temperature increases photosynthesis
• At high temperature → respiration > photosynthesis → net biomass loss
• Fruit crops like tomato, lettuce have narrow optimum ranges

D. Reproductive Development

• Flower induction sensitive to temperature
• High temperature → flower drop in tomato
• Low temperature → pollen sterility in many crops
• Fruit set depends on optimum night temperature

E. Dormancy & Bud Break

• Cold temperature breaks dormancy in apples, peach
• Insufficient winter chill → poor flowering

F. Temperature Stress

• Heat Stress: Sunburn on fruits, Reduced chlorophyll, Membrane instability.
• Chilling Injury (0–15°C): Seen in tropical crops (banana, papaya). Symptoms: browning, water-soaking.
• Freezing Injury (<0 strong=""> Ice crystal formation → cell death.

8. Heat Units (Growing Degree Days – GDD)

Definition: Heat units represent the accumulated temperature required to complete a growth stage.

Formula:

GDD =

T_max + T_min 2

- T_base

Where: T_base = minimum threshold temperature

Importance

• Predicts phenological events
• Scheduling harvesting and flowering
• Crop zoning

📌 Growth progresses only after base temperature is exceeded.

9. Thermoperiodism

Definition: Plant response to daily alternation of temperature—difference between day and night temperatures.

✔ Typical Pattern

Most horticultural crops prefer:

• Warm Day Temperature → increases photosynthesis & carbohydrate production
• Cool Night Temperature → reduces respiration → conserves energy → promotes stem thickening, better flower development

Example of beneficial thermoperiod: Day: 25–30°C, Night: 15–20°C

✔ Positive Effects

• Better biomass accumulation
• Enhanced flowering (e.g., chrysanthemum, tomato)
• Improved fruit coloration (apple, grapes)
• Stronger stems and compact growth

✔ Negative Effects

• No day–night difference → weak, elongated plants
• Very warm nights → excessive respiration → reduced yield
• Very cool nights → delayed growth, nutrient imbalance

✅ UNIT II – QUICK EXAM REVISION

• Annuals complete life in one season
• Perennials show seasonal cycles
• Light affects growth via intensity, quality, duration
• Photoperiodism → flowering response
• Vernalisation → cold-induced flowering
• Temperature controls metabolic rate
• GDD predicts crop maturity
• Thermoperiodism → day–night temperature effect

Click here for notes of other units and subjects