Biological Sciences, Dir (2026)

05/06/2026

✍️Major Controls of (in Detail)

Plant growth is controlled by a combination of environmental factors and internal biological processes. These controls determine how plants germinate, grow, reproduce, and survive.

1. Climate

A. Temperature

Influences photosynthesis, respiration, transpiration, and germination.

Each plant species has a minimum, optimum, and maximum temperature range.

High temperatures may increase water loss and stress.

Low temperatures can slow growth or cause dormancy.

Examples:

Wheat grows well in cool temperatures.

Rice requires warmer conditions.

B. Sunlight

Provides energy for photosynthesis.

Affects flowering, fruiting, and plant form.

Plants are classified as:

Heliophytes (sun-loving plants)

Sciophytes (shade-loving plants)

C. Rainfall and Water Availability

Water is essential for photosynthesis and nutrient transport.

Insufficient rainfall causes drought stress.

Excess water can lead to root rot and poor aeration.

2. Soil Conditions

A. Soil Fertility

Availability of nutrients such as Nitrogen (N), Phosphorus (P), and Potassium (K).

Fertile soils promote healthy growth and high productivity.

B. Soil Texture

Sandy soils drain quickly.

Clay soils retain more water.

Loamy soils are generally best for plant growth.

C. Soil pH

Controls nutrient availability.

Most plants prefer slightly acidic to neutral soils (pH 6–7.5).

D. Soil Aeration

Roots need oxygen for respiration.

Poorly aerated soils restrict root growth.

3. Water

Maintains cell turgidity.

Essential for nutrient transport.

Required for photosynthesis.

Controls transpiration and cooling.

Effects of Water Stress:

Wilting

Reduced photosynthesis

Stunted growth

4. Atmospheric Factors

A. Carbon Dioxide (CO₂)

Raw material for photosynthesis.

Higher CO₂ levels can increase plant growth under suitable conditions.

B. Oxygen

Necessary for root respiration.

Poor oxygen supply limits root development.

C. Wind

Enhances pollination and seed dispersal.

Strong winds may damage plants and increase water loss.

5. Topography and Relief

Altitude

Higher altitudes have lower temperatures and shorter growing seasons.

Slope

Influences drainage, soil depth, and sunlight exposure.

Aspect

South-facing slopes (Northern Hemisphere) receive more sunlight than north-facing slopes.

6. Biotic Factors

A. Competition

Plants compete for sunlight, water, nutrients, and space.

B. Grazing and Herbivory

Animals may reduce plant biomass through feeding.

C. Human Activities

Agriculture, deforestation, irrigation, and conservation practices greatly influence plant growth.

D. Microorganisms

Nitrogen-fixing bacteria and mycorrhizal fungi enhance nutrient availability.

7. Plant Growth Regulators (Hormones)

Auxins

Promote cell elongation and root formation.

Gibberellins

Stimulate stem elongation and seed germination.

Cytokinins

Promote cell division and delay leaf aging.

Abscisic Acid (ABA)

Controls dormancy and helps plants tolerate drought.

Ethylene

Promotes fruit ripening and leaf fall.

8. Genetic Factors

Genes determine growth rate, plant height, drought resistance, and productivity.

Different species and varieties respond differently to environmental conditions.

✍️Summary

The major controls of plant growth are:

1. Climate (temperature, light, rainfall)

2. Soil characteristics

3. Water availability

4. Atmospheric factors (CO₂, oxygen, wind)

5. Topography and relief

6. Biotic influences

7. Plant hormones

8. Genetic factors

Together, these factors regulate plant distribution, productivity, and survival in different ecosystems.

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