Factors Affecting Photosynthesis

The rate of photosynthesis is not constant. It depends on several factors that may work independently or together. These factors are broadly classified into external (environmental) and internal (plant-related) factors. At any given time, the factor in the shortest supply determines the overall rate.

External Factors

1. Light

Light is the primary driving force of photosynthesis.

  • Quality:
  • Red and blue light are most effective because they are strongly absorbed by chlorophyll.
  • Green light is least effective as it is reflected.
  • Intensity:
  • As light intensity increases, the rate of photosynthesis increases linearly at first.
  • After a certain point (light saturation), the rate becomes constant because other factors become limiting.
  • Duration:
  • Longer exposure to light increases photosynthesis until the plant reaches its maximum capacity.

Key Point: More light does not always mean more photosynthesis.

2. Carbon Dioxide (CO₂)

  • CO₂ is the raw material for carbon fixation in the Calvin cycle.
  • Increasing CO₂ concentration increases photosynthesis up to a saturation level.
  • Atmospheric CO₂ (~0.04%) is often a limiting factor, especially in C₃ plants.

Key Point: CO₂ concentration directly controls the rate of sugar formation.

3. Temperature

  • Photosynthesis is controlled by enzymes, so it is temperature-dependent.
  • Optimum temperature:
  • Most plants: 25–35°C
  • At low temperature → enzyme activity is slow.
  • At very high temperature → enzymes denature and photorespiration increases (especially in C₃ plants).

Key Point: Temperature affects enzyme efficiency, not light absorption.

4. Water

  • Water is essential for:
  • Photolysis during light reactions
  • Maintaining cell turgidity
  • Water stress causes:
  • Stomatal closure → less CO₂ enters
  • Reduced chlorophyll formation
  • Reduced enzyme activity

Key Point: Lack of water indirectly reduces photosynthesis.

Internal Factors

  1. Chlorophyll Content:
  • More chlorophyll → greater light absorption → higher photosynthetic rate.
  1. Leaf Anatomy:
  • Leaf size, thickness, number of stomata, and chloroplast distribution affect efficiency.
  1. Protoplasmic Factors:
  • Enzyme concentration, cytoplasmic pH, and overall metabolic state of the cell influence reactions.

Key Point: Internal factors decide the plant’s inherent photosynthetic capacity.

Blackman’s Law of Limiting Factors

Proposed by F.F. Blackman (1905).

Statement:

When a physiological process is controlled by several factors, the rate of the process is determined by the factor which is least favourable or present in minimum amount.

Example:

  • If light and CO₂ are abundant but temperature is low → temperature becomes the limiting factor.
  • Increasing light further will not increase photosynthesis unless temperature improves.

Importance of Blackman’s Law

  • Explains why increasing a single factor does not always increase photosynthesis
  • Helps in greenhouse farming and controlled agriculture
  • Provides a scientific basis for crop yield optimisation

Memory Capsules – Factors Affecting Photosynthesis

External Factors (L-C-T-W):

  • Light → Quality, Intensity, Duration
  • CO₂ → Directly affects carbon fixation
  • Temperature → Enzyme-controlled (25–35°C optimum)
  • Water → Stomatal opening + photolysis

Internal Factors:

  • Chlorophyll content
  • Leaf anatomy
  • Enzyme concentration

Blackman’s Law (One-Liner): The slowest factor controls the speed of photosynthesis.

Exam Tip: If one factor is limiting, increasing others has no effect.

💡 Questions and Answers

Q1. Why does photosynthesis stop increasing beyond a certain light intensity?

A1. Because after light saturation, other factors like CO₂ concentration or temperature become limiting, so extra light has no effect.

Q2. How does water deficiency reduce the rate of photosynthesis?

A2. Water deficiency causes stomata to close, reducing CO₂ entry. It also lowers enzyme activity and chlorophyll formation, decreasing photosynthesis.

Q3. Explain Blackman’s law of limiting factors with an example.

A3. Blackman’s law states that the rate of photosynthesis is controlled by the factor in minimum supply. For example, even if light is abundant, low CO₂ will limit photosynthesis.

Q4: Why is temperature an important factor for photosynthesis?

A4. Photosynthesis is enzyme-controlled. Low temperature slows enzyme activity, while very high temperature denatures enzymes and increases photorespiration.

Q5. Which factor is usually limiting under natural conditions for C₃ plants?

A5. Carbon dioxide is usually the limiting factor because its atmospheric concentration is very low.