5.1.6 Limiting Factors of Photosynthesis

Key Limiting Factors

1. Light Intensity:

• Light provides the energy needed for the light-dependent reaction of photosynthesis.
• As light intensity increases, the rate of photosynthesis also increases until another factor becomes limiting.
• At low light intensities, insufficient ATP and reduced NADP are produced, slowing the light-independent reaction.
• Saturation point: Beyond a certain light intensity, increasing it further has no effect.

2. Carbon Dioxide Concentration:

• $CO₂$ is required for the light-independent reaction (Calvin Cycle) to produce glycerate-3-phosphate ($GP$).
• Increasing $CO₂$ concentration raises the rate of photosynthesis until another factor becomes limiting.
• Optimal $CO₂$ concentration for most plants is about 0.1%.

3. Temperature:

• Photosynthesis involves enzymes, such as Rubisco and ATP synthase, which work best at their optimum temperature.
• Below the optimum, enzyme activity is slower due to reduced kinetic energy.
• Above the optimum, enzymes can denature, reducing the rate of photosynthesis.

Graphical Representation

• Light Intensity:
  • Initially, photosynthesis increases with light intensity but plateaus when another factor becomes limiting.
• Carbon Dioxide Concentration:
  • Shows a similar trend to light intensity, with a plateau at high $CO₂$ levels.
• Temperature:
  • The rate of photosynthesis rises to an optimum temperature but falls sharply beyond this due to enzyme denaturation.

Other Influencing Factors

4. Water Availability:

• Lack of water can cause stomata to close, reducing $CO₂$ uptake.
• Severe water stress may damage chloroplasts, further lowering photosynthesis rates.

5. Chlorophyll Levels:

• A deficiency in chlorophyll, often caused by lack of nutrients like magnesium, reduces the plant's ability to absorb light.

Controlling Limiting Factors in Agriculture

Farmers use controlled environments, such as greenhouses, to optimise conditions:

• Artificial lighting increases light intensity.
• Carbon dioxide enrichment improves $CO₂$ availability.
• Temperature regulation ensures enzymes work at their optimum temperature.