Limiting factors in photosynthesis.

• Both internal (plant) and external factors affect the rate. Internal ones include how many leaves, mesophyll cells and chloroplasts there are and their size, age and orientation, the internal CO₂ concentration and the amount of chlorophyll; external ones are light, temperature, CO₂ concentration and water.
• Blackman's Law of Limiting Factors (1905): when a chemical process depends on more than one factor, its rate is decided by the factor nearest its minimal value; changing that factor alone changes the rate.
• Light: at low intensities the rate rises linearly with light; at higher intensities other factors limit it. Light saturation occurs at about 10 per cent of full sunlight, so light is rarely limiting in nature except for plants in shade or dense forests. Very high light can break down chlorophyll and lower the rate.
• CO₂ is the major limiting factor. Its atmospheric level is very low (0.03 to 0.04 per cent); raising it up to about 0.05 per cent can increase fixation, but higher levels can be damaging over longer periods.
• At low light neither C₃ nor C₄ plants respond to high CO₂; at high light both do. C₄ plants saturate at about 360 μL L⁻¹, while C₃ plants keep responding and saturate only beyond 450 μL L⁻¹, so present CO₂ levels limit C₃ plants.
• Greenhouse crops such as tomatoes and bell peppers are grown in CO₂-enriched air to get higher yields.
• Temperature: the enzymatic carbon reactions are temperature-controlled; the light reactions are also temperature-sensitive but much less so. C₄ plants have a higher temperature optimum than C₃ plants, and tropical plants have higher optima than temperate ones.
• Water affects photosynthesis mainly through the plant: water stress closes stomata, cutting CO₂ supply, and makes leaves wilt, reducing leaf area and metabolic activity.

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