Factors affecting photosynthesis

Many things affect the rate of photosynthesis. Internal factors depend on the plant: the number, size, age and angle of its leaves; the number of mesophyll cells and chloroplasts; the CO₂ inside the leaf; and the amount of chlorophyll. External factors come from outside: sunlight, temperature, CO₂ and water.

Blackman's law of limiting factors (1905): when a process depends on several factors, its rate is set by the factor that is closest to its minimum, the one in shortest supply. Think of a factory making tumblers: each needs a base, a cap and a straw. With 100 bases, 80 straws and only 45 caps, you can make only 45 tumblers: the caps limit you, however many bases there are.

Light: at low light, CO₂ fixation rises in step with light. But it levels off at about 10% of full sunlight. So light rarely limits plants in nature, except in shade or under dense forest cover; a money plant grows even indoors. Very strong light can break down chlorophyll.

CO₂: air holds only 0.03 to 0.04% CO₂, so it is the major limiting factor. Raising it up to about 0.05% increases fixation. C4 plants reach their maximum at about 360 µL per litre, while C3 plants keep responding beyond 450 µL per litre. That is why greenhouse crops like tomatoes and bell peppers are grown in CO₂-enriched air, for higher yields.

Temperature mainly affects the enzyme-driven carbon (dark) reactions. C4 plants work best at higher temperatures than C3 plants, and tropical plants at higher temperatures than those of cold regions. Water shortage closes the stomata, cutting off CO₂, and makes leaves wilt, reducing their surface and activity.