External Factors

Light supplies the energy that drives photosynthesis, and both its quality (wavelength/colour) and its intensity matter. Photosynthesis proceeds fastest in red light, with blue light next; in a forest, plants growing beneath the canopy photosynthesize considerably more slowly because so much light is intercepted above them. Across most plants, the rate is highest in bright, diffused sunlight; it actually falls in very strong direct light and also slows again in very weak/low-intensity light. Interestingly, plants can sustain uninterrupted photosynthesis for fairly long periods without any visible damage.

Carbon dioxide reaches land plants almost entirely from the atmosphere, which holds only about 0.03% CO2 — under normal light and temperature conditions this scarcity makes CO2 the usual limiting factor. Raising CO2 concentration increases the rate of photosynthesis, and concentrations up to about 1% are generally beneficial to most plants; beyond that, higher CO2 levels actually inhibit photosynthesis rather than helping it. …