External factors affecting photosynthesis:

Concentration of carbon dioxide Carbon dioxide is the major limiting factor for photosynthesis. The Increase in the concentration of Co2 in the atmosphere initially increases the photosynthetic rate but higher concentrations of Co2 can be inhibitory to photosynthesis.

The CO2 is present in a very low concentration in the atmosphere. (between 0.03 and 0.04 per cent). Increase in concentration up to 0.05 percent can cause an increase in CO2 fixation rates, beyond this the levels can become damaging over longer periods. The C3 and C4 plants do not respond to the higher concentration of CO2 at low light intensity. At high light intensities, both C3 and C4 plants show increase in the rates of photosynthesis. The C4 plants show saturation at about 360 µlL­­­­­­-1 while C3 show saturation at levels that are beyond 450 µlL­­­­­­-1. Thus, current availability of CO2 levels is limiting to the C3 plants.

The fact that C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.