Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2 is very low in the atmosphere (between 0.03 and 0.04 per cent).
Increase in concentration upto 0.05 per cent can cause an increase in CO2 fixation rates; beyond this the levels can become damaging over longer periods. The C3 and C4 plants respond differently to CO2 concentrations.
At low light conditions neither group responds to high CO2 conditions. At high light intensities, both C3 and C4 plants show increase in the rates of photosynthesis.
What is important to note is that the C4 plants show saturation at about 360 μlL-1 while C3 responds to increased CO2 concentration and saturation is seen only 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.