It is one of the byproducts of photosynthesis. While many plant physiolo­gists have opined that CO2 is necessary for photosynthesis, there does not seem to be any experimental evidence in support of this assumption. The accumulation of oxygen infact retards the rate of photosynthesis. Findings of Warburg (1920) have proved the in hibitory effect of O2 on photosyn­thesis.

Photosynthetic inhibition by O2 is called Warburg effect. Working on Chlorella plants, Warburg (1920) demonstrated such as effect. In Warburg effect, both O2 evolution and C02 assimilation will be inhibited in the presence of atmospheric O2 Gibbs (1970) has demonstrated that high concentration of oxygen brings about competetive inhibition of Ribulose phosphate carboxylase. High concentration of oxygen also inhibits photosynthesis by encourageing photorespiration.