In the majority of plants, including rice, CO2 is first fixed into a compound with three carbons (C3) by the photosynthetic enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco)—this is known as C3 photosynthesis.Rubisco is inherently inefficient because it can also catalyze a reaction with oxygen from the air, in a wasteful process known as photorespiration (rather than photosynthesis). At temperatures above 20°C, there is increasing competition by oxygen (O2), with a dramatic reduction in CO2 fixation and photosynthetic efficiency. While all this is happening, water is escaping from the leaves while the CO2 is diffusing in. Thus, in the hot tropics where most rice is grown, photosynthesis becomes very inefficient.