Oxygen effects on photosynthesis and 14C metabolism in desert plants

The effect of 1% and 21% O2 upon 14CO2 assimilation by desert plants exposed for 10 to 90 seconds has been studied. The plants studied can be divided into three groups with respect to O2. The C3 plants display the usual Warburg effect. No changes could be observed in the intensity of photosynthesis as a function of O2 content in another group of plants (showing signs of Crassulacean acid metabolism). In still another group of plants (C4 plants) the stimulating effect of O2 on photosynthesis could be detected. In C3 plants, O2 inhibits the processing of carbon through the Calvin cycle intermediates. The involvement of carbon in the glycolate pathway fails to explain completely the inhibiting effect of O2 on photosynthesis. It is assumed that O2 inhibits the enzymes of the Calvin cycle. In C4 plants O2 stimulates the incorporation of 14C into malate and aspartate. The incorporation of 14C into the intermediates of the Calvin cycle in C4 plants is inhibited much like that in typical C3 plants.