Effect of CO2, O2, and Light on Photosynthesis and Photorespiration in Wheat

Unidirectional O(2) fluxes were measured with (18)O(2) in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O(2), O(2) uptake was maximum at 60 microliters per liter CO(2). At lower CO(2) concentrations, it was strongly inhibited, as was photosynthetic O(2) evolution. At 2% O(2), there remained a substantial O(2) uptake, even at high CO(2) level; the O(2) evolution was inhibited at CO(2) concentrations under 330 microliters per liter. The O(2) uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO(2). The behavior of the O(2) uptake and the study of the CO(2) compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.