Effect of oxygen concentration on the rates of photosynthesis and photorespiration of some higher plants

Abstract

Carbon dioxide gas exchange of leaf material was studied in the light at different oxygen tensions for two CO₂ concentrations, using an isotope technique. With bean, radish, and tobacco leaves in the leaf chamber, increasing the oxygen tension resulted in a significant alteration in the ¹⁴CO₂/CO₂ ratio of the gas mixture leaving the leaf chamber as compared to that offered to the leaf material. On estimating the rates of “true’ and apparent photosynthesis it was found that below 5% oxygen concentration the rates were not significantly different. However, increasing the oxygen concentration of the gas mixture resulted in a proportional decrease in the rates of true and of apparent photosynthesis. The increasing oxygen tension also resulted in proportional increases in the CO₂ evolution (true photosynthesis minus apparent photosynthesis). The percentage inhibition of apparent photosynthesis was greater at low carbon dioxide concentration, while the inhibition of true photosynthesis was not as sensitive to the carbon dioxide level. The inhibition of apparent photosynthesis was not entirely attributable to the increased photorespiration but was roughly equally divided between an inhibition of true photosynthesis and a stimulation of CO₂ evolution in the light.However, with corn leaf material, there was no effect of oxygen concentration on both the rates of true and apparent photosynthesis; also, no large CO₂ evolution could be detected as emerging from leaf in the light at any of the oxygen concentrations tested.