Photosynthetic Oxygen Reduction in Isolated Intact Chloroplasts and Cells in Spinach

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Abstract
The time course of light-induced O2 exchange by isolated intact chloroplasts and cells from spinach was determined under various conditions using isotopically labeled O2 and a mass spectrometer. In dark-adapted chloroplasts and cells supplemented with saturating amounts of bicarbonate, O2 evolution began immediately upon illumination. However, this initial rate of O2 evolution was counterbalanced by a simultaneous increase in the rate of O2 uptake, so that little net O2 was evolved or consumed during the first .apprx. 1 min of illumination. After this induction (lag) phase, the rate of O2 evolution increased 3- to 4-fold, while the rate of O2 uptake diminished to a very low level. Inhibition of the Calvin cycle, e.g., with DL-glyceraldehyde or iodoacetamide, had negligible effects on the initial rate of O2 evolution or O2 uptake; both rates were sustained for several minutes, and about balanced so that no net O2 was produced. Uncouplers had an effect similar to that observed with Calvin cycle inhibitors, except that rates of O2 evolution and photoreduction were stimulated 40-50%. Higher plant photosynthetic preparations which retain the ability to reduce CO2 may also have a significant capacity to photoreduce O2. With near-saturating light and sufficient CO2, O2 reduction appears to take place primarily via a direct interaction between O2 and reduced electron transport carriers, and occurs principally when CO2-fixation reactions are suboptimal, e.g., during induction or in the presence of Calvin cycle inhibitors. The inherent maximum endogenous rate of O2 reduction is .apprx. 25-50% of the maximum rate of noncyclic electron transport coupled to CO2 fixation. Although the photoreduction of O2 is coupled to ion transport and/or phosphorylation, this proccess does not appear to supply significant amounts of ATP directly during steady-state CO2 fixation in strong light.