Oxygen ist reduced by the electron transport chain of chloroplasts during CO2 reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO2-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO2 reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.

The role of oxygen in the photoinactivation of the photosynthetic apparatus of Spinacia oleracea L. was investigated. Moderate irradiation (1200 μmol photons m(-2)s(-1)) of spinach leaves in an atmosphere of pure nitrogen caused strong inhibition of subsequently measured net CO2 assimilation, whereas considerably less photoinhibition was observed in the presence of low partial pressures (10-20 mbar) of O2. The decrease in activity caused by anaerobiosis in the light was not based on stomatal closure; the decline of assimilation represents a photoinhibition, as activity was not impaired by low irradiation (80 μmol photos m(-2)s(-1)). In contrast, gassing with pure N2 in the dark caused strong inhibition. Electron-transport rates and chlorophyll-fluorescence data of thylakoids isolated from photoinhibited leaves indicated damage to the electron-transport system, in particular to photosystem II reaction centers. In vitro, photoinhibition in isolated thylakoid membranes was also strongly promoted by anaerobiosis. Photoinhibition of electron-transport rates under anaerobic conditions was characterized by a pronounced increase in the initial fluorescence level, F0, of chlorophyll-fluorescence induction, in contrast to photoinhibition under aerobic conditions. The results are discussed in terms of two mechanisms of photoinhibition, one that is suppressed and a second that is promoted by oxygen.

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