Photoinhibition of photosynthesis under anaerobic conditions studied with leaves and chloroplasts of Spinacia oleracea L.

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Abstract
The role of oxygen in the photoinactivation of the photosynthetic apparatus of Spinacia oleracea L. was investigated. Moderate irradiation (1200 μmol photons m-2s-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-2s-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.