Effect of high light intensities on oxygen evolution and the light activation of NADP-malate dehydrogenase in intact...

The factors limiting the photosynthetic carbon metabolism of intact spinach (Spinacia oleracea L.) chloroplasts after a high-light pretreatment have been studied. Photosynthetic CO2 fixation was decreased and became more sensitive to the inhibitory effect of the cyclic-electron-flow inhibitor, antimycin A. Depending on the extent of photoinhibition, changing the balance of linear to cyclic electron flow by adding oxaloacetate and antimycin A either did not relieve, or partially relieved the photoinhibitory effect. The decrease in CO2 fixation appeared to be the consequence of either a limitation by photosystem-II activity (in the case of moderate inhibition) or, at least partially an unfavourable balance between the linear and cyclic electron flows (in the case of strong inhibition). The light activation of NADP-malate dehydrogenase (EC 1.1.1.82) was decreased only in the presence of CO2, i.e. when there was strong competition for reducing power; otherwise, it was unaffected by photoinhibitory treatments, in accordance with its low energy requirement.