COUPLED CYCLIC ELECTRON-TRANSPORT IN INTACT CHLOROPLASTS AND LEAVES OF C3 PLANTS - DOES IT EXIST - IF SO, WHAT IS ITS FUNCTION

Transthylakoid proton transport based on Photosystem I-dependent cycli c electron transport has been demonstrated in isolated intact spinach chloroplasts already at very low photon flux densities when the accept or side of Photosystem I (PS I) was largely closed. It was under stric t redox control. In spinach leaves, high intensity flashes given every 50 s on top of far-red, but not on top of red background light decrea sed the activity of Photosystem II (PS II) in the absence of appreciab le linear electron transport even when excitation of PS II by the back ground light was extremely weak. Downregulation of PS II was a consequ ence of cyclic electron transport as shown by differences in the redox state of P700 in the absence and the presence of CO2 which drained el ectrons from the cyclic pathway eliminating control of PS II. In the p resence of CO2, cyclic electron transport comes into play only at high er photon flux densities. At H+/e = 3 in linear electron transport, it does not appear to contribute much ATP for carbon reduction in C3 pla nts. Rather, its function is to control the activity of PS II. Control is necessary to prevent excessive reduction of the electron transport chain. This helps to protect the photosynthetic apparatus of leaves a gainst photoinactivation under light stress.