The regulatory chain of cyclic electron transport in plant photosystem II

The adaptation of photosystem II to a sharp drop in illumination was studied on Chlorella cells. The state of photosystem II was determined from the kinetics of photosynthetic evolution of oxygen in response to a series of flashes delivered at the end of illumination. Upon weak illumination, the rate of oxygen release shaprly decreased by the action of dark processes; however, after a minute, the rate of dark processes decreased 500-fold and the activity of photosystem II began to rise. Such a behavior was explained by the existence of a regulated cyclic electron transport around photosystem II. Its intensity is proportional to the degree of reduction of the intersystem carrier pool, and steady-state value upon weak illumination is proportional to the rate of light conversions in photosystem II. The oxygen quantum yield in photosystem II reflects the share of its noncyclic flow. The assumption is made that the cyclic transport of two photosystems stabilizes the redox potential of the plastoquinone pool, providing the maximum efficiency of proton transport. It was proposed that plastocyanine acts as a cyclic carrier in photosystem II.