Regulation of the photosynthetic electron transport chain

The regulation of electron transport between photosystems II and I was inve stigated in the plant Silene dioica L. by means of measurement of the kinet ics of reduction of P-700 following a light-to-dark transition. It was foun d that, in this species, the rate constant for P-700 reduction is sensitive to light intensity and to the availability of CO2. The results indicated t hat at 25 degrees C the rate of electron transport is down-regulated by app roximately 40-50% relative to the maximum rate achievable in saturating CO2 and that this downregulation can be explained by regulation of the electro n transport chain itself. Measurements of the temperature sensitivity of th is rate constant indicated that there is a switch in the rate-limiting step that controls electron transport at around 20 degrees C: at higher tempera tures, CO2 availability is limiting, at lower temperatures some other proce ss regulates electron transport, possibly a diffusion step within the elect ron transport chain itself. Regulation of electron transport also occurred in response to drought stress and sucrose feeding. Measurements of non-phot ochemical quenching of chlorophyll fluorescence did not support the idea th at electron transport is regulated by the pH gradient across the thylakoid membrane, and the possibility is discussed that the redox potential of a st romal component may regulate electron transport.