Kinetics of reactions around the cytochrome bf complex studied in intact leaf disks

Electron transfers in the photosynthetic electron transport chain including the cytochrome (cyt) bf and Photosystem ( PS) I complexes were studied in leaves of several plant species by measuring flash-induced absorbancy changes at specific wavelengths. The electrochromic signal (ECS), indicative of a trans-thylakoid membrane electric field, consisted of a fast phase arising from charge separation in both photosystems, and a slow rise usually interpreted as charge transfer in the cyt bf complex ( part of the Q-cycle). The amplitude of the slow phase of the ECS was frequently greater than could be accounted for by the withdrawal of an electron from cyt bf via plastocyanin ( PC) by oxidised P700 in PS I. The 'extra' slow ECS, variable depending on the number of turnovers and plant species, can be attributed to a variable operation of proton-pumping activity of the cyt bf complex. The redox kinetics of cyt f and b were obtained by deconvolution of the signals at three or four wavelengths. Rates of cyt b reduction were very high, and never the same as the onset kinetics of the slow ECS. The cyt f signal suggests that a fraction of the oxidised cyt f was re-reduced only slowly in the time of 5 s between consecutive flashes. Leaf discs in far-red light were given single-turnover flashes to measure the rates of P700(ox) reduction and reoxidation. To simulate the redox kinetics of the ECS, cyt f, cyt b and P700 it was assumed that a Q-cycle normally operated in bf complexes; reasonable values for the appropriate rate coefficients, and for the equilibrium constants for the cyt f/PC and P700/PC reactions were chosen. Close similarity of the observed data with those predicted from the simulation was obtained for cyt b, P700 ( far-red light experiments) and the ECS, but not for cyt f. The results contribute to an understanding of photosynthetic electron transfers in vivo.