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 P700ox 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.