The proton to electron stoichiometry of steady-state photosynthesis in living plants: A proton-pumping Q cycle is continuously engaged

Abstract

A noninvasive technique is introduced with which relative proton to electron stoichiometries (H + /e − ratios) for photosynthetic electron transfer can be obtained from leaves of living plants under steady-state illumination. Both electron and proton transfer fluxes were estimated by a modification of our previously reported dark-interval relaxation kinetics (DIRK) analysis, in which processes that occur upon rapid shuttering of the actinic light are analyzed. Rates of turnover of linear electron transfer through the cytochrome (cyt) b 6 f complex were estimated by measuring the DIRK signals associated with reduction of cyt f and P 700 . The rates of proton pumping through the electron transfer chain and the CF O -CF 1 ATP synthase (ATPase) were estimated by measuring the DIRK signals associated with the electrochromic shifting of pigments in the light-harvesting complexes. Electron transfer fluxes were also estimated by analysis of saturation pulse-induced changes in chlorophyll a fluorescence yield. It was shown that the H + /e − ratio, with respect to both cyt b 6 f complex and photosystem (PS) II turnover, was constant under low to saturating illumination in intact tobacco leaves. Because a H + /e − ratio of 3 at a low light is generally accepted, we infer that this ratio is maintained under conditions of normal (unstressed) photosynthesis, implying a continuously engaged, proton-pumping Q cycle at the cyt b 6 f complex.