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

A noninvasive technique is introduced with which relative proton to electro n 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 o f 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 cyt ochrome (cyt) b(6)f complex were estimated by measuring the DIRK signals as sociated with reduction of cyt f and P-700 The rates of proton pumping thro ugh the electron transfer chain and the CF0-CF1 ATP synthase (ATPase) were estimated by measuring the DIRK signals associated with the electrochromic shifting of pigments in the light-harvesting complexes, Electron transfer f luxes were also estimated by analysis of saturation pulse-induced changes i n chlorophyll a fluorescence yield. It was shown that the H+/e(-) ratio, wi th respect to both cyt b(6)f complex and photosystem (PS) II turnover, was constant under low to saturating illumination in intact tobacco leaves. Bec ause a H+/e(-) ratio of 3 at a low light is generally accepted, we infer th at this ratio is maintained under conditions of normal (unstressed) photosy nthesis, implying a continuously engaged, proton-pumping Q cycle at the cyt b(6)f complex.