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

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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.