The Effect of Transthylakoid Proton Uptake on Cytosolic pH and the Imbalance of ATP and NAPDH/H^+ Production as Measured by CO_2- and Light -Induced Depolarisation of the Plasmalemma

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The light-induced changes of plasmalemma potential and of chlorophyll fluorescence were compared with changes induced by the modulation of O_2- or CO_2-concentration. The fast depolarisation of plasmalemma potential upon illumination as labeled by the time-constant τ_4 (ca. 10 s) is used as a measure of the light-induced alkalinization of the cytosol. Firstly, these experiments on leaves of tabaco, Agopodium podagraria and spinach show that transenvelope fluxes of protons and not the uptake of CO_2 mediate the changes in cytosolic pH. Comparing the light-induced depolarisations of plasmalemma potential with chlorophyll luminescence or fluorescence in Chara excluded alternative explanations like signal flow via putative thylakoid-chloro-lemma connections and alkalinization by PGA-fluxes. Secondly, the occurrence of the pH changes during CO_2 modulation demonstrates the imbalance of ATP and NADPH/H^+ generation by the linear electron flow. Searching for mechanisms furnishing additional ATP was done by measuring heat evolution and O_2-uptake by means of photoacoustic pulse experiments. They show that the Mehler reaction is active under steady-state conditions. It,is stimulated when the transthylakoid pH-gradient increases. Further, the time course of heat evolution gave additional evidence that the time-constant τ_4 is related to transthylakoid proton fluxes and alkalinization in the cytosol.
日本植物生理学会の論文