The Effect of Transthylakoid Proton Uptake on Cytosolic pH and the Imbalance of ATP and NAPDH/H+ Production as Measured by CO2- and Light-Induced Depolarisation of the Plasmalemma

The light-induced changes of plasmalemma potential and of chlorophyll fluorescence were compared with changes induced by the modulation of O2- or CO2-concentration. The fast depolarisation of plasmalemma potential upon illumination as labeled by the time-constant τ4 (ca. 10 s) is used as a measure of the light-induced alkalinization of the cytosol. Firstly, these experimens on leaves of tabaco, Agopodium podagraria and spinach show that transenvelope fluxes of protons and not the uptake of CO2 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-chlorolemma connections and alkalinization by PGA-fluxes. Secondly, the occurrence of the pH changes during CO2 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 O2-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 τ4 is related to transthylakoid proton fluxes and alkalinization in the cytosol.