THE EFFECT OF TRANSTHYLAKOID PROTON UPTAKE ON CYTOSOLIC PH AND THE IMBALANCE OF ATP AND NAPDH H+ PRODUCTION AS MEASURED BY CO2-INDUCED AND LIGHT-INDUCED DEPOLARIZATION OF THE PLASMALEMMA/

The light-induced changes of plasmalemma potential and of chlorophyll fluorescence were compared with changes induced by the modulation of O 2- or CO2-concentration. The fast depolarisation of plasmalemma potent ial upon illumination as labeled by the time-constant tau4 (ca. 10 s) is used as a measure of the light-induced alkalinization of the cytoso l. Firstly, these experiments on leaves of tabaco, Agopodium podagrari a and spinach show that transenvelope fluxes of protons and not the up take of CO2 mediate the changes in cytosolic pH. Comparing the light-i nduced depolarisations of plasmalemma potential with chlorophyll lumin escence or fluorescence in Chara excluded alternative explanations lik e signal flow via putative thylakoid-chlorolemma connections and alkal inization by PGA-fluxes. Secondly, the occurrence of the pH changes du ring CO2 modulation demonstrates the imbalance of ATP and NADPH/H+ gen eration by the linear electron flow. Searching for mechanisms furnishi ng additional ATP was done by measuring heat evolution and O2-uptake b y means of photoacoustic pulse experiments. They show that the Mehler reaction is active under steady-state conditions. It is stimulated whe n the transthylakoid pH-gradient increases. Further, the time course o f heat evolution gave additional evidence that the time-constant tau4 is related to transthylakoid proton fluxes and alkalinization in the c ytosol.