pH regulation in acid-stressed leaves of pea plants grown in the presence of nitrate or ammonium salts: Studies involving P-31-NMR spectroscopy and chlorophyll fluorescence

P-31 nuclear magnetic resonance spectroscopy was used to monitor changes of cytoplasmic and vacuolar pH values in leaf tissues from young pea plants which had been grown in hydroponic culture with either nitrate or ammonium salts as sources of nitrogen. When acid stress was applied by the addition of 15% CO2 to air (5.1 mM CO2 in solution), cytoplasmic pH values decreased fast by 0.5 pH units and then increased slowly without reaching the initial pH, while vacuolar pH values decreased by 0.1 pH units. Under anaerobic conditions, the cytoplasmic pH decreased by one pH unit and the vacuolar pH increased by almost 0.4 pH units. These changes were rapidly reversed when CO2 was removed from air or, after anaerobiosis, by aeration. However, with mannose present during and after anaerobiosis, aeration failed to bring pH values back to the levels observed before anaerobiosis, Simultaneously, mannose phosphates accumulated and cytoplasmic phosphate disappeared. Since loss of phosphate decreases ATP levels, the observations suggest that ATP-dependent pumping of protons into the vacuole restored the cytoplasmic pH partially during acidification by CO2 and fully after anaerobiosis. Photosynthesis was initially inhibited by high CO2 and then restored indicating that protons are exported not only across the tonoplast into the vacuole but also across the chloroplast envelope into the cytosol. No large differences in pH regulation were observed in leaves of pea plants which were grown with either nitrate or ammonium salts, Apparently, retarded growth of ammonium-fertilized plants cannot be attributed to ineffective pH regulation.