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 chang es of cytoplasmic and vacuolar pH values in leaf tissues from young pe a plants which had been grown in hydroponic culture with either nitrat e or ammonium salts as sources of nitrogen. When acid stress was appli ed by the addition of 15% CO2 to air (5.1 mM CO2 in solution), cytopla smic pH values decreased fast by 0.5 pH units and then increased slowl y without reaching the initial pH, while vacuolar pH values decreased by 0.1 pH units. Under anaerobic conditions, the cytoplasmic pH decrea sed by one pH unit and the vacuolar pH increased by almost 0.4 pH unit s. These changes were rapidly reversed when CO2 was removed from air o r, after anaerobiosis, by aeration. However, with mannose present duri ng and after anaerobiosis, aeration failed to bring pH values back to the levels observed before anaerobiosis, Simultaneously, mannose phosp hates accumulated and cytoplasmic phosphate disappeared. Since loss of phosphate decreases ATP levels, the observations suggest that ATP-dep endent pumping of protons into the vacuole restored the cytoplasmic pH partially during acidification by CO2 and fully after anaerobiosis. P hotosynthesis was initially inhibited by high CO2 and then restored in dicating 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, Apparen tly, retarded growth of ammonium-fertilized plants cannot be attribute d to ineffective pH regulation.