Physiological responses of plant leaves to atmospheric ammonia and ammonium

Misting of leaves of several plant species with 3 mM aqueous NH+4 at pH 5, or fumigation with 3000 μg m-3 gaseous NH3 for 1 h, elicits similar biochemical and physiological changes in the species tested. The enzyme glutamine synthetase (GS) was shown to increase its activity in all species, while that of nitrate reductase (NR) was inhibited, at least in those species which possessed the ability to induce foliar NR. At the same time there were marked changes in organic anion concentrations, with malate and citrate in particular being reduced in concentration, following either NH+4 or NH3 application to leaves. The changes in organic anions are also discussed in the light of pH regulation by the cell. A stimulation of photosynthesis was also evident when leaves were treated with either NH3 or NH+4. It is argued that, because of the differences in solution chemistry of the two ammonia forms, the aqueous form applied at pH 5 and the gaseous form being an alkali in solution, these changes can only have occurred through the ability of the leaves to readily assimilate both forms of the ammonia. The biochemical changes might have potential as markers for the onset of physiological perturbation by atmospheric ammonia pollution, particularly changes in organic acid concentration; their use in an index of pollution stress is briefly discussed.