1993. Implications of N acquisition from atmospheric NH, for acid-base and cation-anion balance of Lolium perenne In the atmosphere, ammonia (NH,) is the third most abundant N species which, due to various natural and anthropogenic sources, can locally reach high concentrations. The acquisition of atmospheric NH, by plant shoots will lead to two opposing effects on acid-base balance. Absorption and dissolution of NH, will cause an alkalinisation, while the assimilation of NH, results in an acidification. Different rates of these processes would lead to an acid-base imbalance with consequences for the ionic balance of the plant. As there is only a limited capacity for biochemical disposal of excess H+ in shoots, pH regulation may involve a pattern of (in)organic ion flow between shoots and roots followed by HVOH" extrusion into the media via roots. The acquisition of NH, as additional N source should lead to a reduction in the ratio of mol HVOH" gained per mol N assimilated.We have recently investigated the NH, acquisition by Lolium perenne L. cv. Centurion and studied the effects of gas phase NH3 on growth, acid-base balance and water-use efflciency. The experiments, therefore, included the application of a range of '^NH, to the shoots and of "N as NO3, NHJ or NH4NO, to the roots. After a summary of the main conclusions from those experiments, we discuss the implications of the use of atmospheric NHi for the mineral composition of the plants.Over the range of NH, supplied, plants from all treatments could utilize gas-phase NH3. Plants receiving NO3 via their roots had a higher capacity to use gaseous NH, than those growing with NHt NH3 assimilation in shoots reduced both the acid load with NHJ nutrition and the alkaline load with NOi supply to the roots. The most signiflcant effect of fumigation on the ion balance was an increase in K+ within all treatments, and this effect was highest in the NHj-fed plants. The results of the experiments support predictions of a combination of neutralizing biochemical reactions as well as transport of organic anion salts between shoots and roots as possible acid-base regulation mechanisms of the whole plant.