Uptake and utilization of atmospheric ammonia in three native Poaceae species: leaf conductances, composition of apoplastic solution and interactionswith root nitrogen supply

The physiological potential for acquisition of atmospheric ammonia (NH3) wa s investigated in three European meadow grasses (Arrhenatherum elatius, Bro mus erectus and Brachypodium pinnatum) competing in chalk grasslands. Exper iments were carried out with plants cultivated for about three months on a soil-sand mixture at high root nitrogen supply or in nutrient solutions at low root nitrogen supply. Two different root nitrogen regimes were applied to the solution-grown plants: 130 mu mol NO3 plant(-1) wk(-1) (approx. 50 k g nitrogen ha(-1) in three months); or 130 mu mol NO3- plus 130 mu mol NH4 plant(-1) wk(-1). Each regime was combined with two levels of NH, fumigati on (0 and 70 nmol mol(-1) air for 24 d). Uptake of gaseous NH3 in the shoot s was investigated under controlled environmental conditions including NH3 concentrations ranging from 0 to 30 nmol mol-l air. Concurrently, photosynt hesis, glutamine synthetase activity, nitrogen allocation, biomass allocati on and apoplastic cation composition were measured. For A. elatius, the inf luence of photorespiration on NH3 acquisition was also assessed. Independen tly of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were <0.5 nmol mol-l. The total leaf conductance to NH3 absorption remained constant at increasing NH3 concentrations, showing that the capacity for assimilation was unaltered. Whereas internal factors in t he leaves did not cause differences in the potential for NH3 acquisition be tween the species, other factors of NH3 acquisition were quite different: B . erectus had higher stomatal conductance and, thus, higher NH3 uptake rate s per unit leaf area compared to A. elatius and B. pinnatum; higher stomata l conductances of B. erectus were to a large extent offset by a lower leaf area per plant, resulting from a lower growth rate and thicker leaves than in the two other species. The rate of photorespiration in Arrhenatherum con stituted at least 15% of the net photosynthetic rate. Surprisingly, suppres sion of photorespiration indicated that NH3 uptake was supported by photore spiration. Bromus responded to fumigation with 70 nmol NH3 mol(-1) air for 24 d by lowering the root:shoot ratio and increasing the nitrogen concentra tion in the stem dry matter. The total leaf conductance to NH3 uptake decre ased in all three species upon exposure to NH3, while the stomatal conducta nce was unaffected. The NH, exposure caused lower apoplastic concentrations of H+, Mg2+ and Ca2+ in A. elatius and B. erectus.