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

Abstract

The physiological potential for acquisition of atmospheric ammonia (NH₃) was investigated in three European meadow grasses (Arrhenatherum elatius, Bromus erectus and Brachypodium pinnatum) competing in chalk grasslands. Experiments 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 μmol NO₃ plant⁻¹ wk⁻¹ (approx. 50 kg nitrogen ha⁻¹ in three months); or 130 μmol NO₃⁻ plus 130 μmol NH₄⁺ plant⁻¹ wk⁻¹. Each regime was combined with two levels of NH₃ fumigation (0 and 70 nmol mol⁻¹ air for 24 d). Uptake of gaseous NH₃ in the shoots was investigated under controlled environmental conditions including NH₃ concentrations ranging from 0 to 30 nmol mol⁻¹ air. Concurrently, photosynthesis, glutamine synthetase activity, nitrogen allocation, biomass allocation and apoplastic cation composition were measured. For A. elatius, the influence of photorespiration on NH₃ acquisition was also assessed. Independently of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were <0.5 nmol mol⁻¹. 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 the leaves did not cause differences in the potential for NH₃ acquisition between the species, other factors of NH₃ acquisition were quite different: B. erectus had higher stomatal conductance and, thus, higher NH₃ uptake rates per unit leaf area compared to A. elatius and B. pinnatum; higher stomatal 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 constituted at least 15% of the net photosynthetic rate. Surprisingly, suppression of photorespiration indicated that NH₃ uptake was supported by photorespiration. Bromus responded to fumigation with 70 nmol NH₃ mol⁻¹ air for 24 d by lowering the root[ratio ]shoot ratio and increasing the nitrogen concentration in the stem dry matter. The total leaf conductance to NH₃ uptake decreased in all three species upon exposure to NH₃, while the stomatal conductance was unaffected. The NH₃ exposure caused lower apoplastic concentrations of H⁺, Mg²⁺ and Ca²⁺ in A. elatius and B. erectus.