Leaf-atmosphere NH3 exchange of white clover (Trifolium repens L.) in relation to mineral N nutrition and symbiotic N2 fixation

Abstract

Plant-atmosphere NH3 exchange was studied in white clover (Trifolium repens L. cv. Seminole) growing in nutrient solution containing 0 (N2 based), 0.5 (low N) or 4.5 (high N) mM NO3-. The aim was to show whether the NH3 exchange potential is influenced by the proportion of N2 fixation relative to NO3- supply. During the treatment, inhibition ofN2 fixation by NO3- was followed by in situ determination of total nitrogenase activity (TNA), and stomatal NH3 compensation points (χNH3) were calculated on the basis of apoplastic NH4+ concentration ([NH4+]) and pH. Whole-plant NH3 exchange, transpiration and net CO2 exchange were continuously recorded with a controlled cuvette system. Although shoot total N concentration increased with the level of mineral N application, tissue and apoplastic [NH4+] as well as χNH3 were equal in the three treatments. In NH3-free air, net NH3 emission rates of < 1 nmol m-2 s-1 were observed in both high-N and N2-based plants. When plants were supplied with air containing 40 nmol mol-1 NH3, the resulting net NH3 uptake was higher in plants which acquired N exclusively from symbiotic N2 fixation, compared to NO3- grown plants. The results indicate that symbiotic N2 fixation and mineral N acquisition in white clover are balanced with respect to the NH4+ pool leading to equal χNH3 in plants growing with or without NO3- At atmospheric NH3 concentrations exceeding χNH3, the NH3 uptake rate is controlled by the N demand of the plants.