title: Leaf-atmosphere NH(3) exchange of white clover (Trifolium repens L.) in relation to mineral N nutrition and symbiotic N(2) fixation.
abstract: Plant-atmosphere NH(3) exchange was studied in white clover (Trifolium repens L. cv. Seminole) growing in nutrient solution containing 0 (N(2) based), 0.5 (low N) or 4.5 (high N) mM NO(3)(-). The aim was to show whether the NH(3) exchange potential is influenced by the proportion of N(2) fixation relative to NO(3)(-) supply. During the treatment, inhibition of N(2) fixation by NO(3)(-) was followed by in situ determination of total nitrogenase activity (TNA), and stomatal NH(3) compensation points (chi(NH(3))) were calculated on the basis of apoplastic NH4(+) concentration ([NH4(+)]) and pH. Whole-plant NH(3) exchange, transpiration and net CO(2) 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 chi(NH(3)) were equal in the three treatments. In NH(3)-free air, net NH(3) emission rates of <1 nmol m(-2) s(-1) were observed in both high-N and N(2)-based plants. When plants were supplied with air containing 40 nmol mol(-1) NH(3), the resulting net NH(3) uptake was higher in plants which acquired N exclusively from symbiotic N(2) fixation, compared to NO(3)(-) grown plants. The results indicate that symbiotic N(2) fixation and mineral N acquisition in white clover are balanced with respect to the NH4(+) pool leading to equal chi(NH(3)) in plants growing with or without NO(3)(-). At atmospheric NH(3) concentrations exceeding chi(NH(3)), the NH(3) uptake rate is controlled by the N demand of the plants.
authors.affiliation: Swiss Federal Research Station for Agroecology and Agriculture, Zuerich-Reckenholz and Liebefeld-Bern, Schwarzenburgstrasse 155, Liebefeld, CH-3003 Bern, Switzerland.
authors.forename: Beat
authors.initials: B
authors.lastname: Herrmann
authors.forename: Marie
authors.initials: M
authors.lastname: Mattsson
authors.forename: Jürg
authors.initials: J
authors.lastname: Fuhrer
authors.forename: Jan K
authors.initials: JK
authors.lastname: Schjoerring
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D002473:Cell Wall / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D009566:Nitrates / Q000494:pharmacology; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation / Q000502:physiology*; D009591:Nitrogenase / Q000187:drug effects / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D013559:Symbiosis; D029921:Trifolium / Q000378:metabolism*
chemical_list: D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D000641:Ammonia; D009591:Nitrogenase; D009584:Nitrogen