Influence of nitrogen nutrition and metabolism on ammonia volatilization in plants

Barley (Hordeum vulgare L. cv. Golf) was grown in solution culture with controlled nitrogen availability in order to study the influence of nitrogen nutrition on ammonia emission from the leaves. Ammonia emission measured in cuvettes connected to an automatic NH3 monitor was close to zero for nitrate grown plants but increased to 0.9-1.3 nmol NH3 m(-2) leaf area s(-1) after 3-5 days of ammonium nutrition. Increasing concentrations from 0.5 to 10 mM NH4+ in the root medium increased NH3 emission from the shoots, root glutamine synthetase activity and NH4+ concentrations in apoplast, xylem sap and bulk tissue, while apoplastic pH values decreased. Inhibition of glutamine synthetase in nitrate grown barley plants by addition of 1 mM methionine sulfoximine (MSG) to the root medium caused ammonia emission to increase 5 to 10-fold after 2-3 hours. At the same time shoot tissue ammonium concentrations started to increase. Addition of an inhibitor of photorespiration, 1 mM pyrid-2-yl hydroxymethane sulfonate (HPMS) reduced this increase in ammonia emission showing a relation between NH3 emission and photorespiration. Oil seed rape (Brassica napus L. cv. Global) plants grown at 3 different nitogen levels (2N, 4N and 7N) in a sand/soil mixture showed increasing NH3 compensation points with increasing N level. This increase was highly correlated with increasing NH4+ concentrations in the leaf apoplast and total leaf tissue. The NH3 compensation points could be successfully predicted on basis of the pH and NH4+ concentration in the leaf apoplast.