CHARACTERISTICS OF AMMONIA EMISSION FROM BARLEY PLANTS

Barley (Hordeum vulgare L. cv. Golf) was grown in solution culture wit h controlled nitrogen availability in order to study ammonia emission from leaves. Ammonia emission measured in cuvettes connected to an aut omatic NH3 monitor was close to zero for nitrate grown plants but incr eased to 0.88 nmol m(-2) s(-1) after 3 days of ammonium (2 mM) supply. Ammonium in the root medium also increased tissue ammonium contents a nd xylem sap ammonium concentration. Inhibition of glutamine synthetas e (GS, EC 6.3.1.2.) by 0.5 mM methionine sulfoximine (MSG) caused afte r 2-3 h ammonia emission to increase dramatically (5-10 nmol NH3 m(-2) s(-1)) in both nitrate and ammonium grown plants. After a dark period the ammonia emission was further increased and had reached 30-50 nmol NH3 m(-2) s(-1) after 24 h. At the same time both tissue ammonium con tents and xylem sap ammonium concentrations increased while glutamine synthetase activities of both root and shoot decreased. Compensation p oints for ammonia (the air concentration at which no net in- or out-fl ux of ammonia occurs through stomata) were determined in chamber measu rements with air concentrations from 0 to 40 nmol m(-2) s(-1) of NH3. For two cultivars of barley (Golf and Laevigatum) the compensation poi nts decreased from ca. 5 nmol NH3 mol(-1) air during the vegetative gr owth stage to below 1 nmol NH3 mol(-1) air just after anthesis and the n increased again during senescence.