Ammonia emission from young barley plants: Influence of N source, light/dark cycles and inhibition of glutamine synthetase

Barley (Hordeum vulgare L, cv. Golf) plants were grown at two different relative addition rates; 0.1 and 0.2 d(-1) of nitrate. Three to five days before measurements started the plants were transferred to a nutrient solution with 2 mM nitrate or ammonium. The ammonium-grown plants showed increased ammonium levels in both shoots and roots and also increased ammonium concentrations in xylem sap. Ammonia emission measured in cuvettes connected to an automatic NH3 monitor was close to zero for nitrate-grown plants but increased to 0.59 and 0.88 nmol NH3 m(-2) s(-1) for plants transferred to ammonium after growing at RA = 0.2 and 0.1 d(-1), respectively, In darkness, NH3 emission decreased together with photosynthesis and transpiration, but increased rapidly when the light was turned on again, Addition of 0.5 mM methionine sulphoximine (MSG) to the plants caused an almost complete inhibition of both root and shoot glutamine synthetase (GS) activity after 24 h. Ammonia emission increased dramatically and photosynthesis and transpiration decreased in both nitrate- and ammonium-grown plants as a result of the GS inhibition. At the same time plant tissue and xylem sap ammonium concentrations increased, indicating the importance of GS in controlling plant ammonium levels and thereby NH3 emission from the leaves.