Ammonia exchange between plants and the atmosphere : Effects of ammonium supply to the roots, dark-induced senescence and reduced GS activity

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Ammonia exchange with the atmosphere was studied in barley (Hordeurn vulgate L. cv. Golf) grown in nutrient solution. Ammonia emission from the leaves was evident when NH^<4+> was taken up by the roots or when the plants had been subjected to darkness for 3 to 7 days. Also NH^<4+> concentrations in shoot and root tissues increased with these treatments while the activity of the ammonium assimilating enzyme glutamine synthetase (GS) increased in the roots with increasing NH^<4+> concentrations supplied to the medium and decreased in both shoot and root after 3 days of dark-induced senescence. Barley mutant plants (cv. Maris Mink) with only 66 or 47% of normal GS activity showed higher tissue NH^<4+> concentrations, higher NH_3 emission and a greater sensitivity to increased temperature than wild type barley plants. The 66% GS mutant always showed higher NH_3 emission compared to plants with the lowest GS activity (47%), probably due to a mechanism preventing tissue NH^<4+> concentrations from increasing too much. Apoplastic NH^<4+> and pH also increased in the GS mutants and estimated compensation points for NH_3 were higher compared to wild type plants.