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

Abstract

Ammonia exchange with the atmosphere was studied in barley (Hordeum vulgare L. cv. Golf) grown in nutrient solution. Ammonia emission from the leaves was evident when NH4+ was taken up by the roots or when the plants had been subjected to darkness for 3 to 7 days. Also NH4+ 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 NH4+ 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 NH4+ concentrations, higher NH3 emission and a greater sensitivity to increased temperature than wild type barley plants. The 66% GS mutant always showed higher NH3 emission compared to plants with the lowest GS activity (47%), probably due to a mechanism preventing tissue NH4+ concentrations from increasing too much. Apoplastic NH4+ and pH also increased in the GS mutants and estimated compensation points for NH3 were higher compared to wild type plants.