Ammonium and soluble amide-bound nitrogen in leaves of Brassica napus as related to glutamine synthetase activity and external N supply

Short and long term effects of variations in external N supply on the content of soluble N compounds and glutamine synthetase (GS) activity were investigated in leaves of vegetatively growing oilseed rape (Brassica napus L., cv. Global). Only the chloroplastic isoform of GS had a detectable activity. Plants growing continuously for 5 weeks at elevated nitrogen levels had 3-fold higher levels of NH4+ and soluble amides compared with plants growing at low levels of N supply. In contrast, GS activities were similar at the different levels of N supply. Addition of a pulse of nitrogen to plants of medium N status caused a rapid and large increase in NH4+ and soluble amides within the leaves. Pulse addition of nitrogen also triggered a temporary increase in GS activity, thereby maintaining the concentration of NH4+ below 14 μmol NH4+·g−1 leaf fresh weight (corresponding to 21 mM NH4+ ammonium on tissue water basis). This level of NH4+ was equal to the maximum level observed in leaves of plants growing permanently at elevated N supplies in spite of the fact that the latter plants received a higher total amount of nitrogen. It is concluded that tissue concentrations of NH4+ and soluble amides in B. napus may vary considerably with external N supply, although accumulation of exceedingly high NH4+ concentrations can be avoided by stimulation of GS activity.