Ammonium accumulation and glutamine synthetase activity in C<SUB>3</SUB> and C<SUB>4</SUB> plants

Glutamine synthetase activity was found to be several fold higher in the leaves of a few C3 as compared to C4 plants. Differences in NH4+ levels were marginal. Methionine sulphoximine inhibition of glutamine synthetase resulted in increased in NH4+ levels more in wheat than in sorghum. The decrease in methionine sulphoximine induced accumulation of free NH4+ by photorespiratory inhibitors, isonicotinic acid hydrazide and &#945;-hydroxy methane pyridine sulphate was also several fold higher in wheat than in sorghum. Feeding of glycine and glycolate, intermediates of photorespiration, showed the presence of active glycine decarboxylation system in sorghum but the metabolism of glycolate was more efficient in wheat. Higher glutamine synthetase activity in C3 plants may be a necessary adaptation to a higher photorespiratory NH4+ production and explain the similar or higher N losses in C4 as compared to C3 plants. These results also support the hypothesis that glutamine synthetase is important in the assimilation of photorespiratory ammonia