Ammonium accumulation and glutamine synthetase activity in C₃ and C₄ plants

Abstract

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 α-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.