The roles of glutamate dehydrogenase, glutamine synthetase and three forms of glutamate synthase on nitrogen assimilation in various organs of Pisum arvense L.

Abstract

The activities of GDH, GS and three forms of GOGAT (NADH, NADPH or ferredoxin-dependent) were studied in the leaves, stems and roots of the Pisum arvense. GS and the individual forms of GOGAT dominated in the leaves of 7 day-old plants which were taking up NO₃^- or NH₄⁺ ions, while NADH-GDH dominated in the roots of these plants. In comparison with HNO₃^- , NH₄⁺ ions stimulated the activity of most of the enzymes of the GS/GOGAT and GDH pathways in stems and roots, while in and leaves this effect was age-dependent. The Fd-GOGAT located in leaves and stems was not regulated by NH₄⁺ , which indicates that this enzyme is not likely to be directly involved in the assimilation of NH₄⁺ ions that have been taken up. The obtained data indicate that at lower tissue NH₄⁺ concentration a decisive role in nitrogen assimilation in leaves and stems is played by the GS/GOGAT pathway, while in the roots-by GDH and in less degree by GS, GOGAT. High amounts of accumulated NH₄⁺ ions set off a detoxication mechanism which includes NADH-GDH, common to all tissues. Only in 7 day-old leaves did the detoxication of NH₄⁺ take place with the involvement of NADH-GOGAT and NADPH-GOGAT.