The assimilation of ammonium by barley roots

Enzyme assays of the roots of barley (Hordeum vulgare L.) fed NH 4 (+) show high glutamate-dehydrogenase (GDH; EC 1.4.1.3) activity compared with glutamine-synthetase (GS; EC 6.3.1.2) activity, indicating that GDH may be involved in ammonium assimilation in the root. When (15)NH 4 (+) is fed to barley roots, a high accumulation of (15)N takes place in free amino compounds, particularly glutamine and glutamate. When the GS inhibitor, methionine sulfoximine (MSO), is added to the (15)NH 4 (+) feeding medium the free amino compounds remain unlabelled while (15)NH 4 (+) accumulates rapidly in the roots. Root enzyme assays demonstrate that GS is completely inhibited by MSO treatment, while the activity of GDH remains unaffected. The feeding of (15)N-amido glutamine to the roots in the presence of MSO and the subsequent (15)N enrichment of the free amino compounds of the root show that MSO does not interfere substantially with nitrogen assimilation after the formation of glutamine. These results indicate that in the barley root, ammonium absorbed from the soil is assimilated entirely via the GS-glutamate synthase (GOGAT) pathway, and that GDH plays little, if any, part in this process.