NITRATE AND AMMONIUM ASSIMILATION BY ROOTS OF MAIZE (ZEA-MAYS L) SEEDLINGS AS INVESTIGATED BY INVIVO N-15-NMR

N-15-Nuclear Magnetic Resonance (NMR) was used to study nitrogen assimilation in apices of maize roots in vivo, perfused either with (NO3-)-N-15 or (NH4+)-N-15 for long periods (17 to 20 h). Tissue saps prepared from roots grown in solutions supplied with (NH4+)-N-15 were also analysed by N-15-NMR. Perfusion experiments showed that the N-15 label from nitrate or ammonium is first incorporated into the amide group of glutamine and then into the alpha-amino groups, mainly of glutamate/glutamine and alanine. Resonances assigned to alpha-amino groups of valine and asparagine/aspartate, to the side chain group of asparagine and to gamma-aminobutyric acid could also be detected in vivo. Higher rates of label incorporation were observed when (NH4+)-N-15 was the nitrogen source, Furthermore, intracellular accumulation of (NH4+)-N-15 was observed. The results presented here suggest that the glutamine synthetase/glutamate synthase cycle is operating in this system under the experimental conditions used. Spectra obtained from saps of roots grown in the presence of (NH4+)-N-15 showed an increase of ammonium along the root during elongation and revealed the presence of asparagine together with glutamine in the elongation zone. By comparing spectra recorded in vivo with those obtained with the sap of equivalent root material. the absence of signals due to amino acids derived from glutamate (alanine, gamma-aminobutyric acid) is clear in the latter case.