N-15-NITRATE ASSIMILATION IN ASSOCIATION WITH GLUTAMINE SYNTHESIS IN RICE SEEDLING ROOTS

When rice seedling roots were fed with 15N-nitrate at the steady state of nitrate assimilation, asparagine amido showed the highest 15N abundance, followed by glutamine amido, glutamic acid, aspartic acid and glutamine amino. Ammonia in the roots gave lower 15N abundance than those of main amino acids. In the roots fed with 15N-nitrate after inhibition of their glutamine synthetase by I-methionine-dl-sulfoximine (MS), ammonia showed much higher 15N abundance than that in normal roots. In contrast, all the amino acid nitrogen examined including asparagine amido-N showed remarkably lower 15N abundance in the roots treated with MS. After the treatment with MS, the contents of free nitrogen compounds in roots were changed. Glutamine, asparagine, alanine and glycine continuously decreased respectively. Vigorous and continuous increase was detected in ammonia while, glutamic and aspartic acids, threonine and methionine firstly increased and then, decreased. From these results, the limited locality of nitrate reduction site, action of GS-GOGAT system and a certain active role of asparagine were estimated in the roots fed with nitrate. Comparison of ammonium and nitrate assimilation suggested the different nitrogen flow in roots or root cells according to the form of supplied nitrogen compound. © 1979 Taylor & Francis Group, LLC.