KINETICS OF (NH4+)-N-15 ASSIMILATION IN TOMATO PLANTS - EVIDENCE FOR (NH4+)-N-15 ASSIMILATION VIA GDH IN TOMATO ROOTS

The kinetics of (NH4+)-N-15 assimilation into free amino acids and total reduced nitrogen were monitored in both roots and shoots of two week old tomato seedlings supplied with 5 mM 99% ((NH4)-N-15)2SO4 via the aerated root medium in hydroponic culture, in the presence and absence of a 2 h pre-incubation with 1 mM methionine sulfoximine (MSX). The labeling kinetics of amino acids in roots of tomato plants in the presence of MSX show that continued assimilation of (NH4+)-N-15 can occur when the GS/GOGAT cycle is inhibited. In the presence of MSX, three amino acids [glutamate, alanine, and y-amino butyrate (GABA)] of the root tissue continue to become labeled with N-15 under conditions where labeling of the amino-N moiety of glutamine is completely inhibited. This indicates primary ammonia assimilation via GDH, or alternatively, assimilation of ammonia into alanine via alanine dehydrogenase. Free ammonia accumulates rapidly in both shoots and roots of tomato in response to MSX. The labeled ammonia accumulated in the roots in the presence of MSX must be largely derived from the medium whereas in shoots this ammonia appears to be derived from catabolism of unlabeled amino acids and proteins. The pools of glutamine, glutamate and alanine after 24 h exposure to (NH4+)-N-15 were, on the average, 5- to 10-fold lower in the MSX-treated than in the control (-MSX) shoots and roots. In contrast, the pools of valine, leucine, isoleucine, proline, threonine, phenylalanine, lysine, and tyrosine increased 5- to 10-fold above the control values in the shoots of MSX-treated plants, and 2- to 4-fold above control values in the roots of MSX-treated tomato plants after 24 h. The latter amino acids all exhibited low isotope abundance, and presumably were derived from protein turnover.