02222naa a2200133 a 450000100080000000500110000800800410001910000210006024501350008126000090021652017760022565000110200177300760201214885592018-05-30       1991    bl uuuu     u00u1 u    #d1 aMAGALHAES, J. R.  aKinetics of 15NH4+ assimilation in tomato plantsbevidence for 15NH4+ assimilation via GDH in tomato roots.h[electronic resource]  c1991  aThe kinetics of 15NH4+ 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% 15NH4)2S04 via lhe aerated root medium in hydroponic culture, in lhe 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 lhe presence of MSX show that continued assimilation of 15NH4+ can occur when lhe GS/GOGAT cycle is inhibited. ln lhe presence of MSX, three amino acids [glutamate. alanine, and y-amino butyrate (GABA)] of lhe root tissue continue to become labeled with 15N under conditions where labeling of the amino-N moiety of glutamine is completely inhibited. This indicates primary ammonia assimilation via GDH, or altematively, 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 lhe medium whereas in shoots this ammonia appears to be derived from catabolism of unlabeled arnino acids and proteins. The pools of glutamine, glutamate and alanine after 24 h exposure to 15NH4+ were, on lhe average. 5- to 10-fold lower in lhe MSX-treated than in the control (-MSX) shoots and roots. ln contrast, lhe pools of valine, leucine, isoleucine, proline, threonine, phenylalanine, lysine, and tyrosine increased 5- to 10-fold above lhe control values in lhe 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 alI exhibited low isotope abundance, and presumably were derived from protein turnover.  aTomate  tJournal of Plant Nutrition, New Yorkgv. 14, n. 12, p. 1341-1353, 1991.