THE EFFECT OF GLUTAMINE AND ASPARAGINE ON NET NH4+ UPTAKE IN YOUNG WHEAT PLANTS

Wheat plants grown during 10 days in the absence of N were pretreated with 1.0 eq m(-3) of methionine, asparagine or glutamine and/or 1.0 eq m(-3) MSX(4) or 0.17 eq m(-3) DON. Net NH4+ uptake was measured both in the presence or in the absence of the amino acid or enzyme inhibito r used in the pretreatment. The effect of met, asn and gln on net K+ u ptake was also studied using K+ -depleted plants. Changes in the conte nts of root free NH4+, asn, gln and the activities of GS, PEP-carboxyl ase, NAD(+)-GDH and NADH-GDH were determined. Net NH4+ uptake in gln a nd asn pretreated plants was markedly, and sometimes completely suppre ssed provided uptake was measured in the presence of the amides. On th e other hand, the met pretreated plants absorbed only 35% less NH4+ th an the control. When NH4+ uptake was measured in the absence of the am ino acids, only those plants pretreated with asn showed a marked suppr ession of net uptake during the first 120 min. None of the 3 amino aci ds tested significantly inhibited K+ uptake. Free NH4+ concentration i n roots of N-starved plants increased after 4 h incubation with gln, a sn or MSX in the absence of external NH4+. Nevertheless, no correlatio n was observed between root NH4+ concentration and the extent of net N H4+ uptake suppression. The inhibitory effect exerted by asn decreased when it was supplied together with MSX or DON. Pretreatments with gln or asn in the absence of external NH4+ significantly increased the le vel of asn in the roots, while that of gln remained unchanged. It is c oncluded that asn and gln specifically suppress net NH4+ uptake in whe at, although it is not clear wether they act only from the root exteri or, or through an endogenous pool exhibiting fast turn-over.