EVIDENCE OF INCREASED N-15-AMMONIUM ASSIMILATION IN TOMATO PLANTS WITH EXOGENOUS ALPHA-KETOGLUTARATE

The effect of exogenous alpha-ketoglutarate (alpha-KG) on ammonium (NH4+) assimilation into free amino acids and total reduced nitrogen was monitored in both roots and shoots of 2-week-old tomato (Lycopersicon esculentum Mill.) seedlings supplied with 5 mM 99% ((NH4)2SO4)-N-15 in hydroponic culture. Amino acid pools increased approximately 2-fold and N-15 concentrations averaged 1.5- and 2-fold higher in roots and shoots, respectively, in alpha-KG-treated plants compared with the control. Free NH4+ in tissues decreased sharply with alpha-KG and was almost 90% lower in shoots of treated plants at the same time that the total N-15 assimilation increased 2-fold. This increase of (NH4+)-N-15 assimilation due to exogenous alpha-KG clearly indicates that the availability of carbon skeletons is a key limiting factor for NH4+ assimilation in tomato plants.