Evidence of increased 15N-ammonium assimilation in tomato plants with exogenous α-ketoglutarate

The effect of exogenous α-ketoglutarate (αKG) 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% (15NH4)2SO4 in hydroponic culture. Amino acid pools increased approximately 2-fold and 15N concentrations averaged 1.5- and 2-fold higher in roots and shoots, respectively, in αKG-treated plants compared with the control. Free NH4+ in tissues decreased sharply with αKG and was almost 90% lower in shoots of treated plants at the same time that the total 15N assimilation increased 2-fold. This increase of 15NH4+ assimilation due to exogenous αKG clearly indicates that the availability of carbon skeletons is a key limiting factor for NH4+ assimilation in tomato plants.