Effects of nitrate concentration on growth and nitrogen metabolism of tomato (Lycopersicon esculentum Mill.) under cadmic stress

Nitrate assimilation in higher plants is the main biosynthetic pathway leading to glutamate, required for synthesis of some metabolites that participate in mechanisms of biochemical adaptation to heavy metal such as cadmium. Seven day-old seedlings were grown for 14 days on hydroponic culture medium containing 2 mM or 5 mM of nitrate. Cadmium was added at 25 mu M concentration. The supply of nitrate at an important concentration (5 mM) enhanced growth activity and nitrate content in leaves and roots. In parallel, nitrate and nitrite reductase activities were induced. Regardless the nitrate concentration, presence of cadmium (225 mu M) exerted an inhibitory effect on plant growth, nitrate content, nitrate and nitrite reductase activities. This negative effect was more important when nitrate was added at 2 mM concentration. Aminating GDH activity was enhanced by the presence of cadmium in order to detoxify ammonium. Our results suggest that tomato plant was more sensitive to cadmium toxicity when received a low nitrogen concentration (nitrate).