Changes in growth and nitrogen assimilation in barley seedlings under cadmium stress

Barley (Hordeum vulgare L. cv. Martin) plants grown in solution culture, were exposed to increasing cadmium (Cd) concentration (0, 5, 10, 25, 50, and 100 mu M) for a duration of 12 days. The sequence of important biochemical steps of nitrate (NO3) assimilation were studied in roots and shoots as a function of external Cd concentration. Cadmium uptake in roots and shoots increased gradually with Cd concentration in the medium. This Cd accumulation lowered substantially root and shoot biomass. The nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.6.6.4) activities declined under Cd stress. Concurrently, tissue NO3 contents and xylem sap NO3 concentration were also decreased in Cd-treated plants. These results suggest that Cd could exert an inhibitory effect on the assimilatory NO3 reducing system (NR and NiR) through a restriction of NO3 availability in the tissues. We therefore examined, in short-term experiments (12 h), the impact of Cd on NO3 uptake and the two reductases in nitrogen (N)-starved plants that were pretreated or not with Cd. It was found that Cd induced inhibition of both NO3 uptake and activities of NR and NiR, during NO3 induction period. The possible mechanisms of Cd action on NO3 uptake are proposed. Further, in Cd-grown plants, the glutamine synthetase (GS, EC 6.3.1.2) showed a decreasing activity both in shoots and roots. However, increasing external Cd concentration resulted in a marked enhancement of glutamate dehydrogenase (NADH-GDH, EC 1.4. 1.2) activity, coupled with elevated levels of ammonium (NH,) in tissues. On the other hand, the total protein content in Cd-treated plants declined with a progressive and substantial increase of protease activity in the tissues. These findings indicate that under Cd stress the usual pathway of NH, assimilation (glutamine synthetase/glutamate synthase) can switch to an alternative one (glutamate dehydrogenase). The changes in all parameters investigated were concentration-dependent and more marked in roots than shoots. The regulation of N absorption and assimilation by Cd in relation to growth and adaptation to stress conditions are discussed.