Nickel-induced depression of nitrogen assimilation in wheat roots

The influence of 50 and 100 μM Ni on the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine aminotransferase (AlaAT) and aspartate aminotransferase (AspAT) was studied in the wheat roots. Root fresh weight, tissue Ni, nitrate, ammonium, glutamate and protein concentrations were also determined. Exposure to Ni resulted in a marked reduction in fresh weight of the roots accompanied by a rapid accumulation of Ni in these organs. Both nitrate and ammonium contents in the root tissue were considerably enhanced by Ni stress. While protein content was not significantly influenced by Ni application, glutamate concentration was slightly reduced on the first day after treatment with the higher Ni dose. Treatment of the wheat seedlings with 100 μM Ni led to a decrease in NR activity; however, it did not alter the activation state of this enzyme. Decline in NiR activity observed after application of 100 μM Ni was more pronounced than that in NR. The activities of GS and NADH-GOGAT also showed substantial decreases in response to Ni stress with the latter being more susceptible to this metal. Starting from the fourth day, both aminating and deaminating GDH activities in the roots of the seedlings supplemented with Ni were lower in comparison to the control. While the activity of AspAT remained unaltered after Ni application that of AlaAT showed a considerable enhancement. The results indicate that exposure of the wheat seedlings to Ni resulted in a general depression of nitrogen assimilation in the roots. Increase in the glutamate-producing activity of AlaAT may suggest its involvement in supplying the wheat roots with this amino acid under Ni stress.