Effect of cadmium stress on nitrogen metabolism in nodules and roots of soybean plants

Abstract

The nitrogen metabolism of soybean (Glycine max L.) nodules and roots was studied in plants subjected to two different concentrations (50 and 200 μM) of CdCl₂. Nitrogenase activity was decreased in nodules treated with 200 μM Cd²⁺. In 50 μM Cd²⁺-treated plants, NH₄⁺ content showed similar values to controls in nodules, but increased by 55% in roots. However, after treatment with 200 μM Cd²⁺, NH₄⁺ levels increased in both tissues. Glutamate (Glu) and protein contents remained unaltered in nodules treated with 50 μM Cd²⁺, while at the higher Cd²⁺ concentration both were decreased. Nevertheless, polyamine content was increased at the two Cd²⁺ concentrations. In roots, Glu, polyamine and protein levels were significantly diminished at 50 and 200 μM CdCl₂. For nitrogen-assimilation enzymes, glutamate dehydrogenase activity was moderately increased in nodules and roots following the lower Cd²⁺ treatment, though at the higher Cd²⁺ concentration root enzyme activity returned to control levels. An impressive increase in enzyme activity was found in nodules. In roots, the glutamine synthetase / glutamate synthase pathway was decreased at the two Cd²⁺ concentrations, though in nodules it was diminished only at 200 μM Cd²⁺. No changes in protease activity were found in the two tissues treated with 50 μM Cd²⁺. However, at 200 μM Cd²⁺, nodule and root protease activities decreased and increased, respectively. These results suggest that, in general, treatment with Cd²⁺ affects nitrogen assimilation and metabolism to a greater extent in soybean roots than in nodules.