Cadmium-induced senescence in nodules of soybean (Glycine max L.) plants

The relationship between cadmium-induced oxidative stress and nodule senescence in soybean was investigated at two different concentrations of cadmium ions (50 and 200 µM), in solution culture. High cadmium concentration (200 µM) resulted in oxidative stress, which was indicated by an increase in thiobarbituric acid reactive substances content and a decrease in leghemoglobin levels. Consequently, nitrogenase activity was decreased, and increases in iron and ferritin levels were obtained. Senescent parameters such as ethylene production, increased levels of ammonium and an increase in protease activity were simultaneously observed. Glutamate dehydrogenase activity was also increased. Peroxidase activity decreased at the higher cadmium concentration while the lower cadmium treatment produced changes in peroxidase isoforms, compared to control nodules. Ultrastructural investigation of the nodules showed alterations with a reduction of both bacteroids number per symbiosome and the effective area for N₂-fixation. These results strongly suggest that, at least at the higher concentration, cadmium induces nodule senescence in soybean plants.