Aluminium stress affects nitrogen fixation and assimilation in soybean (Glycine max L.)

Abstract

Nitrogen fixation and assimilation in nodules and roots were studied in soybean (Glycine max L.) exponed to different levels of aluminium (Al) stress (0, 50, 200 and 500 µM). Al at 500 µM induced oxidative stress, which became evident from an increase in lipid peroxidation accompanied by a concomitant decline in antioxidant enzyme activities and leghaemoglobin breakdown. Consequently, there was also a reduction in nitrogenase activity. However, the leghaemoglobin levels and nitrogenase activity were unexpectedly found to be higher in nodules when the plants were treated with 200 µM Al. Of the enzymes involved in nitrogen assimilation, the activity of glutamate dehydrogenase-NADH was reduced in nodules under Al stress, but it was significantly higher in roots at 500 µM Al as compared to that in the control. In nodules, the glutamina synthetase/glutamate synthase-NADH pathway, assayed in terms of activity and expression of both the enzymes, was inhibited at >50 µM Al; but in roots this inhibitory effect was apparent only at 500 µM Al. No significant changes in ammonium and protein contents were recorded in the nodules or roots when the plants were treated with 50 µM Al. However, Al at ±200 µM significantly increased the ammonium levels and decreased the protein content in the nodules. But these contrasting effects on ammonium and protein contents due to Al stress were observed in the roots only at 500 µM Al. The results suggest that the effect of Al stress on nitrogen assimilation is more conspicuous in nodules than that in the roots of soybean plants.