Physiochemical mechanism of nitrate nitrogen stress on root nodulation in common bean and soybean seedlings

Abstract

The physiochemical mechanism of nitrate nitrogen stress on root nodulation was compared in common bean and soybean seedlings. Rhizobium- inoculated common bean and soybean were grown in vermiculite with a mineral nutrient solution containing 5, 30 or 60 mg L-1 15NO3-nitrogen in a growth chamber. Nodulation of common bean was strongly suppressed by 30 mg L-1 nitrate, while 60 mg L-1 nitrate caused distinct suppression in soybean. Common bean absorbed and assimilated larger amounts of medium-derived nitrogen (MDN) than soybean with the same nitrate treatment. Substantial activities of nitrate reduction and assimilation were detected in common bean earlier than in soybean. The percentage of MDN in root amino-nitrogen was closely correlated with the inhibition of nodulation in both common bean and soybean. In conclusion, nitrate stress on nodulation in seedlings of common bean and soybean arose through assimilation of MDN in roots, and the MDN assimilation process proceeded earlier and more actively in roots of common bean than in soybean.