Abstract
ommon bean root nodulation is easily repressed by nitrate application. To clarify the physiochemical backgrounds of the susceptibility, nitrate assimilation and root nodulation were comparatively studied between common bean and soybean seedlings inoculated with rhizobium under different application levels of ^<15>N-nitrate. Common bean assimilated much larger amounts of medium-nitrate derived nitrogen (MDN) than soybean. Furthermore the root-distribution ratio of MDN was larger in common bean than in soybean at 10 d after sowing (DAS), at which time a substantial amount of MDN was detected in both legumes. In addition to these facts, common bean roots were more abundant in free amino acid form MDN (faa-MDN) than soybean roots, revealing higher activity of common bean roots in uptake and assimilation of nitrate. On the whole, the number of primordial and matured root-nodules was much larger in common bean than in soybean. However, at 10 DAS, both formation and development of root nodule primordia were more strongly suppressed in common bean than in soybean by the application of 30 and 60 mg L^<-1> nitrate. The amount of root faa-MDN per unit of root dry weight showed extremely high correlation with an inhibitory index for the rate of primordial and matured root nodule formation in both legumes. This distinctive correlation suggested that so-called nitrate stress on nodulation arose through MDN assimilation in roots in both legumes. Based on these results, it was concluded that higher susceptibility of common bean root nodulation to applied nitrate was due to more active uptake and assimilation of nitrate by its roots.
