High sensitivity of common bean (Phaseolus vulgaris) in root nodule formation and dinitrogen fixation to basally dressed combined-nitrogen: A comparative study with soybean

Rhizobium-inoculated common bean was cultivated with basal application of low-level nitrogen with an andosol soil in a field or small pot. In the field experiment, acetylene reduction activity (ARA), nodule growth and soluble sugar concentration (SSC) in nodules were compared 30 d after sowing (DAS) among three and two respective cultivars of common bean and soybean. Without application of combined nitrogen, ARA and nodule growth of two plant species were comparable and nodule number of determinate and semideterminate type common bean was much larger than that of soybean. By the application of 30 kg N ha(-1), a more drastic decrease of ARA per plant was observed n common bean, especially indeterminate and semideterminate types, than in soybean. The mean value of nodule size also more obviously decreased in determinate and semideterminate type common bean, ARA per nodule weight declined only in common bean by nitrogen fertilization, but did not linearly correspond to SSC in nodules among plant species or cultivars. In the pot experiment, respective cultivars of semideterminate type common bean and soybean was compared with two levels of 15N-ammonium sulfate application. In comparison with soybean, common bean indicated larger amount of 15N uptake per plant dry matter and higher 15N atom% in each organ at 40 DAS revealing active nitrogen-uptake by roots and weak dinitrogen-fixation by nodules through culture period. Dry matter distribution among plant organs was clearly changed only in common bean affected by applied nitrogen level. On the basis of the above results, it was concluded that tested determinate and semideterminate type common bean were highly sensitive in both nodule formation and nitrogen-fixing activity to dressed nitrogen partially attributed to their high capacity in utilizing dressed nitrogen