Abstract
Rhizobium-inoculated common bean was cultivated with basal application of 20 mg nitrogen as the form of ^<15>N-ammonium sulfate in small pot containing artificial vermiculite. Nitrogen uptake and distribution, root nodulation, acetylene reduction activity(ARA) and soluble sugar concentration (SSC) in roots and nodules of the plants were compared after 26 and 42 days of sowing with those of soybean cultured by the same manner. Smaller plant body and shoot/root ratio, much larger nodule number and nodule weight, and earlier uptake of dressed nitrogen characterized common bean in comparison with soybean. Much higher ARA per plant and per nodule weight were detected in common bean at the both stages, thought they were sharply declined in the 42-day-old plants. Also ARA per nodule weight of soybean diminished at the second measurement, but ARA per plant rose in contrast with the case of common bean. In both plant species, SSC based on dry matter was higher in nodules than in roots. And, SSC in nodules of common bean was higher than that of soybean at the 26th day. But the relationship between the two species nodules was reversed at the 42nd day due to that the concentrations were respectively reduced and increased in common bean and soybean nodules, suggesting that the rapid decline of ARA in common bean was related to carbohydrate shortage in nodules. At the 42nd day, apparently three kinds of nodules were observed on the roots of common bean ; these were large red, middle blue and small pink nodules. Although pink one was dominant in number, the red shared more than 70% of total nodule dry weight. SSC was the highest in blue inactive nodules and the lowest in active red nodules, and which may reflect the activity of sugar consumption for nitrogen fixation and assimilation of its product in nodules.
