Abstract
It was reported that a supernodulating soybean mutant, En6500 derived from the variety of Enrei, could produce many root nodules, but the growth rate of the plant was not necessarily high. The purpose of this paper, therefore, is to examine the characteristics of nodulation and also of plant growth in En6500 for elucidation of the mechanism of its low growth rate. The relative growth rate was low in En6500 compared with Enrei. The dry weight of the nodules per unit root mass was greater in En6500 than in Enrei, suggesting that the root of En6500 is supporting too many or an excess of nodules, which presumably leads to a reduction of plant growth rate. Reduced growth rate was associated with low net assimilation rate and further with low leaf photosynthesis. Moreover, the reduced leaf photosynthesis was associated with low partitioning of nitrogen to the leaves in En6500. Much allocation of nitrogen to the plant organs besides the leaves resulted in low photosynthesis, and also in the reduced nitrogen-use efficiency for dry matter production, which might be a potential mechanism for the low growth rate of En6500, a supernodulating soybean mutant.
