Characterization of nitrogen assimilation in a supernodulating soybean mutant En6500

A supernodulating and nitrate-tolerant soybean (Glycine max (L.) Merr.) mutant En6500 and its wild-type parent, cv. Enrei were cultured hydroponically at various nitrate (KNO₃) levels (0.5, 5.0, and 15.0 mol m^-3) to analyze the characteristics of nitrogen fixation, nitrate absorption, and associated characters in this mutant. The mutant produced a larger number of nodules, higher nodule dry weight, and a larger amount of fixed nitrogen per plant, thus depending more on fixed nitrogen at all nitrate levels than its parent. The mutant also exhibited a nitrate tolerance in nodule number and nodule dry matter, though the amount of fixed nitrogen per plant or on a nodule dry weight basis was reduced with increasing nitrate levels. The ability of nitrate absorption and plant growth in the mutant were inherently less active than those of the parent even in the absence of inoculation (non-nodulating conditions). Inoculation enhanced the low performance of nitrate absorption and growth in the mutant, which appeared to be related to the high consumption of carbohydrates by the nodules and limited root surface for absorption.