Effects of nitrate application on growth, modulation, and nitrogen fixation of nitrate-tolerant mutants of soybean

Nitrate-tolerant mutants of soybean (Glycine max L. Merr.) cv. Bragg, nts1116 and nts1007, were cultured hydroponically to study the effects of nitrate on plant growth, nodulation, and nitrogen fixation compared to the parent cultivar Bragg. The vegetative growth of nts1116 was the most active. Absorption rate of nitrate increased with the duration of the treatment. The absorption rate was slightly higher in nts1116 compared to ntsl007 and Bragg. Nitrogen accumulation of the nitrate-treated plants through biological nitrogen fixation was larger in the order of ntsl007, nts1116, and Bragg. However, N accumulation from nitrate did not differ among the 3 lines. Nitrate reductase activities in the nodules were the highest in Bragg followed by nts1116 and nts1007, regardless of the nitrate treatment and assay time, and were negatively correlated with the nodule masses. Ureide concentration was the highest in nts1007 followed by nts1116 and Bragg, and was positively correlated with the nodule masses and negatively with the sugar concentration in the leaves in the 3 lines. Total-N concentration was higher in nts1007 than in nts1116 in the. presence of nitrate, while plant growth was more active in nts1116 than in nts1007. The above results indicated that the repression of plant growth in nts1007 was related to the high ureide production of plant associated with the super-abundant nodulation.