Systemic Effects of Nitrate, Asparagine, and Glutamine on Nodulation and Biological Nitrogen Fixation in Soybean

Although it is well established that nitrate exerts an inhibitory effect on nodulation and biological nitrogen fixation in soybean, the underlying mechanism remains unclear. In soybean plants, nitrate is assimilated into asparagine (Asn) and glutamine (Gln); their systemic circulation within the plant may contribute to the reduced N-fixation capacity of nodules. To investigate the effects of nitrate, Asn, and Gln on soybean nodulation and biological N fixation, a unilateral nodulated double-root soybean system was used. The non-nodulated side roots were supplied with nitrate (14 mM), Asn (20 mM), or Gln (20 mM), while the nodulated side roots were not supplied with N. Changes in nodule number, nodule dry weight, nitrogenase activity, and N compound content were analyzed after 4 and 10 days of treatment. The results showed that all three exogenous N sources significantly reduced nodule number, nodule dry weight, and nodule nitrogenase activity after both 4 and 10 days of treatment, while promoting the accumulation of ureides, Asn, and Gln in leaves. Nitrate and Asn treatments increased the accumulation of ureides and Asn in nodules, whereas Gln had no significant effect on nitrogenous compounds in nodules. These results suggest that nitrate inhibits nodulation and biological N fixation primarily through its conversion to Asn in soybean plants. The accumulation of Asn in shoots and nodules may suppress nodulation and biological N fixation by promoting ureide accumulation.