**6. Conclusion**

Rapid and reversible repression of nodule growth and nitrogen fixation activity of nodulated soybean were observed when nitrate was supplied in the culture solution. This may be caused by the decrease in the photo-assimilate partitioning to the nodules and not by the transport of N compounds from applied N in the solution. Transcriptome and metabolome analysis supported the above hypothesis. Conversely, the C and N metabolism in the roots was promoted by the application of nitrate. A similar rapid and reversible repression of nodule growth and nitrogen fixation activity was also observed when ammonium, urea, or glutamine was supplied as same as nitrate, however, the inhibitory effect was stronger in nitrate compared with ammonium, urea, or glutamine. Urea was detected in xylem sap and all parts of soybean, and some of the urea originated from ureide degradation.

The plant shoots and roots exchange C and N through the xylem and phloem transport systems [35]. The C and N metabolism is regulated by complex mechanisms to optimize plant organ development and growth. So, understanding the CN metabolism can be related to the agricultural crop production, and maintenance of the agroecosystem.
