Application of Nitrate, Ammonium, or Urea Changes the Concentrations of Ureides, Urea, Amino Acids and Other Metabolites in Xylem Sap and in the Organs of Soybean Plants (Glycine max (L.) Merr.).

Yuki Ono,Akihiro Saito,Norikuni Ohtake,Sayuri Tanabata,Takashi Sato,Kuni Sueyoshi,Kyoko Higuchi,Takuji Ohyama,Ryo Toyota,Masashige Fukasawa

doi:10.3390/ijms22094573

Abstract

Soybean (Glycine max (L.) Merr.) plants form root nodules and fix atmospheric dinitrogen, while also utilizing the combined nitrogen absorbed from roots. In this study, nodulated soybean plants were supplied with 5 mM N nitrate, ammonium, or urea for 3 days, and the changes in metabolite concentrations in the xylem sap and each organ were analyzed. The ureide concentration in the xylem sap was the highest in the control plants that were supplied with an N-free nutrient solution, but nitrate and asparagine were the principal compounds in the xylem sap with nitrate treatment. The metabolite concentrations in both the xylem sap and each organ were similar between the ammonium and urea treatments. Considerable amounts of urea were present in the xylem sap and all the organs among all the treatments. Positive correlations were observed between the ureides and urea concentrations in the xylem sap as well as in the roots and leaves, although no correlations were observed between the urea and arginine concentrations, suggesting that urea may have originated from ureide degradation in soybean plants, possibly in the roots. This is the first finding of the possibility of ureide degradation to urea in the underground organs of soybean plants.

Highlights

A group of soybean plants was cultivated without soil microorganisms; urea may be directly absorbed by the roots, bradyrhizobium inoculation, and the non-nodulated plants were cultivated by supplying plants were not aseptically cultivated
Nodulated soybean plants were supplied with 5 mM N as sodium nitrate (5 mM) N of nitrate, ammonium, or urea for 3 d, and the concentrations of ureides, urea, amino acids, anions, sugars, and organic acids in the xylem sap and each organ were analyzed
A positive correlation was observed between the concentrations of ureides and urea in the xylem sap as well as in the leaves and roots, correlations were not observed between urea and arginine, except for in leaves

Summary

Introduction

Soybean plants form root nodules with symbiotic soil bacteria such as bradyrhizobium, and they can fix atmospheric nitrogen (N2 ). Soybean plants absorb N from soil and fertilizers through their roots. Nodule formation, nodule growth, and N2 fixation activities are repressed when the nodulated roots are directly exposed to high concentrations of the combined forms of nitrogen, especially nitrate, a major form of inorganic nitrogen in upland fields [1,2,3,4]. Multiple effects of nitrate inhibition have been reported, such as the decrease in nodule number, mass, and N2 fixation activity as well as the acceleration of senescence or disintegration of established nodules, so nitrate inhibition cannot be explained by a simple mechanism [2,5]

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Conclusion