Effects of Nodulation on Metabolite Concentrations in Xylem Sap and in the Organs of Soybean Plants Supplied with Different N Forms

Abstract

The effects of nodulation on N metabolism in soybean plants supplied with various forms of N are not fully understood. Ureides are the principal forms of N transported from nodules, but nitrate and asparagine are the primary N compounds transported from roots supplied with NO₃^-. In this research, the effects of 1-day treatments of NO₃^-, NH₄⁺, urea, or NO₃^- + NH₄⁺ on N metabolite concentrations in xylem sap and each organ were compared between nodulated and non-nodulated soybeans. Capillary electrophoresis and colorimetry were used for the analysis. In the xylem sap of the nodulated plants with an N-free solution, ureides were the major N metabolites, followed by asparagine and glutamine. Ureides concentrations were much lower in the xylem sap of the non-nodulated soybeans. In the NO₃^- treatment, the concentrations of ureides in the xylem sap of the nodulated plants decreased compared to the control plants. In the NH₄⁺, urea, and NO₃^- + NH₄⁺ treatments, the concentrations of asparagine and glutamine increased significantly compared with the control and NO₃^- treatments. Similar changes with the N treatments were observed between the nodulated and non-nodulated soybeans, suggesting that nodulation does not have significant effects on the metabolism of absorbed N in roots.

Keywords: allantoate; allantoin; asparagine; capillary electrophoresis; glutamine; nitrate; nodule; soybean; urea; xylem sap.

Figure 1

Schematic outline of the absorption and assimilation of NO₃⁻, NH₄⁺, and urea in a root cell. Transporters. ASNS: asparagine synthetase; AST: aspartate aminotransferase; GlyAT: glycine aminotransferase; GOGAT: glutamate synthase; GS: glutamine synthetase (referred to [3,15]).

Figure 2

Concentrations of the principal N metabolites in the xylem sap of nodulated (A) and non-nodulated (B) soybean plants treated with various N compounds. Treatments: Cont, control with N-free solution; NO₃⁻, 5 mM KNO₃; NH₄⁺, 2.5 mM (NH₄)₂SO₄; urea, 2.5 mM urea; NO₃⁻ + NH₄⁺, 2.5 mM KNO₃ + 1.25 mM (NH₄)₂SO₄. n = 3. Average ± standard error. The different letters on the top of the columns indicate significant differences in the N concentration among the treatments based on Tukey’s test (p < 0.05). Asterisks (*, **) designate significant changes according to Student’s t-test results for the same compound in the same treatment between nodulated and non-nodulated plants at p < 0.05 and p < 0.01, respectively. ND; not detected.

Figure 3

Comparison of the concentrations of the principal N metabolites in the nodules of nodulated soybean plants treated with various N compounds. Treatments: Cont, control with N-free solution; NO₃⁻, 5 mM KNO₃; NH₄⁺, 2.5 mM (NH₄)₂SO₄; urea, 2.5 mM urea; NO₃⁻ + NH₄⁺, 2.5 mM KNO₃ + 1.25 mM (NH₄)₂SO₄. n = 3. Average ± standard error. The different letters on the top of the columns indicate significant differences in the N concentration among the N treatments based on Tukey’s test (p < 0.05).

Figure 4

Comparison of the concentrations of the principal N metabolites in the lower roots of the Nod and Non-nod soybean plants treated with various N compounds. The symbols, abbreviations and the explanations of the statistics are the same as in Figure 2.

Figure 5

Comparison of the concentrations of the principal N metabolites in the upper roots of the Nod and Non-nod soybean plants treated with the various N compounds. The symbols, abbreviations and the explanations of the statistics are the same as in Figure 2.

Figure 6

Comparison of the concentrations of the principal N metabolites in the stems of the Nod and Non-nod soybean plants treated with the various N compounds. The symbols, abbreviations and the explanations of the statistics are the same as in Figure 2.

Figure 7

Comparison of the concentrations of the principal N metabolites in the leaves of the Nod and Non-nod soybean plants treated with various N compounds. The symbols, abbreviations and the explanations of the statistics are the same as in Figure 2.

Figure 8

Comparison of the concentrations of N metabolites among the organs of the Nod and Non-nod soybean plants treated with the various N compounds: (A,B) NO₃⁻; (C,D) allantoate; (E,F) allantoin; (G,H) Asn; (I,J) Gln; (K,L) Asp; (M,N) Glu; (O,P) urea; (Q,R) NH₄⁺. (A,C,E,G,I,K,M,O,Q) nodulated plants; (B,D,F,H,J,L,N,P,R) non-nodulated plants. Treatments: Cont, control with N-free solution; NO₃⁻, 5 mM KNO₃; NH₄⁺, 2.5 mM (NH₄)₂SO₄; urea, 2.5 mM urea; and NO₃⁻ + NH₄⁺, 2.5 mM KNO₃ + 1.25 mM (NH₄)₂SO₄. n = 3. Average ± standard error. Different letters on the top of the columns indicate significant differences in the N concentrations among the organs based on Tukey’s test (p < 0.05).

Figure 8

Comparison of the concentrations of N metabolites among the organs of the Nod and Non-nod soybean plants treated with the various N compounds: (A,B) NO₃⁻; (C,D) allantoate; (E,F) allantoin; (G,H) Asn; (I,J) Gln; (K,L) Asp; (M,N) Glu; (O,P) urea; (Q,R) NH₄⁺. (A,C,E,G,I,K,M,O,Q) nodulated plants; (B,D,F,H,J,L,N,P,R) non-nodulated plants. Treatments: Cont, control with N-free solution; NO₃⁻, 5 mM KNO₃; NH₄⁺, 2.5 mM (NH₄)₂SO₄; urea, 2.5 mM urea; and NO₃⁻ + NH₄⁺, 2.5 mM KNO₃ + 1.25 mM (NH₄)₂SO₄. n = 3. Average ± standard error. Different letters on the top of the columns indicate significant differences in the N concentrations among the organs based on Tukey’s test (p < 0.05).

Figure 8

Comparison of the concentrations of N metabolites among the organs of the Nod and Non-nod soybean plants treated with the various N compounds: (A,B) NO₃⁻; (C,D) allantoate; (E,F) allantoin; (G,H) Asn; (I,J) Gln; (K,L) Asp; (M,N) Glu; (O,P) urea; (Q,R) NH₄⁺. (A,C,E,G,I,K,M,O,Q) nodulated plants; (B,D,F,H,J,L,N,P,R) non-nodulated plants. Treatments: Cont, control with N-free solution; NO₃⁻, 5 mM KNO₃; NH₄⁺, 2.5 mM (NH₄)₂SO₄; urea, 2.5 mM urea; and NO₃⁻ + NH₄⁺, 2.5 mM KNO₃ + 1.25 mM (NH₄)₂SO₄. n = 3. Average ± standard error. Different letters on the top of the columns indicate significant differences in the N concentrations among the organs based on Tukey’s test (p < 0.05).