Nitrogen Assimilation in Nitrogen‐Fixing Soybean Plants during Phosphorus Deficiency

Abstract

This study evaluated the hypothesis that P deficiency alters N assimilation in N2‐fixing soybean plants (Glycine max L. Merr.) through effects on nodule carbohydrate status. Transport of N constituents in xylem sap, concentrations and total quantities of N fractions in plant organs, and nonstructural carbohydrate status of nodules were compared in P‐sufficient (1.0 mM P) and P‐deficient (0.05 mM P) greenhouse‐grown soybean plants dependent solely on symbiotic N2 fixation. Increased ureide concentration in nodules and decreased ureide concentration in the xylem sap indicated that P deficiency restricted ureide transport from nodules to the xylem. Decreased ureide and total N concentration and flux in xylem sap, and decreased concentrations of ureide and soluble and insoluble reduced N in leaves and stems indicated that P deficiency decreased N delivery to the shoot. Phosphorus deficiency decreased the sap exudation rate per unit root mass by 75%. Thus, decreased exudation rate, as well as decreased constituent concentrations, contributed to decreased flux of N compounds in the xylem. The decreased exudation rate may have resulted from decreased hydraulic conductivity of the root system and/or decreased ion transport into the xylem. Phosphorus deficiency caused decreased hexose and sucrose concentrations and increased starch concentration in nodules, indicating impaired carbohydrate utilization. This impaired carbohydrate utilization and the resulting decrease in energy status may have contributed to the decreased movement of ureide, and other N constituents, from host cells into the xylem of nodules.