Evaluation of the Relative Ureide Content of Xylem Sap as an Indicator of N2 Fixation in Soybeans

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Abstract
The use of the relative ureide content of xylem sap [(ureide-N/total N) .times. 100] as an indicator of N2 fixation in soybeans was examined under greenhouse conditions. Acetylene treatments to inhibit N2 fixation were imposed upon the root systems of plants totally dependent upon N2 fixation as their source of N and of plants dependent upon both N2 fixation and uptake of exogenous nitrate. Significant decreases in the total N concentration of xylem sap from plants of the former type were observed, but no significant decrease was observed in the total N concentration of sap from the latter type of plants. In both types of plants, acetylene treatment caused significant decreases in the relative ureide content of xylem sap. The results provided further support for a link between the presence of ureides in the xylem and the occurrence of N2 fixation in soybeans. The relative ureide content of xylem sap from plants totally dependent upon N2 fixation was shown to be insensitive to changes in the exudation rate and total N concentration of xylem sap brought about by diurnal changes in environmental factors. There was little evidence of soybean cultivars or nodulating strains affecting the relative ureide content of xylem sap. ''Ransom'' soybeans nodulated with Rhizobium japonicum strain USDA 110 were grown under conditions to obtain plants exhibiting a wide range of dependency upon N2 fixation. The relative ureide content of xylem sap was shown to indicate reliably the N2 fixation of these plants during vegetative growth using a 15N method to measure N2 fixation activity. The use of the relative ureide content of xylem sap for quantification of N2 fixation in soybeans should be evaluated further.