Re-evaluation of the role of ureides in the xylem transport of nitrogen in Arachis species

There are conflicting reports in the literature of the possible role of the ureides, allantoin and allantoic acid, in the nitrogen economy of Arachis species. Therefore, xylem sap composition in food peanut ( Arachis hypogaea L.), and two forage peanuts ( A. pintoi L. and A. glabrata Benth.) has been studied in detail. Xylem saps were collected from peanuts grown under different nutritional regimes and environmental conditions in the glasshouse and field in Australia, Malaysia and Indonesia, and the N-containing solutes analysed. The relative amounts and concentrations of ureides in these peanut exudates were compared with those of soybean ( Glycine max [L.] Merr.) – a species known to export ureides in its xylem stream as the major product of N₂ fixation.
Xylem concentrations of ureides in soybean were high in N₂-fixing plants (2.9 to 3.7 μmol ml⁻¹), representing 60 to 88% of xylem solute nitrogen, but it contributed only 9% (0.7 μmol ml⁻¹) if plants were unnodulated and supplied nitrate. In all species of peanut, concentrations of ureides measured in xylem sap were generally much smaller (0.02 to 0.37 μmol ml⁻¹; 1–7% of xylem nitrogen) and were unaffected by peanut species or cultivar, rhizobial strain, plant size, growth rate, or stage of development, and were not related to N₂ fixation (less than 0.1% of currently fixed nitrogen exported as ureides) or the assimilation of nitrate. Apparently high levels of ureides in sap from some field-grown plants were shown to be due to interference with the ureide colorimetric assay by some contaminating compound rather than represent increased ureides per se.