Ureide Production by N2-fixing and Non-n2-fixing Leguminous Trees

Xylem-sap and stem and leaf extracts from 35 species, comprising 14 genera, of leguminous trees were analyzed for ureides, nitrate and a-amino acids. Trees were either inoculated with Rhizobium or fertilized with NH4NO3. The dominant form of soluble N in stem and leaf extracts and xylem sap was a -amino acids. Certain non-N2-fixing species, i.e. Tamarindus indica and Adenanthera pavonina, produced significant amounts of ureides. Several N2 fixing species, Mimosa scabrella, Sesbania grandflora, Acacia mearnsii and Gliricida sepium, grown on mineral-N had higher absolute amounts of ureides in both extracts and exudates than did most nodulated species. Nodulated A. mearnsii and S. grandiora, had the highest amounts of ureides in xylem sap. The relative abundance of ureides in stem and leaf extracts was lower than in xylem sap, but was correlated. Results indicated that the presence of ureides, per se, was not a reliable indicator of N2-fixing activity. Moreover, the relative abundance of ureides in most of the species tested was too low to use as a presumptive test for, or as a means of, estimating N2 fixation.