Xylem sap composition was examined in nodulated and nonnodulated cowpea ( Vigna unguiculata (L.) Walp.) plants receiving a range of levels of NO 3 and in eight other ureide-forming legumes utilizing NO 3 or N 2 as sole source of nitrogen. A 15 N dilution technique determined the proportions of plant nitrogen derived from N 2 in the nodulated cowpeas fed NO 3 . Xylem sap composition of NO 3 -fed, nodulated cowpea varied predictably with the relative extents to which N 2 and NO 3 were being utilized. The ratios of asparagine to glutamine (N/N) and of NO 3 to ureide (N/N) in xylem sap increased with increasing dependence on NO 3 whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO 3 (N/N) in xylem sap increased with increasing dependence on N 2 fixation. The amounts of NO 3 and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO 3 and the presence or absence of N 2 fixation. All species showed higher proportions of organic nitrogen as ureide and several-fold lower ratios of asparagine to glutamine in their xylem sap when relying on N 2 than when utilizing NO 3 . In nodulated (minus nitrate) cowpea and mung bean ( Vigna radiata (L.) Wilczek) the percentage of xylem nitrogen as ureide remained constant during growth but the ratio of asparagine to glutamine varied considerably. The biochemical significance of the above differences in xylem sap composition was discussed.
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