Xylem sap composition was examined in nodulated and nonnodulated cowpea ( Vigna unguicd L.I Walp.) plants receiving a range of levels of NO3 and in eight other ureide-forming legumes udlizing NO3 or N2 as sole source of nitrogen. A 15N dilution technique determined the proportions of plant nitrogen derived from N2 in the nodulated cowpeas fed NO. Xylem sap composition of N03-fed, nodulated cowpea varied predictably with the relative extents to which N2 and NO3 were being utilized. The ratios of asparagine to glutamine (N/N) and of NO3 to ureide (N/N) in xylem sap increased with increasing dependence on NO3 whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO3 (N/N) in xylem sap increased with increasing dependence on N2 fixation. The amounts of NO3 and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO3 and the presence or absence of N2 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 N2 than when utilizing NO3. In nodulated (minus nitrate) cowpea and mung bean (Vigna radiata [L.1 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 compositdon was discussed.A number of legumes, especially tropical species, synthesize allantoin and allantoic acid from currently fixed N2 in nodules and use these compounds for transport and storage of N (1, 3, 6, 10). In soybean (Glycine max [L.] Merr.) the ureides are more prominent in nodulated plants than in plants grown with NO3 or NH4 salts (8, 11), suggesting a special association between N2 fixation and ureide formation (9), and the possibility of using ureide levels in plant tissues or transport fluids to determine the extent to which a field-grown crop is relying on symbiosis as opposed to combined forms of N from soil or fertilizer. This paper describes xylem sap composition of N2 and N03-fed cowpea (Vigna unguiculata [L.] Walp.) and of other grain legumes which form ureides, and suggests how compositional features of the sap might be used to evaluate the N2-fixing status of nodulated plants grown in the presence of NO3. MATERIALS AND METHODSThe species studied were cowpea (V. unguiculata [L.] Plants were grown in pots of heat sterilized sand during summer in a naturally lit glasshouse maintained within the temperature range 20-35 C by supplementary heating (night) and evaporative cooling (day). One set of plants of each species was inoculated with an effective Rhizobium (strain CB437, for soybean, strain NGR234 for horse gram, and strain CB756 for all other species) and grown with a N-free culture solution. A matching set of uninoculated plants received culture solution containing 10 mM NO3. By using this level of NO3, sterile sand and effective surface sterilization of s...

The response of non-nodulated cowpea ( Vigna suguicalata (L.) Walp. cv Caloona) to a wide range of NO3 levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO3 reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO3 showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66,62, and 58% of the total N of their xylem sap as NO3. These data, and the absence in the plants of significant pools of stored NO3, indicated that shoots were major organs of NO3 assimilation, especially at levels of NO3 (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO3 plants and non-nodulated plants fed 10 or 20 millimolar NO3, the levels of NO3 which gave rates of growth and N assimilation closest to those of the symbiotic plants. The