Effect of NO3 on N2 fixation and nitrogenous solutes of xylem in two nodulated West African geocarpic legumes,Kersting's bean (Macrotyloma geocarpum L.) and Bambara groundnut (Vigna subterranea L.)

Nodulation, nitrogen (N₂) fixation and xylem sap composition were examined in sand cultured plants of Bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.) inoculated with Bradyrhizobium strain CB756 and supplied via the roots for a 4 week period from the third week onwards with different levels of (¹⁵N)-nitrate (0–15 mM). The separate contributions of nitrate and N₂ to plant nitrogen were measured by isotope dilution. Increasing levels of nitrate inhibited nodule growth (measured as dry matter or nodule N) of both species parallel with decreased dependence on symbiotically-fixed N. Specific nodule activity (N₂ fixed g nodule dry⁻¹ d⁻¹ of nodules) was reduced progressively with time in V. subterranea at higher (5 or 15 mM) levels of NO₃, but this was not so for M. geocarpum. Root xylem bleeding sap of both species showed ureides (allantoin and allantoic acid) as predominant (>90%) solutes of nitrogen when plants were relying solely on atmospheric N. Levels of ureide and glutamine decreased and those of asparagine and nitrate in xylem increased with increasing level of applied nitrate. Relative levels of xylem ureide-N were positively correlated (R²=0.842 for M. geocarpum and 0.556 for V. subterranea), and the ratio of asparagine to glutamine in xylem exudate negatively correlated (R²=0.955 for M. geocarpum and 0.736 for V. subterranea) with plant reliance on nitrogen fixation. The data indicate that xylem sap analyses might be useful for indirect field assays of nitrogen fixation by the species and that Kersting's bean might offer some potential as a symbiosis in which N₂ fixation is relatively tolerant of soil N.