Nodule function in symbiotic bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.) is tolerant of nitrate in the root medium

Nitrogen-fixing activity in two nodulated African legumes, Bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.), was assessed in the presence of nitrate (NO3-) ions in the rooting medium. Nitrogenase activity was unimpaired by the supply of 5 mol m(-3) NO3 to both species. Also, large concentrations of ureides dominated the transpiration stream of NO3-fed plants. Compared to other symbiotic legumes cultured with similar NO3 concentrations, nodule functioning in the tested landraces of Bambara groundnut and Kersting's bean is tolerant of NO3 ions in the rhizosphere. The potential benefits of such naturally occurring NO3-tolerant symbioses are substantial, as they would permit inorganic N fertilizer application in intercropping systems without inhibiting N-2 fixation in the associated legumes. (C) 1998 Annals of Botany Company.