Symbiotic functioning and bradyrhizobial biodiversity of cowpea (Vigna unguiculata L. Walp.) in Africa

Field measurements of N2 fixation revealed significant differences in plant growth, δ15N values, %Ndfa and amounts of N-fixed between and among the 9 cowpea genotypes in Ghana and South Africa. Following DNA analysis of 270 nodules from the 9 genotypes, 18 strain IGS types were found. Relating nodule function to the 18 IGS types revealed significant differences in IGS type N2-fixing efficiencies. Sequencing the 16S - 23S rDNA gene also revealed 4 clusters, with cluster 2 forming a distinct group that may be a new Bradyrhizobium species. Taken together, our data indicated greater biodiversity of cowpea bradyrhizobia in South Africa relative to Botswana and Ghana.We have shown that cowpea is strongly dependant on N2 fixation for its N nutrition in both South Africa and Ghana. Strain IGS type symbiotic efficiency was assessed for the first time in this study, and a positive correlation was discernible where there was sole nodule occupancy. The differences in IGS type diversity and symbiotic efficiency probably accounts for the genotype × environment interaction that makes it difficult to select superior genotypes for use across Africa. The root-nodule bacteria nodulating cowpea in this study all belonged to the genus Bradyrhizobium. Some strains from Southern Africa were phylogenetically very distinct, suggesting a new Bradyrhizobium species.Cowpea (Vigna unguiculata L. Walp.) is a major food crop in Africa, where its leaves, green pods and grain are eaten as a dietary source of protein. The cowpea grain contains about 23% protein and 57% carbohydrate, while the leaves contain between 27 - 34% protein [1]. The leaves and grain are also supplied as high protein feed and fodder to livestock. Cowpea is the most commonly grown food legume by traditional farmers in Sub-Saharan Africa, possibly because of its relatively wide adaptation to drought and low-nutrient environments. Cowpea freely forms root nodules with some members of the Rhizobiaceae such as Rhizobium and Bradyr