Sequences 104, 27 and 36 showed little variation with B. yuanmingense (98% similarity), while IGS sequence 103 showed a 79% similarity with Bradyrhizobium sp ORS 3409 and CIRADAc12. IGS sequences 115 and 68 were found to be similar to Bradyrhizobium species ORS 188, ORS 190 and Bradyrhizobium genospecies VIII of [20]. Another cluster was formed by IGS sequences 5, 201, 22, 117, 153

and 146 around Bradyrhizobium japonicum USDA 38, Bradyrhizobium genospecies V of PARP signaling [20] and Bradyrhizobium liaoningense. The third cluster was made up of IGS sequence 106 with B. elkani, with the two having 98 – 99% similarities (Figure 3). The root-nodule bacteria nodulating cowpea in this study all belonged to the genus Bradyrhizobium. Figure 3 Phylogenetic relationship

among 16S-23S rDNA IGS types of from cowpea nodules, reference strains and more closed isolates based upon aligned 16S-23S rDNA IGS region sequences constructed as rooted tree using neighbour-joining method. The bootstrap values (expressed as percentage of 1000 replications) shown at nodes are those greater than 70%. Discussion Field measurements of N2 fixation using the 15N natural abundance revealed significant differences in plant growth and symbiotic performance of the 9 cowpea genotypes tested in South Africa and Ghana (Tables 2 and 3). The marked variation in plant growth (measured as dry matter yield) was linked to differences in overall nodule functioning. At Wa, for example, Omondaw and Glenda, which were among the highest in nodulation (nodule number and Selleckchem PS341 mass), showed the lowest ∂15N

values, the highest %Ndfa, the highest amount of N-fixed, and thus produced the largest amount of plant growth and dry matter (Table 2). This was in contrast to Mamlaka and Fahari, which exhibited low nodulation and low N-fixed, and therefore produced the least shoot biomass at Wa (Table 2). At Taung in South Africa, Fahari which showed the best nodulation and the highest amount of N-fixed, recorded the highest amount of shoot biomass relative to Apagbaala, which exhibited the least nodulation, lowest amount of N-fixed, and thus produced the smallest plant biomass (Table 3). Of the 9 cowpea genotypes planted at Wa, Apagbaala was among the top 3 genotypes in N2 fixation (Table 2) due to its high specific nodule activity (Figure 2A). Ribonucleotide reductase Yet in South Africa, Apagbaala and Omondaw were among the least in N2 fixation, even though they were the highest fixers in Ghana. The better symbiotic performance of genotypes at one location (e.g. Omondaw and Apagbaala at Wa in Ghana) and their poor performance at another site (e.g. Taung in South Africa) could be attributed to the quality of nodule occupants (i.e. the resident IGS types inside root nodules, see Tables 4 and 5). As shown in Figure 2, when nodule functioning was related to nodule occupants, differences in N2-fixing efficiency were found among the resident IGS types, especially where there were clear cases of sole occupancy.