Functional and genetic diversity of native rhizobial isolates nodulating cowpea (Vigna unguiculata L. Walp.) in Mozambican soils

Identification and symbiotic characterization of indigenous rhizobial isolates are the basis for inoculant formulations needed for sustainable grain legume production. This study screened for morpho-genetic diversity of indigenous cowpea nodulating rhizobia in farmers’ fields across two contrasting agroecological zones of Northern Mozambique. The photosynthetic function induced by the isolates in their homologous cowpea was assessed. The results showed high genetic variability among the isolates based on morphology and ERIC-PCR fingerprinting. The trap cowpea genotype did not influence the diversity of isolates collected from the two different agroecologies, suggesting that the cowpea-rhizobia compatibility may be conserved at species level. Phylogenetic analysis of the 16S rRNA gene assigned representative rhizobial isolates to species in the Bradyrhizobium and Rhizobium genera, with some isolates showing high divergence from the known reference type strains. The isolates from both agroecologies highly varied in the number and biomass of nodules induced in the homologous cowpea, resulting in variable plant growth and photosynthetic activities. A total of 72% and 83% of the isolates collected from the agroecological zones 7 and 8 were respectively classified as highly effective candidates with > 80% relative effectiveness compared to plants fertilized with nitrate, indicating that elite native strains populated the studied soils. Moreover, the top 25% of high N(2)-fixing isolates from the two agroecologies recorded relative effectiveness ranging from 115 to 154%, values higher than the effectiveness induced by the commercial Bradyrhizobium sp. strain CB756. These strains are considered as having potential for use in inoculant formulations. However, future studies should be done to assess the ecologically adaptive traits and symbiotic performance under field conditions.