Genetic diversity and relationship between Bradyrhizobium strains isolatedfrom blackgram and cowpea

The genetic diversity of bradyrhizobial strains associated with blackgram a nd cowpea grown in two different agricultural soils (non-saline and saline) along the coastline of Tamil Nadu has been analysed. Phenotypically indist inguishable isolates were analysed for DNA polymorphism using random amplif ication of polymorphic DNA (RAPD) and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of 16S rDNA and nifD. Although the se bacteria belong to a group with a broad host range, RAPD analysis showed a considerable level of genetic diversity among the strains isolated from different host plants. Soil pH and salinity seem to have an effect on the s election of natural populations as revealed by PCR-RFLP of 16S rDNA. A comb ination of PCR-RFLP genotyping with nodulation studies indicates that monoc ropping of blackgram and the salinity of the soil have made ineffective rhi zobia the dominant genotype, thereby creating an ecological burden on their other compatible hosts. A group of strains and a type strain sharing three different 16S PCR-RFLP types were shown to have the same set of symbiotic genes as inferred from the PCR-RFLP pattern of nifD. Another group of cowpe a rhizobia that were found to be effective nitrogen fixers and sharing dist inct 16S profiles were found to have a different set of symbiotic genes.