Nitrogen fixation and nodule occupancy by native strains of Rhizobium on different cultivars of common bean (Phaseolus vulgaris L.)

A field experiment under rainfed conditions was conducted in Durango, México, to assess N₂-fixation of three cultivars of common bean (Phaseolus vulgaris L.) using ¹⁵N-methodology. In addition, diversity of rhizobial isolates obtained from nodules of the different plant genotypes was evaluated by intrinsic antibiotic resistance (IAR), PCR using enterobacterial repetitive intergenic consensus (ERIC) primers, PCR-RFLP analysis of the 16S rRNA gene and multilocus enzyme electrophoresis (MLEE). Selected isolates were used to determine acetylene reduction and competitive ability under greenhouse conditions. The three cultivars tested did not show high variation in N₂-fixation, the %Ndfa values ranged from 19 to 26%. Variability in N₂-fixation efficiency among various native rhizobial isolates was very high and our results indicate that differences in competitive abilitiy exist also. PCR-RFLP of the 16S rRNA gene and MLEE revealed that most of the isolates belong to the species Rhizobium etli. Intrinsic antibiotic resistance analysis and ERIC-PCR showed high diversity among isolates. In contrast, our results using MLEE show low genetic diversity (H = 0.105).