Competition among Bradyrhizobium strains for nodulation of green gram (Vigna radiata): use of dark-nodule strain.

The competitiveness of dual-strain inoculum of Bradyrhizobium strains S24 and GR4 was demonstrated for nodulation of green gram (Vigna radiata). Strain S24 formed pink nodules, GR4 produced visually distinguishable dark-brown nodules. When a mixture of these Bradyrhizobium strains was applied as inoculum, nodules of both pink and dark-brown types were formed on the same root. The strain GR4, which was less competitive than strain S24, was mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine to obtain pigment-diverse mutants and six selected mutants were screened for symbiotic parameters. One mutant produced pink nodules and appreciably increased plant dry mass. The competitive ability of this mutant lacking brown pigment was compared with that of strain S24 by using antibiotic resistance markers; it showed increased nodulation competitiveness than its parent strain GR4. The dark-brown nodule-phenotype could be useful in evaluating nodulation competitiveness of "cowpea miscellany" bradyrhizobia in soil where dark-brown nodule-forming strains are not indigenous.