A New Approach to the Screening of Efficient Strains of Rhizobia

Maximum benefit through Rhizobium inoculation is accrued when the strain of Rhizobium used for inoculation is a highly efficient nitrogen fixer and is also able to compete successfully with native rhizobial strains occupying infection sites on the host root hairs. In India, the problem of competition is more acute, especially for the Rhizobia specific for cowpea group of legumes: groundnut and bengal gram. Conventional methods of screening of rhizobial strains does not take into account the competitive ability of the strains. That is why, quite often inoculation with a highly efficient nitrogen fixing strain fails to register significant increase in yield due to its inability to dominate over native rhizobial strains. This means that for such promiscuous legume crops, efficient rhizobial strains can be obtained only through field tests, which however are not always practical.

Serology has been found to offer an useful tool to examine the competitive ability of the rhizobial strains in different soils, each of which have been found to be dominated by rhizobial population of a particular sero-group, specific for different host legumes. With the help of this technique, dominant sero-groups of Rhizobium of Cicer in different soil lypes of India have been determined. Selecting the most efficient nitrogen fixing strains from among those belonging to the dominant sero-group of a particular soil and using the strain in the soil which is dominated by rhizobial population of the same sero-group resulted in significant increase in yield (112 to 120%) in case of bengal gram (Cicer arietinum) grown in Delhi soil (I.A.R.I. Farm).