Competitiveness and symbiotic efficiency of potential inoculant strains of Bradyrhizobium sp. (Vigna)

Abstract

Screening of ten Bradyrhizobium sp. ( Vigna) strains for symbiotic efficiency on green gram ( Vigna radiata (L) Wilczek) cultivar Pusa 105 in sterile soil revealed significant differences among strains with respect to nitrogenase activity and dry mass accumulation. Strain M-11-85 gave the highest nitrogenase activity while strain COG-15 resulted in maximum dry matter accumulation. On overall basis, strains COG 15, M 10, M 11 were found to be symbiotically efficient. All strains were tested for their efficiency and competitiveness both in pot and field conditions. Strain S 24 was found to be the best competitor followed by COG 15 in pot and field trials. The maximum grain yield was produced in plots inoculated with COG 15 (833 Kg ha -1) followed by M-ll-85 (717 Kg ha -1). The results conclusively demonstrated that crop productivity is a sum total of proportions contributed by efficiency and competitive ability of a strain. A less efficient strain (S24) with good competitive ability (54% nodule occupancy) produced a crop yield equal to that of an efficient strain (M-ll-85) with poor competitive ability (21% nodule occupancy). Strains COG 15, M-ll-85, M 10 and S 24 appear to have potential for inoculant production. Spatial differences in competitive ability in nodulating tap and lateral roots were also studied but no significant differences were obtained.