Competitiveness of introduced and indigenous cowpea Bradyrhizobium strains for nodule formation on cowpeas [ Vigna unguiculata (L.) walp.] in three soils

Abstract

Streptomycin-resistant mutant strains of cowpea Bradyrhizobium. designated CP3-S, CBD-S and CKF-S were inoculated into three soils (Tikobo, Amo and Akuse) containing indigenous bradyrhizobia and assessed for relative nodule occupancy on cowpea plants [ Vigna unguiculata (L.) Walp.]. The nodule-forming strains were identified by the ability to grow on streptomycin-containing yeast extract mannitol medium and by serological differences. Each strain competed successfully against the native soil bradyrhizobia. Except for CKF-S in a few cases, the addition of about 10 5 cells of each strain g − soil, or various strain combinations, always increased nodule numbers, N yield and shoot dry weight of plants in all three soils, compared with the uninoculated soil. The magnitude of these inoculation responses was influenced by the soil type, with the highest effect occurring in the Amo soil. The mixed strain inoculation frequently gave plant responses intermediate to those obtained using the individual strains constituting the particular mixture. At 10 2 cells of CP3-S or CBD-S g −1 soil, the inoculation responses disappeared. suggesting that the observed increases in harvest measurements were due to the large cell populations used. The strains differed in competitiveness for nodule formation, with strain CP3-S being the most competitive. Inoculated alone, or in competition with the other strains and native soil bradyrhizobia, CP3-S formed between 60 and 94% of the nodules sampled. Strain CP3-S formed more nodules than strain CBD-S even when the CBD-S population was ten times greater. However, when the relative CBD-S :CP3-S population was increased to 1000:1, CBD-S occupied more nodules than CP3-S. These results indicate the possibility of increasing nodulation, N3 fixation and yield of field grown cowpeas through inoculation with selected strains.