Variation in symbiotic effectiveness of Bradyrhizobium sp. through plant passage in peanut (Arachis hypogaea L.

The soil bacteria classified as Bradyrhizobium sp. (Arachis) comprises a heterogeneous group with extensive variability in nitrogen fixation traits when in association with the host plant. The purpose of this work was to evaluate the potential use of plant passage as a method for selection of Bradyrhizobium sp. isolates with superior symbiotic effectiveness on peanut genotypes.;The effects of repeated plant passage of Bradyrhizobium sp. strains on nitrogen fixation in peanut cultivars were investigated in two greenhouse experiments. First, a series of 4 selection cycles were conducted to obtain nodule isolates (germplasm) from each strain/cultivar combination at each cycle. Bradyrhizobium strains 3G4b20, CB756 and 3G4b5 were repeatedly passed on two Arachis hypogaea cultivars Argentine and Florigiant. Each nodule isolate from the cultivar/strain combination were pooled and used as an inoculum source for the next passage. In the second phase, an overall evaluation of plant passage was conducted which included all plant passage isolates and the original strains. The main objective was to compare the symbiotic effectiveness of the Bradyrhizobium isolates obtained in each cycle without the seasonal effect on plant yield observed during the individual selection cycles.;The plant passage isolates derived from Bradyrhizobium strains 3G4b20 and CB756 significantly increased the total nitrogen and plant shoot dry weight of the peanut cultivars Argentine and Florigiant when compared with the original strains. In general, these increases were not correlated with nodule number or nodule mass. The effectiveness of Bradyrhizobium strain 3G4b5 did not significantly change with the plant passages performed.;In the final evaluation of the plant passage study, the influence of plant age on symbiotic effectiveness of plant passage isolates and the amino acid content of nodule cytosol were also investigated. Generally, most of the significant increases in symbiotic effectiveness due to plant passage were confined to cultivar Florigiant when strains 3G4b20 and CB756 were used as inocula. These changes were correlated with the sampling date. Strain 3G4b20 expressed significant changes between 8 and 10 weeks after planting, whereas changes induced by strain CB756 were expressed between 4 and 8 weeks after planting. The levels of the amino acid asparagine were correlated with the increases in nitrogen fixation due to plant passage.