Interaction effects between Rhizobium strain and bean cultivar on nodulation, plant growth, biomass partitioning and xylem sap composition

The interaction Rhizobium/Phaseolus was studied under controlled conditions using commercial bean cultivars and different Rhizobium strains. Plant growth (Shoot and pods), symbiotic parameters (nodule number and nodule mass) and N solutes (ureides; amino acids and nitrate) in xylem sap were determined. A significant variation of plant growth in response to inoculation with Rhizobium strains was observed. Inoculation with different strains affected biomass (shoot and pod growth) and N concentration in shoots, as a result of differences in N2 fixation rates, but also affected the harvest index, significant differences were observed in the concentration of N solutes in the xylem sap. The genotype of bean cultivar had an important effect on the symbiotic effectiveness of different strains, affecting variables like number and mass of nodules, and the concentration of N solutes in xylem sap. An interaction cultivar x strain effect was observed for plant growth, symbiotic parameters and ureides and aminoacids in sap. The results suggest the need to search for specific strain/cultivar combinations in order to maximize the N2 fixation capacity in beans. Because of the variation in sap ureide and amino acid concentration, the ureide method for N2 fixation assessment would require calibration for each Rhizobium/Phaseolus combination.