Varability of the plant-Rhizobium symbiosis can be attributed to additive effects of the plant genotype and the Rhizobium strain and the nonadditive effects of specific plant and Rhizobium combinations. The relative contribution of these sources of variability is important in adopting the best procedure to maximize nitrogen fixation. Six peanut (Arachis hypogaea L.) genotypes were grown in all possible combinations with 10 Rhizobium strains in order to estimate the relative importance of the three genetic components of symbiotic variability. Additive genetic effects of host and Rhizobium genotype were significant for plant color, nodule number and weight, N₂ (C₂H₂) fixed, and shoot dry weight. Nonadditive variation attributable to specific host-strain combinations was significant for all traits measured except for shoot dry weight. The large additive effects of the host genotype for nodule weight and shoot weight suggest that the variability for these traits can best be exploited by selection of host plants. However, the large nonadditive effects for nodule number and N₂ (C₂H₂) fixed suggest that these traits can best be improved by simultaneous selection of both host and bacterium. Rhizobial strains NC123 and 3G4b21 were found to have significant stability variances indicating that these strains show host specificity, whereas strain RP182-13 exhibited a nonsignificant stability variance with a high mean for all traits in symbiosis with all host genotypes.

The poor response to inoculation of the Spanish genotypes in comparison to the nitrogen control suggests that superior strains for these genotypes must be identified.