Gene centres,a source for genetic variants in symbiotic nitrogen fixation: Host-induced ineffectivity inPisum sativum ecotype fulvum

Summary Pisum sativum ecotype fulvum forms ineffective nodules with Rhizobium strains, isolated from effective nodules of the cultivated pea in Europe. Rhizobium strains isolated from nodules of fulvum peas in Israel are fully effective on this host plant, but in association with the cultivated pea they induce nodules of poor N₂-fixing activity. The distribution of these fulvum-specific Rhizobium strains is restricted to regions where the fulvum pea occurs naturally. Rhizobium strains from other geographical regions induce mainly ineffective, or partially effective nodules on fulvum plants.A wide genetic variation, with regard to symbiotic response to a standard set of Rhizobium strains, was demonstrated in the fulvum plants collected in Israel. Based on variation in N₂-fixation three groups of plants can be distinguished. These plants offer the possibility for the study of host-genetic control on symbiotic nitrogen fixation.