Symbiotic competence of Sinorhizobium fredii on twenty alfalfa cultivars of diverse dormancy

Effective nodulation and efficient symbiotic biological nitrogen fixation w ith two alfalfa cultivars, 'ARC' and 'Giza 4' by some strains of the fast-g rowing soybean microsymbiont species Sinorhizobium fredii was recently obse rved for the first time. However, the host specificity and symbiotic compet ence of these strains on a range of genetically diverse alfalfa cultivars h ad not yet been investigated. Therefore, in this study, twenty genetically distinct alfalfa cultivars that differ in origin and fall dormancy, from fa ll non-dormant to completely dormant, were inoculated with each of four S, fredii strains or inoculated with S, meliloti strain USDA 1936, grown in th e growth-chamber or in the greenhouse for six weeks and then examined for g rowth vigor, nodulation and nitrogenase activities. Whereas S, fredii strai ns USDA 205 and USDA 208 effectively nodulated all of the alfalfa cultivars , strains USDA 201 and USDA 214 nodulated only two and six cultivars, respe ctively. The latter strains nodulated some of the semi-dormant and dormant alfalfa genotypes, but failed to nodulate any of the non-dormant genotypes. In many cases, strains USDA 205 and USDA 208 were as symbiotically compete nt with the alfalfa cultivars tested as was the alfalfa microsymbiont S. me liloti strain USDA 1936. Therefore, this study clearly shows that nodulatio n of alfalfa by S. fredii type strain USDA 205 and strain USDA 208 is not c ultivar specific.