1998 Volume 43 Issue 4 Pages 391-397
Eleven species or subspecies of Medicago widely distributed in the world were evaluated in association with six strains of R. meliloti, isolated from nodules of six Medicago species. Significant differences in acetylene reduction ability and in plant dry matter among Medicago species were recognized, indicating considerable genetic variation in the symbiotic nitrogen fixation among species in the genus of Medicago. Perennial species M. sativa ssp. sativa and M. sativa ssp. falcata produced effective nodules to all of the six strains of R. meliloti, but annual species M. rigidula, M. arabica, M. aculeata and M. rugosa were effective to only four strains, suggesting that the latter was more specific to strains of R. meliloti in nitrogen fixation than the former. The six strains of R. meliloti isolated from six species of Medicago have differentiated in the compatibility with the plant species; GU 94-1, GU 95-1 and GU 96-1 were more specific to host plant species than the other three strains. The highest acetylene reduction ability and plant dry matter was obtained when a strain of R. meliloti was inoculated to the Medicago species from which the strain was isolated, indicating that the best compatibility was in the combination of the strain of R. meliloti with the species from which the strain was isolated. It can be estimated that the host plant produced very effective large nodules when associating symbiotically with the strain of R. meliloti which has the best compatibility. We can conjecture that it is the start which leads to coevolution of R. meliloti with Medicago species.
