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Medicago sativa, a perennial species of the genus Medicago, is particularly cultivated in the Tunisian oases. Three M. sativa cultivated varieties (Magali A, Gabes2355, ABT805) and 1 genotype from a local provenance (Widhref) were cultivated on 4 Tunisian soil samples exhibiting contrasting pedo-climatic characteristics. Results showed that nodulation and aerial growth were significantly dependent on both variety and soil factors. Magali A showed the highest number of nodules, and varieties grown in Amra and Mateur soils produced the largest nodulation and aerial growth.

A collection of 250 rhizobial isolates was obtained from the 4 soils by trapping them on M. sativa. Molecular characterisation by PCR/RFLP of 16S genes showed that almost all isolates ( 158 of 160 isolates) belong to Sinorhizobium meliloti. The subsequent analysis of isolates from 2 contrasting soils ( Mateur, in the north of Tunisia, and Rgim Maatoug, in the south) demonstrated that strains harbour polymorphism at symbiotic genes, with 4 nodC genotypes, 2 nif DK genotypes and 2 nodA genotypes. The 2 soils contained different symbiont genotypes. The distribution of various profiles types was different according to geographic sites. The soils of Mateur and Rgim Maatoug do not contain the same genotypes. Eleven representative strains from this collection were evaluated for their nodulation and nitrogen fixation capacities. Results showed a high degree of variability among strains for nodulation and aerial growth parameters, and only 2 strains, GII/M1 and LII/M1, seem to be effective with all 4 varieties of M. sativa tested. Strains isolated from the soil of northern Tunisia were more efficient and produced fewer nodules than those trapped on southern soil.