Abstract

Soil bacteria Sinorhizobium meliloti are of enormous agricultural value, because of their ability to fix atmospheric nitrogen in symbiosis with an important forage crop legume-alfalfa. The main aim of this study was (i) to isolate indigenous S. melilotistrains from different field sites in Saudi Arabia, (ii) to assess genetic diversity and genetic relationships amongst strains of natural populations and (iii) to provide information about nodulation and symbiotic efficiency of indigenous S.  melitlotistrains.  Nineteen strains isolated from alfalfa nodules collected from different field sites and one reference strain were analyzed. Genetic characterization by rep-PCR and RAPD-PCR was applied to study the status of S. melitloti populations inhabiting nodules of alfalfa. Cluster analysis of rep-PCR profiles showed significant differences among S. meliloti isolates.  Both methods resulted in almost identical grouping of strains.  Among indigenous strains two divergent groups could be determined. The biggest differences were detected among the reference strains along with two local isolates and all field isolates. Quantitative expression of symbiotic efficiency was evaluated by measurement of total nitrogen in plants and dry matter yield of plants.  All strains nodulated alfalfa cultivar CAF 101 but with different efficiency.  Significant differences in dry mater yield of alfalfa as well as total nitrogen content were determined depending on the strain used. The results indicate that sixty one percent of indigenous S. meliloti strains can be characterized as low efficient strains.

Key words: Alfalfa, Sinorhizobium meliloti, symbiotic efficiency, genetic diversity.