La Symbiose Rhizobium-luzerne : Étude De La Diversité Rhizobienne Et Essais D’inoculation En Vue D’améliorer Les Rendements En Fourrage Dans Le Sahara Septentrional Est-algérien

Résumé: This work was carried out to evaluate genetic and phenotypic diversity in a collection of forty-eight bacterial strains isolated from the root nodules of cultivated alfalfa (Medicago sativa L.) and to test their symbiotic efficacy against the host plant by inoculation tests. Phenotypic characterization showed a diversity of responses. The strains exhibited a broad tolerance spectrum to NaCl and alkaline and neutral pH. In general, all strains showed good growth at 28 °C and 37 °C and poor growth at extreme temperatures (4 °C and 45 °C). Strains tested for different types of antibiotics were visibly different. The numerical taxonomy has placed strains from Ouargla and El Oued in one class and those from Ghardaïa in another. The reference strains Sm1021 and Sm2011 are classified in a class by themselves, rather close to that of Ghardaïa. The evaluation of the taxonomic position of the strains showed their affiliation to the genus Sinorhizobium. Phylogenetic analysis has shown the existence of two distinct groups where the first group strains, originating in Ghardaïa, belong to the species S. meliloti and S. kummerowiae. Group II strains, originating in Ouargla and El Oued, have been separated from the reference strains and suggest that they may represent a different lineage from the already defined species. The resulting classification of the nodC gene analysis reflects host specificity, while phylogeny based on chromosomal genes is independent of the host plant. Water and salt stress were found to significantly reduce nodulation, growth and yield of alfalfa. Inoculation with strains of Sinorhizobium improved the plant's ability to resist stress. The responses of the plants were clearly different according to the inoculant strains. Our results showed that strains Sm2011 and O152 formed the best symbiotic pairs under waterlimiting conditions, and the O172 strain, halotolerant in vitro, also confirmed it in vivo provided the plants with major advantages that allowed them to withstand high stress of 80 mM NaCl. We obtained a large amount of information on the local rhizobial strains nodulating M. sativa and on the use of these as effective inoculants for the improvement of forage yields in a Saharan context marked by major pedoclimatic constraints.

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