Diversités phénotypique et moléculaire des microsymbiotes du Sulla du nord (Hédysarum Coronarium L. ) et sélection de souches rhizobiales efficientes

Summary

In order to promote the diversity of rhizobial genetic resources specific to sulla (Hedysarum coronarium L.), a prospective study was undertaken in 14 sites of seed production in northern Tunisia. The analysis of growth and production of cultivated sulla revealed variability of intra-specific behavior depends on both edapho-climatic parameters, and the indigenous rhizobial population. The exploration of sulla's nodulation in various sites unveiled the existence of symbiotic associations with high fixation (Oued Zarga and Tunis) or a failure of this activity due to the absence of specific rhizobial strains (Goubellat). Isolation of bacteria from sulla nodules has allowed the establishment of a collection of 45 isolates presenting a positive nodulation testing.

Analysis of the genotypic diversity of these strains by PCR/RFLP followed by the sequencing of the ADNr16S showed that 71% of strains have a similarity of more than 99% with R. sullae strain IS 123T. Phenotypic characterization by tests of tolerance to abiotic stress (salinity, pH, drought) revealed a tolerance limit varying respectively from 150 to700 mM for NaCl; 9 to10.5 for alkaline pH and -0,5 to -0,95 MPa for drought simulated by PEG 6000.

The assessment of symbiotic isolates efficiency tested with sulla has identified 19 strains particularly efficient. Among these, 2 strains HC5 and HC14, respectively classified as highly and moderately tolerant to osmotic stress, were evaluated in pots at different levels of water stress in comparison with sensitive strain (HC1). At 100% of useful reserves (UR), inoculated sulla with HC14 (200 mM NaCl and -0.9 MPa) has provided the best dry forage yield (4.75 g / plant). At 75% and 50% UR HC14 provided yields similar to those recorded with the strain HC5 (400 mM NaCl and -0.95 MPa). By contrast, plants inoculated with sulla HC1 (150 mM NaCl and -0.5 MPa) induced the lowest nodulation and forage yields.

In a non-irrigated field trial Tunis and Goubellat, rhizobial strains HC5 and HC14 provided competitive and efficient. The improvement yields of crude protein at flowering stage of sulla inoculated with HC14 ranged from 40 to 277% respectively in Tunis and Goubellat.

Keywords: Hedysarum coronarium L., Rhizobium sullae, Nitrogen fixation, Inoculation, Abiotic stress.

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