Effect of salinity on Rhizobium growth and survival

This study examines the effect of salinity on the growth and survival of Rhizobium spp. in culture media and soil. Eleven isolates from saline and nonsaline environments were compared. The growth (mean doubling time) of all strains and species tested decreased when the electrical conductivity of the culture medium (yeast extract-mannitol) was raised from 1.2 mS cm to 6.7 mS cm (15% seawater equivalent) or to 13.1 mS cm (28% seawater equivalent). Three of eleven strains failed to grow at 13.1 mS cm. Although growth was affected by salinity, four strains selected from the growth rate study could survive in extremely high concentrations of salt. Two strains with growth rates sensitive to salt and two strains with growth rates relatively unaffected by salt were inoculated into solutions with electrical conductivities of up to 43.0 mS cm (92% seawater equivalent). Not only did all four strains survive the initial osmotic shock (at 5 h after inoculation), but it was not until 27 days after inoculation that the sensitive strains exhibited a significant reduction in viable numbers. The salt-tolerant strains survived for more than 65 days with no reduction in viable counts. The interaction between soil moisture tension and soil salinity in relation to Rhizobium survival in gamma-irradiated soil was also examined. Six treatment combinations were used, ranging from -0.1 bars and 0.2 mS cm to -15 bars and 12 mS cm. Sensitive strains declined from 10 to 10 organisms per g of soil after 84 days of incubation at -15 bars and 12 mS cm. Tolerant strains survived for the same period with no loss in viable numbers. The results of these experiments indicate that many strains of Rhizobium can grow and survive at salt concentrations which are inhibitory to most agricultural legumes. The emphasis of research concerning the effects of salinity on symbiotic nitrogen fixation should, therefore, be directed to aspects of the symbiosis other than the survival of the Rhizobium spp.