SURVIVAL OF CLOVER RHIZOBIA AND THEIR PLASMID-CURED DERIVATIVES IN SOIL UNDER HEAT AND DROUGHT STRESS

Strains of Rhizobium leguminosarum bv. trifolii often contain plasmids other than the symbiotic plasmid. The ecological function of these plasmids is not known. Three strains of R. leguminosarum bv. trifolii containing from 3 to 5 plasmids were used to obtain derivatives cured of individual plasmids and in some cases cured of more than one plasmid. The ability of the strains and their piasmid-cured derivatives to survive in soil under heat and moisture stress was tested. Resistance to heat stress was tested by incubating these bacteria in soil at 38 or 42-degrees-C and drought resistance was tested by subjecting them to a soil moisture tension of -1.5 MPa at 28-degrees-C. The results suggested an involvement of some plasmids in mediating the response to the stress conditions. Some cured derivatives survived better than the parent but for others the parent survived better than cured derivatives. Evidence for involvement of plasmids was stronger for resistance to heat stress than moisture stress.