IDENTIFICATION OF SALT-TOLERANT AND DROUGHT-TOLERANT RHIZOBIUM-MELILOTI L STRAINS

The first set of experiments identified sodium chloride (NaCl) tolerance of 92 accessions of Rhizobium meliloti L. from various rhizobia collections and arid and saline areas of the Intermountain West. Accessions were incubated in salinized (0, 176, 352, 528, 616, 704 or 792 mM) yeast extract mannitol (YEM) medium. Growth was measured by turbidity at 420 nm after 3 d in culture. Rhizobial strains were classified by their growth response at an optical density (OD) of 704 mM; Groups One and Two did not exceed 0.10 and 0.33, respectively. Forty three different rhizobial strains were identified as salt-sensitive and 49 as salt-tolerant at 704 mM NaCl. None grew in a saline solution of 792 mM NaCl. The second set of experiments investigated the drought tolerance of R. meliloti accessions that exhibited differential salt tolerance. Fifteen salt-sensitive and 15 salt-tolerant strains of R. meliloti from the first experiment were exposed to simulated drought stress by adding polyethylene glycol 6,000 (PEG-6,000) to the YEM medium at concentrations of 0, -0.4, -0.8 or -1.0 MPa. Rhizobium strains were incubated for 10 days at 25-degrees-C and growth turbidity was measured at 420 nm. Growth turbidity of the 30 accessions ranged from 100% at -0.4 MPa to 0% at -1.0 MPa. With one exception, strains that were more drought-tolerant (at -1.0 MPa) were also more salt-tolerant (616 mM). However, some of the more salt-tolerant strains at 616 mM were not the more drought-tolerant stains at -1.0 MPa. These salt- and drought-tolerant Rhizobium accessions are excellent models to study the mechanism(s) of such resistance, and to elucidate the role of genetics of NaCl and drought tolerance.