Nodulation, biomass production, and nitrogen fixation in alfalfa under drought

The effect of drought on nodulation, biomass production, and symbiotic nitrogen fixation was evaluated for seven strains of Rhizobium meliloti Dang. and three alfalfa (Medicago sativa L. and M. falcata L.) accessions. Alfalfa plants were grown for 10 weeks in plastic pots containing a sterilized mixture of sand and peat moss maintained at -0.03, -0.5, and -1.0 MPa. Number of nodules, total plant dry weight, and shoot nitrogen pool declined as drought increased from -0.03 to -1.0 MPa. Rhizobial strains varied markedly in their ability to form nodules and fix nitrogen under drought. Plants inoculated with strains UL 136, UL 210, and UL 222 formed nodules at -0.5 and -1.0 MPa and had plants with greater total dry weight and shoot nitrogen pool than plants with no nitrogen. These three rhizobial strains may be beneficial for increasing alfalfa production under water-limited conditions.