Acetylene reduction activity in salt‐tolerant alfalfa andrhizobium

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Abstract
Twenty‐nine alfalfa, Medicago sativa L., populations and 14 Rhizobium meliloti L. strains, which had survived sodium chloride (NaCl) concentrations of 264 and 528 mM, respectively, were tested for nodulation and acetylene reduction activity under in vitro zero NaCl conditions. The most efficient strains were 1027, 1111, (USDA‐Agricultural Research Service collection), RMZ1, and D7, (Utah State University collection). In a second experiment, the 29 alfalfa populations were inoculated with thesefour strains and grown in 0, 44, 88,132, or 176 mM NaCl. Nodule number seedling'’ and acetylene reduction activity differed significantly among alfalfa populations, rhizobium strains and NaCl concentrations; there were also highly significant interactions among these factors. Nodulation of all alfalfa populations was delayed 48–72 h by salt concentrations higher than 132 mM. Sodium chloride concentrations that affected symbiosis were lower than those that affected growth and survival of individual alfalfa genotypes or Rhizobium. The significant decrease in nodule number seedling−1 and rate of acetylene reduction in all 29 populations as NaCl concentrations increased suggests that responses to salt levels may be genetically controlled. The mechanism(s) governing such inheritance requires further research and may aid in the development of salt‐tolerant alfalfa and rhizobium strains.