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Rhizobial Strain Preference of Alfalfa Populations Selected for Characteristics Associated with N₂ Fixation¹

Preference for strains of Rhizobium meliloti by alfalfa (Medicago sativa L.) genotypes was studied in a series of experiments to determine whether selection for physiological and morphological traits associated with N₂ fixation capability modified the preference of alfalfa for rhizobial strains. In the first two experiments, an unselected alfalfa population and eight subpopulations were inoculated with antibiotic-resistant mutants of Rhizobium to study the effect of plant selection on preference for strains of Rhizobium. A series of indigenous isolates of Rhizobium was obtained from nodules of alfalfa subpopulations grown in nonsterile field soil and tested for effectiveness. A second set of experiments was similarly conducted with five populations: two unselected alfalfa populations, a population derived from each unselected population by three cycles of selection for enhanced nitrogenase activity, and the F₁ between the two third-cycle populations.

Plant selection for high and low levels of N₂ fixation potential modified the preference of alfalfa for antibiotic resistant mutant strains of R.meliloti. Selected alfalfa subpopulations showed increased nodule occupancy by indigenous and by effective mutant strains compared with the unselected original population. In other experiments, nodule occupancy by an effective mutant increased on selected alfalfa populations compared with that of the unselected populations, while nodule occupancy by indigenous Rhizobium decreased.

Generally, the indigenous isolates from the alfalfa entries selected for high levels of N₂ fixation potential were more effective than were the isolates of Rhizobium from unselected populations and subpopulations selected for low levels of N₂ fixation potential. It was concluded that plant selectlon programs should be conducted with the most effective and competitive strains of Rhizobium available. This should increase the likelihood that selected alfalfa populations will show preference for very effective strains of indigenous Rhizobium.

Key Words: Medicago sativa • Rhizobium meliloti • Effectiveness assay • Competitiveness • Antibiotic-resistant mutant • Inoculum

² Microbiologist, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (Present address: Joint FAO/IAEA Divison, International Atomic Energy Agency, Seibersdorf Laboratory, A-2444 Seibersdorf, Austria), plant physiologist, research geneticist and plant physiologist, USDA/SEA/AR in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Ave., St. Paul, MN 55108.

Received for publication February 9, 1981.

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