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Dinitrogen Fixation, Herbage Yield, and Rhizobial Preference of Selected Alfalfa Clones¹

Improvement of N₂ fixation in forage legumes requires development of methods for the identification of genetic variability in this trait and measurement of its expression in the field. Our objective was to investigate the N₂ fixation of clones of alfalfa (Medicago sativa L.) initially selected from adapted germplasm, over 4 years of growth in the field, with evaluation of the rhizobial preference of the clones in specific years. Dinitrogen fixation in the field was measured by the ¹⁵N isotope dilution method. Rhizobial preference was established by serology or by use of antibiotic resistant mutants. While rates of N₂-fixation varied substantially among clones in each year, three clones consistently ranked high across years, and three clones ranked low. Performance of the remaining five clones showed considerable variation with year of growth. Dinitrogen fixation was highly correlated (r = 0.70-0.99) with herbage yield within each year and across years. There were significant differences in nodulation among the clones in 1978 and in 1979. Nodule number per plant was correlated with milligrams of N₂ fixed per plant in both years (r = 0.63*, 0.65*) (*Significant at the 0.05 level) with herbage yield (r = 0.66*, 0.83*). Use of antibiotic resistant rhizobial strains, but not serology, showed significant differences among clones in preference for indigenous strains and for one mutant strain. However, both methods showed that most of the nodules were formed by indigenous strains. These results suggest that the capability for N₂ fixation of some clones derived from adapted alfalfa germplasm can be reproducibly determined in the field by the ¹⁵N isotope dilution method, even when specific knowledge of rhizobial preference of the host is unavailable. This may facilitate the identification of plant material potentially useful in breeding forage legumes for improved N₂ fixation capability.

Key Words: Physiological traits • Stability of performance • ₁₅N isotope dilution • Serology • Antibiotic resistance • Medicago sativa L.

² Plant physiologist, USDA-ARS; former research specialist, Univ. of Minnesota; research geneticist, and plant physiologist, in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Ave., St. Paul, MN 55108. Hardarson's present address is FAO/IAEA Agric. Biotech. Lab., A-2444 Seibersdorf, Austria.

Received for publication December 27, 1983.