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Relationships Between Apparent Nitrogen Fixation and Carbon Exchange Rate in Alfalfa¹

The distribution of genetic potentials for symbiotic N fixation and carbon exchange rate (CER) was examined in ‘Vernal’ alfalfa (Medicago sativa L.). Symbiotic fixation capability was correlated with whole-plant CER in plants grown without combined N but was not correlated with innate photosynthetic efficiency measured as CER dm^–2 leaf area in the presence of combined N. Values for individual leaf CER of 35 seedlings grown on combined N ranged from 24 to 82 mg CO₂•dm^–2•hour^–1 at 300 ppm CO₂ with 1,400 µE•m^–S•see -1 photosynthetically active radiation. Carboxylation efficiencies based on either internal or external CO₂ concentrations, light and dark respiration of leaves, and leaf resistance also varied over a wide range, but there was little variation in the apparent quantum efficiencles and CO₂ compensation points. Twenty-five seedlings representative of the variation of CER observed in this initial screening were cloned, inoculated with Rhizobium meliloti strain 102F28, and grown without combined N. Leaf area, acetylene reduction, dry weight, and total N were measured for plants from each of the 25 clones. Whole-plant CER was determined for eight selected clones that had shown high, medium, or low individual leaf CER when grown with combined N. Whole-plant CER was correlated positively with leaf area, acetylene reduction, and total N of the eight selected clones. Leaf area was correlated positively with both acetylene reduction and total N of all 25 clones. Rankings of acetylene reduction, leaf area, total dry weight, or total N content for plants grown on N₂, however, were not correlated with individual leaf CER ranking of the same genotypes grown on combined N. Six genotypes representing a wide range of photosynthetic efficiency and N fixation potential showed few corresponding differences in top growth through three harvests in the absence of combined N.

Key Words: Medicago sativaL. • Photosynthesis • Genotypic variation • Rhizobium Meliloti

² Visiting scientist, graduate research assistant, and associate agronomist, respectively, Dep. of Agronomy and Range Science, Univ. of Calif., Davis, CA 95616. Present address of J. E. S.: Grassland Research Institute, Hurley, Maidenhead, Berks., U.K

Received for publication October 22, 1979.