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Carbon Costs of Dinitrogen Fixation Associated with Dry Matter Accumulation in Alfalfa

Dep. of Agronomy & Plant Genetics, Univ. of Minnesota, 1991 Buford Circle, St. Paul, MN 55108
Dep. of Agronomy, Univ. of illinois, 1102 S. Goodwin Ave., Urbana, IL 61801, Athens, GA 30602

^* Corresponding author.

Dinitrogen fixation of modulated legumes theoretically may limit crop yield because of nodule needs for products of photosynthesis; however, this concept is poorly supported by experimental evidence. To determine how nodule C use might limit legume dry-matter accumulation, the C costs of N2 fixation of alfalfa (Medicago sativa L.) were investigated in the glasshouse daring two harvest-regrowth cycles. Plants were inoculated with either of two contrasting rhizobiai strains and grown in nil-N hydroponics to provide differing symbioses. Dry matter accumulation, total reduced N accumulation, and nodule gaseous exchange (respiration and nonphotosynthetic CO₂ fixation) were measared, and C costs were calculated. Dinitrogen fixation and nodule gaseous exchange varied with plant development and harvest. Plants nodulated by Strain I02F51 averaged a 68% higher rate of N₂ fixation on a unit nodule-mass basis than those nodulated by strain P207. Total N₂ fixation per plant, however, was similar due to the contrasting nodulation characteristics of the two symbioses. Nodule CO₂ fixation on a unit nodule-mass basis was greater in nodules formed by strain I02F51 than in those formed by strain P207; nodule CO₂ fixation per plant was similar for the two symbioses. Nodule CO₂ fixation apparently assimilated 25 to 30% of the CO₂ respired by nodules. Over the course of the experiment, the average C cost of N₂ fixation in nodules formed by strain P207 (9.4 mol C mol -~ N) was 59% greater than that of nodules formed by strain I02F51 (5.9 mol C mol^–1 N). Total dry matter accumulation during the experiment by plants nodulated by Strain P207 (with higher C costs) was 21% greater than that of plants nodulated by Strain I02F51 (with lower C costs). The results indicated that, in our experimental system, dry matter accumulation was unrelated to the C costs of N₂ fixation. These results contrast with what theory suggests, despite the association between specific activity of N₂ fixation and C costs. The results have implications to understanding symbiotic limitations to legume productivity and to transforming nonlegumes to fix N₂.

Received for publication February 9, 1990.