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Carbon costs of dinitrogen fixation associated with dry matter accumulation in alfalfa

, : Carbon costs of dinitrogen fixation associated with dry matter accumulation in alfalfa. Crop Science 31(4): 985-992

Dinitrogen fixation of nodulated legumes theoretically may limit crop yield because of nodule needs for products of photosynethesis; 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 during two harvest-regrowth cycles. Plants were inoculated with either of two contrasting rhizobial 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 measured, and C costs were calculated. Dinitrogen fixation and nodule gaseous exchange varied with plant development and harvest. Plants nodulated by Strain 102F51 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 102F51 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-1 N) was 59% greater than that of nodules formed by strain 102F51 (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 102F51 (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.

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