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

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

Accession: 002042997

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