Efficiency of N2 fixation in Vicia faba L. in combination with different Rhizobium leguminosarum strains

The efficiency of symbiotic dinitrogen (N-2) fixation in Vicia faba L in combination with 3 different Rhizobium leguminosarum strains was studied in a pot experiment during vegetative and reproductive growth. The objective of the experiments was to assess variability among Rhizobium strains inoculated on single legume species and determine possible reasons for observed variations. Dry matter formation, N-2 fixation and the carbon (C) costs of N-2 fixation were determined in comparison with nodule free plants grown with urea. Nodule number and the capacity of different respiratory chains in the nodules were also measured. The plants inoculated with the Rhizobium strain A 37 formed less dry matter and fixed less N compared to the other two Rhizobium Strains (Vic 1 and A 150). This coincided with a lower number of nodules and higher C costs of N-2 fixation. The C costs for N-2 fixation were in all cases significantly lower during reproductive growth compared to vegetative growth. Neither the latter nor the differences in C expenditure for N-2 fixation between the Rhizobium strains could he explained in terms of differences or shifts in the capacity of different respiratory chains in the nodules.