Evaluation of effectiveness in mutant strains of Rhizobium by acetylene reduction relative to other criteria of N2 fixation.

TLDR: There was generally good agreement between the rate of acetylene reduction (assayed at a stage of peak activity for effective nodules only) and plant dry weight, plant nitrogen, or visual ratings of effectiveness in Rhizobium.

Abstract: The feasibility of the acetylene reduction technique for evaluation of comparative effectiveness inRhizobium was tested inR. leguminosarum, R. trifolii, andR. meliloti with strains which were closely related but differed widely in effectiveness. Several variables in sampling and handling of nodules were found to introduce significant error into this sensitive assay. Freezing of nodules destroyed all reducing activity. Removal of nodules from the roots, storage of detached nodules for several hours before assay, and the dry-wet condition of nodules during assay contributed to lowered ethylene production. The time pattern of appearance, increase, and decline of acetylene reducing activity paralleled that of leghemoglobin content in effective pea nodules assayed at different stages of development. In a comparison of strains from all 3 species, there was generally good agreement between the rate of acetylene reduction (assayed at a stage of peak activity for effective nodules only) and plant dry weight, plant nitrogen, or visual ratings of effectiveness. Several exceptional mutants which were rated as partly effective on the basis of nodule type or leghemoglobin content showed little acetylene reduction or N2 fixation. Suitability of the acetylene assay for strain comparison and the question of units of comparison with other criteria of N2 fixation are considered.