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Effect of Temperature on H₂ Evolution and Acetylene Reduction in Pea Nodules and in Isolated Bacteroids

Department of Molecular Biology and Plant Physiology, University of Aarhus, DK-8000 Aarhus C, Denmark

Nitrogenase (EC 1.7.99.2) activity in pea (Pisum savitum) nodules formed after infection with Rhizobium leguminosarum (lacking uptake hydrogenase) was measured as acetylene reduction, H₂ evolution in air and H₂ evolution in Ar:O₂. With detached roots the relative efficiency, calculated from acetylene reduction, showed a decrease (from 55 to below 0%) with increasing temperature. With excised nodules and isolated bacteroids similar results were obtained. However, the relative efficiency calculated from H₂ evolution in Ar:O₂ was unaffected by temperature. Measurements on both excised nodules and isolated bacteroids showed a marked difference between acetylene reduction and H₂ evolution in Ar:O₂ with increased temperature, indicating that either acetylene reduction or H₂ evolution in Ar:O₂ are inadequate measures of nitrogenase activity at higher temperature.