The Relationship between H(2) Evolution and Acetylene Reduction in Pisum sativum-Rhizobium leguminosarum Symbioses Differing in Uptake Hydrogenase Activity

Peas (Pisum sativum L.) were inoculated with strains of Rhizobium leguminosarum having different levels of uptake hydrogenase (Hup) activity and were grown in sterile Leonard jars under controlled conditions. Rates of H₂ evolution and acetylene reduction were determined for intact nodulated roots at intervals after the onset of darkness or after removal of the shoots. Hup activity was estimated using treatment plants or equivalent plants from the growth chamber, by measuring the uptake of H₂ or ³H₂ in the presence of acetylene. In all cases, the rate of H₂ evolution was a continuous function of the rate of acetylene reduction. In symbioses with no demonstrable Hup activity, H₂ evolution increased in direct proportion to acetylene reduction and the slopes were similar with the Hup⁻ strains NA502 and 128C79. Hup activity was similar in strains 128C30 and 128C52 but significantly lower in strain 128C54. With these strains, the slopes of the H₂ evolution versus acetylene reduction curves initially increased with acetylene reduction, but became constant and similar to those for the Hup⁻ strains at high rates of acetylene reduction. On these parallel portions of the curves, the decreases in H₂ evolution by Hup⁺ strains were similar in magnitude to their H₂-saturated rates of Hup activity. The curvilinear relationship between H₂ evolution and acetylene reduction for a representative Hup⁺ strain (128C52) was the same, regardless of the experimental conditions used to vary the nitrogenase activity.