Energetics of Symbiotic Nitrogen Fixation: The Relationship Between Oxygen,Malate and Hydrogen

The energetics of symbiotic nitrogen fixation was Investigated by measuring on Intact soybean plant, with an open flow device, the Influence of the root medium oxygen and malate content on the nodule nitrogenase acetylen reducing activity. It was shown that the maximum nodule activity required more oxygen than in air and was higher in presence of malate, but the photosynthesis modification by light intensity manipulation had no effect on nodule activity, although it affected the total mass of nodule per plant. The presence of an hydrogenase, which oxidized the hydrogen produced by the nitrogenase, had no beneficial effect on nitrogen fixation and growth of soybean, probably because of the oxygen limitation of nodule activity. In the absence of hydrogenase, the hydrogen production was higher than the minimum one measured on the purified enzyme; it varied with the environment and the host-plant cultivar.