Hydrogen evolution: A major factor affecting the efficiency of nitrogen fixation in nodulated symbionts

Abstract

A survey revealed that H evolution is a general phenomenon associated with N fixation by many nodulated N-fixing symbionts. An evaluation of the magnitude of energy loss in terms of the efficiency of electron transfer to N, via nitrogenase, in excised nodules suggested that H production may severely reduce N fixation in many legumes where photosynthate supply is a factor limiting fixation. With most symbionts, including soybeans, only 40-60% of the electron flow to nitrogenase was transferred to N. The remaining was lost through H evolution. In situ measurements of H evolution and acetylene reduction by nodulated soybeans confirmed the results obtained with excised nodules. In an atmosphere of air, a major portion of the total electron flux available for the reduction of atmospheric N by either excised nodules or intact nodulated plants was utilized in the production of H gas. Some nonleguminous symbionts, such as Alnus rubra, and a few legumes (i.e., Vigna sinensis) apparently have evolved mechanisms of minimizing net H production, thus increasing their efficiency of electron transfer to N. The extent of H evolution during N reduction is a major factor affecting the efficiency of N fixation by many agronomically important legumes.