Hydrogen measurements provide direct evidence for a variable physical barrier to gas diffusion in legume nodules

A considerable weight of evidence has accumulated to show that O2 diffusion into legume nodules is controlled by a variable physical barrier which balances the influx of this gas against its respiratory consumption. Recently, however, the existence of such a barrier has been challenged on the grounds of lack of evidence for structural changes and on the possibility that there are metabolic and/or biochemical alternatives which might substitute for the barrier. Such speculation may be justified for the apparent diffusion resistance to O2 and CO2 where a range of potential metabolic reactions make it difficult to separate physical and chemical processes. However, this ambiguity does not apply to H2 within nodules formed by rhizobium strains lacking an uptake hydrogenase (Hup-ve). Within such nodules H2 generated as a by-product of N2 fixation cannot be further metabolized. Thus the steady-state relationship between internal concentration of H2 and its rate of efflux from the nodule can only be determined by the resistance of a physical barrier to diffusion. Data are presented here on H2 concentrations and rates of efflux from nodules of soyabean (Clarke/USDA16 symbiosis) subjected to detopping and stepped increases in pO2, which provides incontrovertible evidence for the existence of such a variable physical barrier.