HYDROGEN MEASUREMENTS PROVIDE DIRECT EVIDENCE FOR A VARIABLE PHYSICALBARRIER TO GAS-DIFFUSION IN LEGUME NODULES

A considerable weight of evidence has accumulated to show that O-2 dif fusion into legume nodules is controlled by a variable physical barrie r which balances the influx of this gas against its respiratory consum ption. Recently, however, the existence of such a barrier has been cha llenged on the grounds of lack of evidence for structural changes and on the possibility that there are metabolic and/or biochemical alterna tives which might substitute for the barrier. Such speculation may be justified for the apparent diffusion resistance to O-2 and CO2 where a range of potential metabolic reactions make it difficult to separate physical and chemical processes. However, this ambiguity does not appl y to H-2 within nodules formed by rhizobium strains lacking an uptake hydrogenase (Hup(-ve)). Within such nodules H-2 generated as a by-prod uct of N-2 fixation cannot be further metabolized. Thus the steady-sta te relationship between internal concentration of H-2 and its rate of efflux from the nodule can only be determined by the resistance of a p hysical barrier to diffusion. Data are presented here on H, concentrat ions and rates of efflux from nodules of soyabean (Clarke/USDA16 symbi osis) subjected to detopping and stepped increases in pO(2), which pro vides incontrovertible evidence for the existence of such a variable p hysical barrier.