REGULATION OF SOYBEAN NODULE GAS PERMEABILITY AND NITROGEN FIXATION RATE

The gas permeability of leguminous nodules plays an important role in protecting nitrogenase from oxygen inactivation. The purpose of this research was to test the hypothesis that the nodule gas permeability is under active physiological regulation and that changes in the nodule gas permeability can in turn affect nitrogen fixation rates.; The first condition under which this hypothesis was tested was changing rhizosphere oxygen concentrations. Nitrogen fixation rates were immediately affected by alterations in oxygen concentration in a range of 0.06 mm('3) mm('-3) to 0.40 mm('3) mm('-3). These effects were transitory as several hours after altering the oxygen concentration acetylene-reduction rates in both intact field- and hydroponically grown soybean plants returned to rates similar to those observed under ambient conditions. Nodule respiration (oxygen uptake) responded to the external oxygen concentration in a similar fashion. The nodule gas permeability, however, varied in response to the alteration in the external oxygen concentration and reached steady-state values significantly different from those observed under ambient conditions.; The second condition under which this hypothesis was tested was changing soil temperature. Diurnal acetylene-reduction rates of intact field- and hydroponically grown soybean plants were found to be related to the soil temperature even when the diurnal cycles of photosynthetically active radiation and soil temperature were completely out of phase with each other. Changes in soil temperature also resulted in proportional changes in the nodule gas permeability which could not be explained in terms of a passive plant process.; It was concluded that in soybean the nodule gas permeability is a dynamic variable under active physiological control and which in addition to protecting nitrogenase from oxygen inactivation may play an important role in the regulation of symbiotic nitrogen fixation.