An osmotic hypothesis for the regulation of oxygen permeability in soybean nodules

Nodule permeability (P) controls the amount of O₂ entering the nodule, and thereby the rates of both nodule respiration and N₂ fixation. P may be regulated by changes in the effective thickness of a water-filled diffusion barrier in the nodule cortex. Regulation of diffusion barrier thickness was hypothesized to result from changes in the water content of intercellular spaces. Modulation of intercellular water would be a response to osmotic potential gradients in the tissue. To test this hypothesis, preliminary experiments examined three classes of solutes (soluble sugars, free amino acids, and ureides) in nodules of intact plants exposed to 10 or 21 kPa O₂ for 24 h. Neither soluble sugars nor free amino acids in nodules were responsive to O₂ treatments. However, nodule ureides accumulated after exposure to 10kPa O₂ for 24 h. A symplastic increase in nodule ureides under the 10kPa O₂ treatment compared to the 21 kPa O₂ treatment may have removed water from intercellular spaces in the nodule cortex and increased P. In addition, the nodule cortex of intact plants was infiltrated with water, polyethylene glycol (PEG), KC1, or Na-succinate solutions to determine the effect of intercellular water and osmoticants on dinitrogenase activity and P. Results from infiltrating the apoplast of the nodule cortex with osmotic solutions indicated that both increases in intercellular water and decreases in the apoplastic water potential decrease dinitrogenase activity and P. Furthermore, the inability to recover dinitrogenase activity and P following the infiltration of the cortex with PEG compared to either KCl or Na-succinate treatments may indicate that recovery was dependent upon removal of the solute from the apoplast.