DIMENSIONS AND DISTRIBUTION OF INTERCELLULAR SPACES IN CRYO-PLANED SOYBEAN NODULES

The ability of legume nodules to regulate their permeability to gas di ffusion has been attributed to physiological control over the size and distribution of gas-filled intercellular spaces within the nodule cor tex. To examine the size and distribution of intercellular spaces and to determine whether they were filled with gas (high diffusion permeab ility) or liquid (low diffusion permeability), whole nodules were froz en in liquid nitrogen slush (-210-degrees-C), and then either cryo-fra ctured or cryo-planed before being examined by cold-stage scanning ele ctron microscopy (SEM). The cryo-planed tissue was found to have many advantages over cryo-fractured nodules in providing images which were easier to interpret and quantify. Intercellular spaces throughout the nodule were examined in both tangential and medial planed faces. Since no differences were observed between views in either the size or shap e of the open intercellular spaces, it was concluded that the intercel lular spaces of nodules were not radially oriented as assumed in many mathematical models of gas diffusion. The inner cortex region in the n odules had the smallest intercellular spaces compared to other zones, and less than 10% of the intercellular spaces were occluded with any t ype of material in the central zone regions. Vacuum infiltration of no dules with salt solutions and subsequent cryo-planing for SEM examinat ion showed that open and water-filled intercellular spaces could be di fferentiated. The potential is discussed for using this method to stud y the mechanism of diffusion barrier regulation in legume nodules.