Gaseous diffusive properties of soybean nodules cultured with non‐ambient pO₂

Measurements of the short‐term response of nodulated roots of soybean ( Glycine max L. Merr, cv. Harosoy: Bradyrhizobium japonicum USDA 16) to rapid changes in surrounding pO 2 indicate that their ability to reversibly adjust gaseous diffusive resistance is retained whether plants are cultured in rhizospheres of very low (2.8%) or very high (61.2%) pO 2 . Thus the capacity for reversible short‐term diffusion adjustment is additional to structural changes in the fixed diffusional barriers of nodules which allow their continued fixation of N 2 in unfavourably high or low external pO 2 . Anatomical evidence, involving quantitative measurement of intercellular spaces in the cortical tissues using electron microscopy of thin sections, indicates that the major fixed diffusional barrier is a boundary layer of cells in the inner cortex which may be as small as one cell thick in nodules from 2.8% O 2 to 5 or 6 cells thick, and almost completely devoid of intercellular spaces, in those from 61.2% O 2 . The data are interpreted to indicate that the variable diffusion harrier is distinct from the boundary layer and is most likely to be a property of cells and/or intercellular spaces inside the boundary layer of the nodule cortex.