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

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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 pO2 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%) pO2. 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 N2 in unfavourably high or low external pO2. 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% O2 to 5 or 6 cells thick, and almost completely devoid of intercellular spaces, in those from 61.2% O2. 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.