Evidence that short‐term regulation of nodule permeability does not occur in the inner cortex

Abstract

Regulation of nodule permeability in response to short‐term changes in environmental and physiological conditions is thought to occur by occlusion of intercellular spaces in the nodule inner cortex. To test this hypothesis, the permeability of legume nodules was altered by adapting them to either 20 or 80% O2 over a 2.5‐h period. The nodules were then rapidly frozen, cryo‐planed and examined under cryo‐scanning electron microscopy for differences in the number, area or shape factor of intercellular spaces. Comparisons were made between whole nodules and specific nodule zones (outer cortex, middle cortex, inner cortex and central zone) in each treatment. Gas analysis measurements indicated that nodules equilibrated at 20% O2 had a 6.6‐fold higher permeability than those equilibrated at 80% O2 However, no significant differences were observed between pO2 treatments in the number of open intercellular spaces, the cross‐sectional area of those spaces, or the proportion of the tissue area present as open space in whole nodules or any nodule zone. Also, although nodules in both treatments possessed a boundary layer of tightly packed cells in the inner cortex, the total area of intercellular spaces between cells bordering this layer did not differ between treatments. Together these observations do not support the currently favored hypothesis that nodule permeability is regulated by opening or occlusion of intercellular spaces in the nodule inner cortex. Highly significant differences (P= 0.0006) were observed between O2 treatments in the shape factor of the open intercellular spaces in all nodule zones. Nodules equilibrated at 80% O2 had significantly more isodiametric spaces while those equilibrated at 20% O2 had more long, narrow spaces. This observation suggests that the critical step in the regulation of nodule permeability to O2 may be localized in the central, infected zone and involve changes in the ratio of the surface area of the intercellular space to the volume of the infected cell.