EVIDENCE THAT SHORT-TERM REGULATION OF NODULE PERMEABILITY DOES NOT OCCUR IN THE INNER CORTEX

Regulation of nodule permeability in response to short-term changes in environmental and physiological conditions is thought to occur by occ lusion of intercellular spaces in the nodule inner cortex. To test thi s hypothesis, the permeability of legume nodules was altered by adapti ng them to either 20 or 80% O-2 over a 2.5-h period. The nodules were then rapidly frozen, cryo-planed and examined under cryo-scanning elec tron 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 t hat nodules equilibrated at 20% O-2 had a 6.6-fold higher permeability than those equilibrated at 80% O-2. However, no significant differenc es were observed between pO(2) treatments in the number of open interc ellular spaces, the cross-sectional area of those spaces, or the propo rtion of the tissue area present as open space in whole nodules or any nodule zone. Also, although nodules in both treatments possessed a bo undary layer of tightly packed cells in the inner cortex, the total ar ea 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 c ortex. Highly significant differences (P=0.0006) were observed between O-2 treatments in the shape factor of the open intercellular spaces i n all nodule zones. Nodules equilibrated at 8O% O-2, had significantly more isodiametric spaces while those equilibrated at 20% O-2, had mor e long, narrow spaces. This observation suggests that the critical ste p in the regulation of nodule permeability to O-2, may be localized in the central, infected zone and involve changes in the ratio of the su rface area of the intercellular space to the volume of the infected ce ll.