Anatomy of the Legume Nodule Cortex With Respect to Nodule Permeability

Abstract

We describe the anatomy of the cortex of a range of legume nodules (43 species of 31 genera of 13 tribes, principally of the Papilionoideae). To our knowledge, the nodule anatomy of 19 of the genera examined (30 species) was previously unreported. All nodules have a layer(s) in the inner cortex with distorted radial walls ('boundary layer') and occlusions of the intercellular spaces within the middle cortex. The 'nodule endodermis' as defined by Frazer (1942) was difficult to identify unless modified by sclerification or tannin deposition. In other species, the outer cortex contains a well developed periderm, often with several layers of tannin-containing cells. When present, sclereids contain a living cytoplasm, and are interspersed with parenchyma cells to form a water-filled barrier c. 50 μm in width about the nodule. Nodule sclereids are apparent within 5 days of inoculation in Glycine max. Sclerification of the common endodermis did not significantly increase following growth at 50 or 80% rhizosphere pO₂. We suggest that the intercellular space deposition, the common endodermal layer and the periderm layer will act as a fixed resistance to gas diffusion into the nodule, allowing for a fine control of a variable resistance (boundary) layer within the inner cortex.