Stem and root nodules on the tropical wetland legume Aeschynomene fluminensis

Aeschynomene fluminensis Veil., originally obtained from flooded areas of the Pantanal Matogrossense region of Brazil, was grown under stem-flooded or non-flooded conditions for 70 d after inoculation with isolates of photosynthetic stem nodule rhizobia obtained from native A. fluminensis. Stem nodules formed only on submerged stems of flooded plants (mean of 25 per plant), and did not form on aerial parts, although they were capable of growing and fixing N₂ after drainage of the stems. Root nodules formed on both non-flooded and flooded plants but were usually decreased in number by flooding (from means of 124 to 51 per plant, respectively). Flooding (and stem-nodulation) resulted in an increase in shoot (and a decrease in root) dry weight, regardless of rhizobial isolate. Stem nodules were attached by a wide collar of aerenchymatous tissue at the base of the nodule. There were large air spaces in the stem where nodules were subtended and these were continuous with nodule aerenchyma/outer cortex. In addition, aerenchyma and spongy tissue at the base of the nodule connected both flooded and non-flooded root nodules to large intercellular spaces in the root cortex. The stem and root nodules were ovoid in shape, and essentially aeschynomenoid in type, i.e. the central infected tissue was without uninfected, interstitial cells. Root nodules had a similar structure to stem nodules (although stem nodules were generally larger), and flooded root nodules were approximately twice the size of non-flooded nodules. The infected tissue of root and stem nodules consisted of spherical, bacteroid-containing cells containing one or two rod-shaped bacteroids per peribacteroid unit and prominent organelles. Infection threads were observed in root but not in stem nodules. The cortex of stem and root nodules had an apparent oxygen diffusion barrier, consisting of concentric layers of small cells with interlocking cell walls and few intercellular spaces. Cell layers external to these consisted of larger cells and intercellular spaces, with some spaces being occluded with an electron-dense material that contained a glycoprotein recognized by the monoclonal antibodies MAC236 and MAC265. The amount of glycoprotein occlusions did not appear to differ between nodule types or treatments, although stem nodules contained intracellular glycoprotein vesicles adjacent to cell walls. The exterior of the nodules consisted of an epidermis of thin flattened cells with occasional lenticels. Amyloplasts were common in lower stem and hypocotyl nodules, but fewer in flooded or non-flooded root nodules. Upper stem nodules (i.e. those within 6 cm of the water surface) differed from more profoundly submerged stem nodules by having chloroplasts throughout the cortex. Root nodules did not contain chloroplasts, and undifferentiated plastids were found mainly in lower stem nodules.