NITROGEN-FIXING STEM NODULES OF THE LEGUME, DISCOLOBIUM-PULCHELLUM BENTH

We report on the structure of N-2-fixing nodules formed on the stem of Discolobium pulchellum Benth., an aquatic legume in the subfamily Pap ilionoideae, tribe Aeschynomeneae, from the flooded areas in the 'Pant anal Matogrossense' region of Brazil. The stem (and root) nodules were obligately aquatic, requiring permanent submergence in water or flood ed soil, and receive oxygen via profuse aerenchyma covering the lower stem. Of the 69 isolates of rhizobia isolated from stem and root nodul es, 70 % were fast-growing acid producers and 30 % were slow growers. The rhizobia were not photosynthetic. Nodules were connected to the st em, and the vascular system from the stem branched throughout the nodu le, penetrating the infected tissue within finger-like ingrowths of co rtex. In both stem and root nodules, infected tissue was aeschynomenoi d or desmodioid, that is, without uninfected (interstitial) cells. The infected cells in stem nodules were vacuolate, with visible infection threads. The inner cortex was rich in amyloplasts and contained the c omponents of an oxygen diffusion barrier (a boundary cell layer withou t intercellular spaces and glycoprotein occlusions of intercellular sp aces in other cell layers). The mid-cortex, external to the boundary l ayer, consisted of loosely-packed cells and these were continuous with stem aerenchyma. The outer part of the nodules was made up of phellog en-derived cells forming a periderm, or 'corky' layer of cells. The pe riderm formed large lenticels above cortical vascular bundles. These l enticels also connected with the stem aerenchyma. Root nodules differe d only in that infected cells were not vacuolate, bacteroids were larg er and contained more poly-P-hydroxybutyrate (PHB) and there was less aerenchyma/lenticellular tissue. Stem and root nodule structure is dis cussed in terms of adaptations to O-2 constraints in an aquatic enviro nment.