THE STRUCTURE OF NITROGEN-FIXING ROOT-NODULES ON THE AQUATIC MIMOSOID LEGUME NEPTUNIA-PLENA

Nodules of the aquatic mimosoid legume Neptunia plena (L.) Benth. were always found associated with roots but not stems. They appeared macroscopically 10 and 20 d after inoculation on plants grown hydroponically and in vermiculite, respectively. The development of nitrogen-fixing cells occurred in a series of stages not yet reported in legume nodule formation: initial infection was apparently intercellular and rhizobia spread between cells and through intercellular spaces before penetrating individual host cells by means of infection threads. Subsequently nodule development was broadly similar to that described for indeterminate papilionoid nodules. The infection threads of Neptunia and pea nodules contained a matrix with a common epitope, which was, in Neptunia, extruded from the infection thread at the point of bacterial release.The central tissue contained infected and interstitial cells and was surrounded by a three-layered cortex and a phellem. Bounding the infected region was a layer two to three cells thick with large, unoccluded intercellular spaces. External to this was a layer, one or more cells thick, in which the cell walls were interlocked, reducing the number of radially oriented intercellular spaces. The outer layer, several cells thick, contained intercellular spaces many of which were occluded. These features did not vary with growth conditions in a way which might influence oxygen diffusion characteristics. However, the phellem of water-cultured nodules was much more aerenchymatous than that of vermiculite-grown nodules. © 1992 Annals of Botany Company.