Nitrogen‐fixing stem nodules of the Legume,Discolobium pulchellumBenth.

Abstract

SUMMARYWe report on the structure of N^-fixing nodules formed on the stem of Discolobium pulchellum Benth., an aquatic legume in the subfamily Papilionoideae, tribe Aeschynomeneae, from the flooded areas in the ' Pantanai Matogrossense' region of Brazil. The stem (and root) nodules were ohligately aquatic, requiring permanent submergence in water or flooded soil, and receive oxygen via profuse aerenchyma covering the lower .stem. Of the 69 isolates of rhizobia isolated from stem and root nodules, 70 % were fast-growing acid producers and 30 "'o were slow growers. The rhizobia were not photosynthetic. Nodules were connected to the stem, and the vascular system from the stem branched throughout the nodule, penetrating the infected tissue within finger-hke ingrowths of cortex. In both stem and root nodule.s, infected tissue was aeschynomenoid 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 components of an oxygen diffusion barrier (a boundary cell layer without intercellular spaces and glycoprotein occlusions of intercellular spaces in other cell layers). The midcortex, e.\ternaJ to the boundary layer, consisted of loosely-packed cells and these were continuous with stem aerenchyma. The outer part of the nodules was made up of phellogen-derived cells forming a periderm, or 'corky' layer of cells. The periderm formed large lenticels above cortical vascular bundles. These lenticels also connected with the stem aerenchyma. Root nodules differed only in that infected cells were not vacuolate, bacteroids were larger and contained more poK-/y-hydroxybutyrate (PHB) and there was less aerenchyma/ienticellular tissue. Stem and root nodule structure is discussed in terms of adaptations to O^ constraints in an aquatic environment.

Development of N2‐fixing nodules on the wetland legume Lotus uliginosus exposed to conditions of flooding

“…Moreover, in common with previous studies of legumes that are tolerant to flooding (Arber, 1920 ;Walker et al, 1983 ;James et al, 1992a,b ;Loureiro et al, 1994Loureiro et al, , 1995, as well as of terrestrial legumes that were subjected to flooding and\or artificially lowered pO # (Pankhurst & Sprent, 1975 ;Parsons & Day, 1990 ;Dakora & Atkins, 1990 ;Shiferaw et al, 1992 ;Arrese-Igor et al, 1993 ;Pugh et al, 1995), stems and roots of L. uliginosus became highly aerenchymatous after flooding. In addition, lenticels, which in Lotus nodules have recently been identified as strategic choke points for gaseous diffusion (Jacobsen et al, 1998), were very prominent in the flooded L. uliginosus nodules (Fig.…”

Stem and root nodules on the tropical wetland legume Aeschynomene fluminensis

“…Stem nodulation is a relatively rare phenomenon that, until recently, has been confirmed only in some species of the hydrophytic genera Aeschynomene and Sesbania (Arora, 1954;Yatazawa & Yoshida, 1979;Dreyfus & Dommergues, 1981;Eaglesham & Szalay, 1983;Alazard, 1985;Vaughn & Elmore, 1985), Reports that stem nodules also occur on aquatic Neptunia species are likely to be erroneous (James et al, 1992a) and a misquote of Schaede (1940). However, the short list of stem nodulating legumes has just been increased to three genera by the discovery of nodules on stems of Discolobium (Loureiro et al, 1994), a relative of Aeschynomene.…”

Photosynthetic oxygen evolution within Sesbania rostrata stem nodules

“…However, in recent years, attention has also focussed on the ability of a limited number of legumes to form N 2 -fixing nodules on their stems (see recent review by Boivin et al, 1997). This is known to occur in only three legume genera: Aeschynomene, Discolobium and Sesbania (Alazard, 1985;Boivin et al, 1997;Dreyfus and Dommergues, 1981;Eaglesham and Szalay, 1983;Loureiro et al, 1994). All stem-nodulated species are tropical or subtropical hydrophytes, frequenting swamplands and river/ lake margins (Boivin et al, 1997;Dreyfus and Dommergues, 1981;Eaglesham and Szalay, 1983;Ladha et al, 1992;Loureiro et al, 1994Loureiro et al, , 1995.…”

“…This is known to occur in only three legume genera: Aeschynomene, Discolobium and Sesbania (Alazard, 1985;Boivin et al, 1997;Dreyfus and Dommergues, 1981;Eaglesham and Szalay, 1983;Loureiro et al, 1994). All stem-nodulated species are tropical or subtropical hydrophytes, frequenting swamplands and river/ lake margins (Boivin et al, 1997;Dreyfus and Dommergues, 1981;Eaglesham and Szalay, 1983;Ladha et al, 1992;Loureiro et al, 1994Loureiro et al, , 1995. The formation of nodules on their stems is considered to be an adaptation to allow them to fix N 2 under flooded conditions by reducing the oxygen constraints that flooded root nodules would normally experience (Becker and George, 1995;James et al, 1992aJames et al, , 1992bPugh et al, 1995).…”

“…Stem nodules can be formed on the aerial stem, above the water level, as on A.afraspera, A.indica, A.scabra and S.rostrata (Alazard, 1985;Boivin et al, 1997;Dreyfus and Dommergues, 1981;Eaglesham and Szalay, 1983;Ladha et al, 1992), or on the flooded stem, as with A.elaphroxylon, A.fluminensis and Discolobium pulchellum (Alazard, 1985;Loureiro et al, 1994Loureiro et al, , 1995. An interesting feature of aerial, and some submerged stem nodules, is the occurrence of chloroplasts in the uninfected, cortical, cells which surround the central, rhizobial-infected tissue (de Bruijn, 1989;Dreyfus and Dommergues, 1981;Duhoux, 1984;de Lajudie and Huguet, 1988;Loureiro et al, 1995;Vaughn and Elmore, 1985).…”