Flooding-tolerant legume symbioses from the Brazilian Pantanal

Nodulated legumes in some of the flooded and seasonally flooded areas of th e Pantanal Mato-Grossense wetlands in Brazil are described here. In the permanently flooded lakes (baias) of the Caracara national park Disc olobium pulchellum, Mimosa pellita, Sesbania exasperata and Vigna lasiocarp a (syn. Phaseolus pilosus) were the most abundant, whereas close to Corumba , at the edges of the river Paraguai, Neptunia spp. were also common. Adapt ations that allow these legumes to fix N-2 in a flooded environment include d a floating growth habit, aerenchyma and nodulated adventitious roots. By contrast, Aeschynomene spp. (A. ciliata, A. denticulata, A. fluminensis, and A. sensitiva) were the dominant nodulated legumes in the seasonally fl ooded pastures of Nhumirim. Stem-nodulation was commonly observed, particularly on seasonally flooded A eschynomene and seasonal/permanently flooded Discolobium spp., but also, in a modified form, on floating stems of L: lasiocarpa. The structures of ste m and/or root nodules on Aeschynomene spp., Discolobium leptophyllum and V. lasiocarpa are described in detail, and nodulation by D. leptophyllum and Neptunia pubescens is reported for the first time.