Flooding-induced adventitious rooting in Rumex: morphology and development in an ecological perspective

The formation of aerenchymatous adventitious roots is a morphological acclimation to soil flooding in many wetland plants. The large air spaces in these roots facilitate the diffusion of gases between shoot and root. This study investigated adventitious root formation in species of the genus Rumex, and related the differences between species to the natural habitat of the species. Large differences in both the number and the morphology of the adventitious roots were found between closely related species. R. hydrolapathum, a species of almost permanently waterlogged soils, showed only a slow growth of adventitious roots, but these roots were extremely porous and very thick. R. palustris, which is flooded very frequently but not permanently, developed a large number of adventitious roots with a high content of aerenchyma. On the other hand, adventitious rooting in R. thyrsiflorus and R. acetosa, which are species of dry and seldom flooded habitats, was rather poor and the adventitious roots contained only very little air space. A group of species occurring in regularly flooded grasslands ( R. crispus, R. conglomeratus and R. obtusifolius) showed an intermediate response to soil flooding. Two contrasting species, R. palustris and R. thyrsiflorus, showed emergence of adventitious roots within 2 days after the onset of waterlogging. Although the time of onset of adventitious rooting was similar in these two species, the primary lateral root system of R. palustris was completely replaced by adventitious roots in 1 week, independently of the age of the plants. In contrast, the biomass of adventitious roots in R. thyrsiflorus after 1 week of flooding was never more than one-third of the biomass of the primary lateral root system. We concluded that the ability of species to form adventitious roots during flooded soil conditions and the adaptive characteristics of these roots were very closely related to the flooding frequency of these species in their natural habitat. Differences in adventitious rooting between species mainly occurred in the number, growth rate, morphology and anatomy of the roots, and not in the timing of emergence.