PHYSIOLOGICAL ECOLOGY OF RIVERSIDE SPECIES - ADAPTIVE RESPONSES OF PLANTS TO SUBMERGENCE

In river floodplains, variation in flooding conditions results in successional stages in colonization ranging from annual pioneers to long-lived perennials. Reactions to submergence of species from the mid-successional zone are compared with adaptive responses of species from other zones. Presence and abundance are related to elevation and can be explained by characteristics of biomass production, and recovery in response to various submergence intensities. Rumex species, from early to late successional stages, serve as models to elucidate, in more detail, mechanisms of adaptation. Flooding-resistant species develop large numbers of adventitious roots upon submergence and exposure to low oxygen conditions. Due to internal oxygen transport through aerenchyma, soil around these roots is reoxidized, which stimulates bacterial nitrification. Ethylene and auxin promote adventitious rooting. Increased petiole elongation is also an adaptive feature of submergence-resistant Rumex species. Differences between species in submergence-induced growth are not only controlled by variation in endogenous levels of ethylene but also by different sensitivities to this hormone. Auxin does not affect Rumex petiole elongation, but a clear positive effect of gibberellin is demonstrated. Apparently, submergence induces a higher sensitivity to gibberellin and ethylene in the petioles of flooding-resistant Rumex Many of the submergence reactions can also be induced by restricting the oxygen supply, suggesting that low-oxygen might be a triggering factor. The Rumex species we study represent various distinct communities. Thus, the ecophysiological phenomena observed in these model plants may explain processes and patterns in other species too and thus are interpretable at the riverside community level.