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

In river floodplains, variation in flooding conditions results in succ essional stages in colonization ranging from annual pioneers to long-l ived perennials. Reactions to submergence of species from the mid-succ essional zone are compared with adaptive responses of species from oth er zones. Presence and abundance are related to elevation and can be e xplained by characteristics of biomass production, and recovery in res ponse to various submergence intensities. Rumex species, from early to late successional stages, serve as models to elucidate, in more detai l, mechanisms of adaptation. Flooding-resistant species develop large numbers of adventitious roots upon submergence and exposure to low oxy gen conditions. Due to internal oxygen transport through aerenchyma, s oil around these roots is reoxidized, which stimulates bacterial nitri fication. Ethylene and auxin promote adventitious rooting. Increased p etiole elongation is also an adaptive feature of submergence-resistant Rumex species. Differences between species in submergence-induced gro wth are not only controlled by variation in endogenous levels of ethyl ene but also by different sensitivities to this hormone. Auxin does no t affect Rumex petiole elongation, but a clear positive effect of gibb erellin is demonstrated. Apparently, submergence induces a higher sens itivity to gibberellin and ethylene in the petioles of flooding-resist ant Rumex Many of the submergence reactions can also be induced by res tricting the oxygen supply, suggesting that low-oxygen might be a trig gering factor. The Rumex species we study represent various distinct c ommunities. Thus, the ecophysiological phenomena observed in these mod el plants may explain processes and patterns in other species too and thus are interpretable at the riverside community level.