ADAPTATIONS TO FLOODING IN PLANTS FROM RIVER AREAS

Flooding frequencies and intensities determine both species composition and the behaviour of individual plants along many rivers in the world. In this context, this paper describes the vegetation zonation associated with the fluctuating water levels of the river Rhine in the eastern part of The Netherlands. To obtain insight into the morphological and physiological processes of plants that have been shown to possess contrasting tolerances to flooding, certain species were chosen as being representative of the vegetation types from the river foreland and subsequently used in experimental studies. These species, of the genera Rumex and Chenopodium, were subjected to various flooding regimes in order to study their adaptive responses upon waterlogging of the soil, submergence and related changing environmental factors. This study has revealed that differential responses towards flooding in plants can be explained, at least partly, by an ability to elongate petioles and stems in order to protrude above the water surface. The hormone ethylene is involved in the regulation of this process. Renewed contact between leaves and the open air after submergence stimulates the formation of a new aerenchymatous root system in the flood-tolerant species. Increased porosity enables the plants to perform longitudinal transport from aerial and photosynthetic oxygen to the rhizosphere. The internal aeration systems of flood-in-olerant species is not sufficient to sustain an aerobic state in the root-soil environment. Another mechanism plants use to survive flooding is to change their timing of reproduction. Two strategies are involved: some species delay their flowering and seed production during flooding periods and survive as vegetative plants; others are able to accelerate flowering during short dry periods in order to produce seeds in the short intervals between two successive floods. The model system in which individual plants serve as indicator species, while additionally representing large groups of co-occurring plants, appears to be a manageable tool in studies on adaptations to flooding. © 1990.