Adaptations to flooding stress: From plant community to molecule

This review highlights four major topics in plant flooding research: the processes underlying vegetation zonation in the floodplain, the challenges of using model species to reveal adaptive responses in shoots and roots, the role of micro-organisms in flooded soils in relation to plant growth, and the molecular regulation of the hormone ethylene which is heavily involved in the adaptation reaction of flood-resistant plants. Model species and vegetation strategies are used to unravel mechanisms of vegetation zonation in the river floodplain. In the case of woodlands, hydrological conditions determine to a large extent their zonation patterns under natural conditions. For softwood species, such as Salicaceae, the interaction between water levels and timing of seed dispersal is the dominating process determining their establishment success on river banks. Their strategy is well adapted to irregular, high and prolonged floods. Hardwood species, Quercus, Fraxinus, Ulmus and Acer, are hood-sensitive and inhabit the higher sites. They mainly have heavy seeds and germinate under shaded conditions. The most shade-tolerant hardwood species are the least well adapted to flooding. Anthropogenically influenced parts of the floodplain are characterized by grasslands with elevation level and management practices determining the species composition. Low-lying grasslands have flood-tolerant species; elevated zones are seldom flooded and have flooding-sensitive species. Following Grime (1998([59])), plant species of major vegetation types within the floodplain zone can be divided into three categories - dominants, subordinates and transients - illustrating the diversity in plant species in relation to environmental properties. Model species that are indicative of the different conditions in the various zones are chosen to help in the understanding of morphological and physiological adaptations at the plant level. The formation of aerenchymatous roots and the capacity to elongate shoot parts upon submergence are among the main responses of surviving plants. The role of hormones in the adaptation reaction is emphasized. Owing to high porosities in roots of flood-tolerant plants, radial oxygen loss greatly influences nitrification and denitrification processes in the flooded soil. Nutrient cycles are restored by root-derived oxygen and the oxygenated rhizosphere is detoxified. A new development in flooding ecology is the unravelling of the molecular regulation of hormonally controlled processes. The expression of an ethylene receptor gene in Rumex palustris is highlighted. This paper ends with some suggestions for future flooding research.