Adaptations to flooding stress: From plant community to molecule

This review highlights four major topics in plant flooding research: the pr ocesses 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 t he molecular regulation of the hormone ethylene which is heavily involved i n the adaptation reaction of flood-resistant plants. Model species and vege tation strategies are used to unravel mechanisms of vegetation zonation in the river floodplain. In the case of woodlands, hydrological conditions det ermine to a large extent their zonation patterns under natural conditions. For softwood species, such as Salicaceae, the interaction between water lev els and timing of seed dispersal is the dominating process determining thei r establishment success on river banks. Their strategy is well adapted to i rregular, high and prolonged floods. Hardwood species, Quercus, Fraxinus, U lmus and Acer, are hood-sensitive and inhabit the higher sites. They mainly have heavy seeds and germinate under shaded conditions. The most shade-tol erant hardwood species are the least well adapted to flooding. Anthropogeni cally influenced parts of the floodplain are characterized by grasslands wi th elevation level and management practices determining the species composi tion. 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 c an be divided into three categories - dominants, subordinates and transient s - illustrating the diversity in plant species in relation to environmenta l properties. Model species that are indicative of the different conditions in the various zones are chosen to help in the understanding of morphologi cal and physiological adaptations at the plant level. The formation of aere nchymatous roots and the capacity to elongate shoot parts upon submergence are among the main responses of surviving plants. The role of hormones in t he 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 rest ored by root-derived oxygen and the oxygenated rhizosphere is detoxified. A new development in flooding ecology is the unravelling of the molecular re gulation 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.