Mechanisms of flood tolerance in plants

It has been estimated that wetlands occupy approximately 6% of the earth’s land surfaces (Maltby 1991). They comprise fresh, brackish and salt-water marshes, inland and coastal swamps, flood plains, and lowland and upland mires (fen, bogs), as well as agricultural wetlands such as rice paddy. On a smaller scale there are constructed wetlands valued for their role in the purification of domestic, agricultural and industrial effluents (Cooper & Findlater 1990). In addition to these examples, occasional soil flooding and even a degree of submergence can be a common though unwelcome feature of non-wetland agriculture. Wetlands are usually characterized by permanent or long-term soil flooding, but often the vegetation may be wholly or partly submerged; in the lower reaches of coastal marshes this may be a daily or twice daily occurrence. Indeed, since wetlands are found world-wide and at most altitudes, flooding regimes can vary enormously; not only do the seasonal timing, duration and depth of flooding differ, but so too do light and temperature regimes and sediment type—factors which strongly interact with flooding to influence vegetation. The large numbers and wide variety of vascular plants indigenous to wetlands reflect this variety in habitat features; species range from wholly submerged aquatics of pools, rivers, coastal and lake margins through to the large tree species of the flood plain forests of the Americas and Africa. In view of this variety in both plant and habitat, a whole spectrum of flood-related stresses might be anticipated; similarly one might expect to find a number of flood-tolerance strategies.