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Water is essential for all plant life. Clearly, however, water is not uniformly distributed across the planet: the success of some vascular plants has been achieved through colonising waterlogged and flooded lands, and even totally aquatic habitats. The reviled wetlands discovered by Byrd are just one such waterlogged environment. Oceans, saltwater and fresh-water lakes, rivers, canals and ponds all provide permanent water bodies to which plants have acclimated through a wide range of physiological processes. Seagrasses are a spectacular case of tolerance to complete submergence throughout an entire life cycle. These plants have evolved from their terrestrial ancestors to colonise seabeds, overcoming salinity and anoxic sediments to out-yield all but the most luxuriant tropical forests. Even sexual reproduction is possible in seagrasses through major modifications to pollination and seed dispersal mechanisms.

The swamp paperbark (Melaleuca halmaturorum) is presented as a land plant that can grow in saline, anoxic wetlands providing its stems are not inundated. Bodies of fresh water are also colonised by higher plants, with unrelated species having independently evolved mechanisms of rapid stem and leaf growth to maintain contact with the atmosphere (e.g. waterliles — Nymphaea spp.). Wetland plants have, in other words, evolved a genetic makeup that allows them to survive deepwater habitats and minimise competition from land plants.

Many other higher plants are periodically waterlogged through flooding of the land. Roots are often primarily affected but leaves and stems might also be inundated, leading after several days to tissue damage, slower growth and some-times plant death. Tolerance to these conditions varies greatly, with species that evolved in flood-prone environments better able to acclimate than those which are rarely inundated. How-ever, surprisingly little is known about flood tolerance, even of common species. Against the common view, soybean is shown in Case study 18.2 to be relatively flood tolerant, reflecting the ancient practice of mutual cultivation with rice. This chapter describes the factors that damage plants during and after flooding, focusing especially on disruption of energy metabolism. A suite of adaptive changes that enable land plants to overcome some effects of waterlogging are illustrated, showing the importance for plant survival of coordinated structure (e.g. air space formation) and physiology (e.g. fermentative pathways).