Plants as amphibians

From the poles to the tropics flooding is a powerful discriminator in plant distribution. Although plants can be divided globally as to whether or not they are tolerant of high water tables, it does not follow that all flood-tolerant species achieve their ability to survive flooding by similar adaptations. Flooding implies a periodic but temporary rise of the water table, hence plants that live in such areas have an amphibious life style. Amphibious plants have to adjust, not only to inundation and the dangers of oxygen deprivation, but also to the eventual lowering of water tables and often sudden re-exposure to a fully aerated environment and the lack of the physical support that is provided by flooding. In this respect they are distinct from aquatic species that live constantly in water. It is often tacitly assumed that for amphibious species flooding is the stressed condition and non-flooding the norm. This pre-judgement is not appropriate, particularly as in many habitats the flooded condition predominates for a longer part of the year than the unflooded. For amphibians, re-adapting from the aquatic to the terrestrial habitat requires specialised adaptations, just as much as a change from unflooded to flooded. Many flood-tolerant species, including surface-rooting grasses and sedges, may not be tolerant of anoxia, and instead prevent the accumulation of an oxygen debt in submerged organs by aeration mechanisms, including oxygen diffusion through aerenchyma, thermally induced mass movement of air, and the elongation of submerged shoots. In other species, and particularly in perennial plants with buried perennating organs, flooding can impose prolonged periods of anaerobiosis (anoxia). Being able to survive such oxygen deprivation requires (1) energy reserves sufficient for cell maintenance, (2) the prevention of cytoplasmic acidosis under anoxia, and (3) the anaerobic mobilisation of starch reserves. Re-entry to the aerobic habitat is facilitated by (4) the dispersal and excretion of products that transfer hydrogen from anoxic or hypoxic tissues, either to the external environment, or to parts of the plant with access to oxygen, before the anaerobic tissues return to air, and (5) anti-oxidative activity to minimise post-anoxic injury.