Ecophysiological studies of plants in marginal flood-prone habitats have demonstrated that there are differences between adaptations to oxygen deficiency as it operates in plants in winter and in summer. The winter adaptations may not be as active as those operating in summer. However, they operate under different conditions and are well suited to function in a prolonged and sustainable manner which will satisfy, at least in part, the lower demands for oxygen in overwintering plant organs.
Throughout the world plants are divided ecologically as to whether or not they can survive with wet feet. This does not mean that all plants that survive flooding endure similar stresses or adopt the same adaptive strategies. Flooding in the moving tidal zones of tropical mangrove swamps or even bottomland trees of South Carolina or Louisiana is a very different stress from the long wet winters and short summers encountered by plants that grow in bogs at higher latitudes. Even within the flora of any one particular bog there can be found a variety of adaptations in plants that grow in close proximity to one another. Surface rooting plants with evergreen shoot bases, as in the rushes (Juncus spp.), survive by ventilating their underground organs by diffusion supplemented by internal generation of oxygen from photosynthesis. Deeper-rooted plants endure these adverse conditions by ensuring that their perennating organs have the ability to tolerate total oxygen deprivation and resume growth once the winter water tables subside.
The tree habit appears to be particularly disad-vantaged by flooded soils in areas where flooding in winter is prolonged but not cold enough to ensure complete dormancy of the roots. Flooding of non-dormant root systems early in the winter can induce severe carbohydrate depletion which is then associated with dieback of the root systems on re-exposure to air when water tables drop. Re-exposure to air is potentially hazardous for tissues that have been denied access to oxygen for a prolonged period. Most plant organs that can endure long-term tolerance anoxia also have to be resistant to post-anoxic injury and this incurs extra metabolic costs in ensuring adequate carbohydrate reserves.
Metabolically, the pathways that are available under anoxia to plants that can endure anaerobiosis are not substantially different from intolerant dry land species. The major differences appear to be in regulation, in accessing and translocating carbohydrate reserves under anoxia, together with the minimization and dispersal of the oxygen debt and avoidance of anoxia-induced cytoplasmic acidosis. In terms of biochemistry, the modifications may not appear to be particularly remarkable, yet they are probably sufficient to enable tolerant species to do just a little bit better than dry land species when deprived of oxygen. These minor biochemical differences, however, when translated into anoxia and hypoxia survival time limits, can make all the difference between life and death at the margins for plants with wet feet.
Fig. 9.1 Coastal heathland, Papa Stour, Shetland (UK) in August with dwarf heather (Calluna vulgaris) in flower.
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