Tolerance desiccation

Water is, of course, essential for plant growth, but one of the themes of this chapter is that it may not be necessary for plant survival. Although most agronomically important plants are very sensitive to internal water deficits, the majority of plants at some stage of their life cycle are tolerant of desiccation. Few of these have vegetative parts which are desiccation tolerant, but the survival of even so-called drought-evading species, such as the ephemeral desert annuals, rests on the tolerance of their seeds to desiccation. 

Desiccation tolerant species may exhibit little or no metabolic activity depending upon the extent of dehydration. In this anhydrobiotic or ametabolic state we are concerned not with metabolic perturbation but with the stability of organelles, membranes and macromolecules in a dehydrated state. However, in the initial period of rehydration, the passage to a metabolically active state poses particular problems if metabolic mayhem is to be avoided. 

There are representatives of desiccation tolerant species amongst all of the major plant divisions. The water content of many bacterial and fungal spores is low (<25%) and they exhibit great tolerance of desiccation (see Ross Billing, 1957 Bradbury et ai, 1981). Desiccation tolerant cyanobacteria are found in a diverse range of drought-prone habitats. 

The drying protoplast will be subjected to tension as the result of volume contraction and its adherence to the cell wall. Early observations (Steinbrick, 1900) on desiccation tolerant species showed that the protoplasm does not separate from the wall, but rather that it folds and cavities develop in the wall. Where there are thick-walled cells, localised separation of the plasmalemma from the wall may occur. It seems unlikely, however, that rupture of the plasmalemma normally occurs during desiccation. A more subtle form of membrane damage may arise from dehydration-induced conformational changes. Certainly it is relatively easy to demonstrate that dehydrated membranes exhibit a loss of functional integrity... 

Desiccation tolerance and injury avoidance The remarkable tolerance to prolonged anhydrobiosis in resurrection plants suggests they are able to maintain essential structure and physiological integrity in the dry state or are able to repair dehydration-induced damage rapidly following rehydration. 

Among resurrection plants two seemingly very different kinds of response occur at the ultrastructural level. In many desiccation tolerant seeds, pollens, mosses and vascular plants, dehydration brings about rather... 

These extensive alterations in cell structure and the biochemical machinery are indicative of entry into an ametabolic condition. In this condition damage from free radicals is potentially decreased, certainly the loss of chlorophyll and chloroplast structure removes a major source of free radical generation. About 50% of the extremely desiccation tolerant monocots exhibit extensive loss of chlorophyll and ultrastructural organisation when desiccated. Dicots, ferns and bryophytes retain most of their chlorophyll and exhibit small changes in structure when dry (see Gaff,... 

Polyols are present in desiccation tolerant lichens and liverworts, although not in mosses (Lewis, 1984). More generally starch hydrolysis and sugar accumulation occur in many plants experiencing severe water deficits (Hsiao, 1973). It is tempting to speculate that the accumulation of low molecular weight solutes in reponse to water stress represents a mechanism for the protection of membranes and proteins in the dry state. 

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