Membrane behaviour in seeds and other systems at low water content: the various effects of solutes

A common feature of desiccation-tolerant organisms, such as orthodox seeds, is the presence of large quantities of sugars, especially di- and oligosaccharides. These sugars may be one component of the suite of adaptations that allow anhydrobiotes to survive the loss of most of their cellular water. This paper describes the physical effects of dehydration on cellular ultrastructure, with particular emphasis on membranes, and explains quantitatively how sugars and other solutes can influence these physical effects. As a result of dehydration, the surfaces of membranes are brought into close approach, which causes physical stresses that can lead to a variety of effects, including demixing of membrane components and fluid-to-gel phase transitions of membrane lipids. The presence of small solutes, such as sugars, between membranes can limit their close approach and, thereby, diminish the physical stresses that cause lipid fluid-to-gel phase transitions to occur during dehydration. Thus, in the presence of intermembrane sugars, the lipid fluid-to-gel phase transition temperature (T-m) does not increase as much as it does in the absence of sugars. Vitrification of the intermembrane sugar solution has the additional effect of adding a mechanical resistance to the lipid phase transition; therefore, when sugars vitrify between fluid phase bilayers, T-m is depressed below its fully hydrated value (T-o). These effects occur only for solutes small enough to remain in very narrow spaces between membranes at low hydration. Large solutes, such as polymers, may be excluded from such regions and, therefore, do not diminish the physical forces that lead to membrane changes at low hydration.