There are a few biological specimens that tolerate overall water concentrations below 0.01 g/g without incurring any obvious damage. Encysted embryos of Artemia constitute one example (Clegg, 1986), while spores of Aspergillus niger showed no reduction in viability when stored at 1% RH (Walters et al., 2005a). Seeds of Welwitschia mirabilis Hook. f. (which is endemic to the Namib Desert) survived after drying to 0.007 g/g (Whitaker et al., 2004) and showed no reduction in viability when stored at this low water concentration in sealed foil containers for 2 years at 5.5°C or -20°C. However, in all these cases the structures are heavily encapsulated, and it is feasible that the living tissues were actually at water concentrations somewhat higher than the surrounding non-living structures. Nevertheless, from the results of ultra-dry seed storage experiments, it seems unlikely that viability could be retained in the complete absence of intracellular water (Walters and Engels, 1998).
From a wide-ranging study across species, Walters et al. (2005b) found that even when stored under near-ideal gene-banking conditions, the viability of desiccated seeds declines. Characteristic P50 values were not correlated with content of sucrose or oligosaccharides (Walters et al., 2005b), supporting the view that the stability of the intracellular glassy state is not primarily dependent on these compounds (Buitink and Leprince, 2004).
It is, however, apparent that the properties of intracellular glasses do change at very low water concentrations (Buitink and Leprince, 2004), supporting Walters' (1998) identification of temperature-dependent critical water contents, below which the deterioration rate of dry seeds increases. As relaxation (i.e. movement) within the glassy matrix may occur with time, Walters (1998) also concluded that the nature and kinetics of chemical deteriorative processes might be expected to change, which could underlie the kinetics of seed ageing. Walters et al. (2004) concluded that, even below -130°C, ageing reactions are enabled because molecules remain sufficiently mobile, which has serious implications for cryostorage of desiccated material. It is pertinent that free radicals could persist within the glassy matrix, despite the significant extension of lifespan afforded by cryostorage (Benson and Bremner, 2004).
The nature of deteriorative reactions in seeds at water concentrations or ¥ below the critical level does change (Vertucci and Farrant, 1995; Walters et al., 2005a), which, at water concentrations <0.08 g/g (i.e. ¥ -150 to -1000 MPa), include free radical production via autoxidation, evolution of carbonyls and desta-bilization of protein and membrane structure. Concomitantly, increased seed ageing rates that typify ultra-dry seed storage occur (Walters et al., 2005a).
What might occur at water concentrations <0.08 g/g, to account for increased molecular mobility, change in the nature of deleterious events that occur and increased rate of deterioration that characterizes ultra-dry seeds? Water is suggested to play a critical role in the integrity of the glassy matrix by hydrogen bonding between components: on withdrawal of water below the critical level, the structural continuity of the glass is suggested to be perturbed, and, as suggested by Walters (1998), structural integrity of macromolecules may be compromised.
Loss of glass integrity could result in increased porosity, with the potential for greater molecular or free radical mobility. Hence, the rate of deleterious reactions might well increase while the desiccated state prevails, the consequences of which would be cumulative with time. Additionally, if free radicals do persist in the glassy state (Benson and Bremner, 2004), the increase in water in the seed during post-storage imbibition is also potentially hazardous.
Recent progress has furthered the understanding of DT, the commonality of certain traits across a range of organisms, and the statics and dynamics of the desiccated state. In the context of orthodox seeds, the major challenge is translating this progress into best practices for plant germplasm conservation.
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