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Title: Extreme biology: probing life at low water contents and temperatures

item Walters, Christina

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2013
Publication Date: 6/22/2014
Citation: Walters, C.T. 2014. Extreme biology: probing life at low water contents and temperatures. Acta Horticulturae. 1039:49-56.

Interpretive Summary: Germplasm that is preserved by dry or cold conditions lives in a state of ‘suspended animation.’ On a normal time scale, it doesn’t appear to change. However, the purpose of germplasm preservation in genebanks is to maintain viability of germplasm for decades, maybe centuries. Under this time frame, germplasm viability will likely succumb. A major problem is that genebank operators have no predictive tools to determine which accession will age fastest and when viability will decline. This paper describes the physical principles governing change in preserved biological materials and how these principles are currently applied to develop techniques that can monitor aging rates and predict longevity. With these tools, genebanks will reduce the costs of monitoring viability, reliably schedule regenerations, and avoid depletion of valuable samples through needless testing.

Technical Abstract: Germplasm that is dried or cryopreserved appears quiescent. However, changes occur in preserved germplasm, albeit slowly. Viability time courses follow a sigmoidal curve where there is a lag phase when changes can’t be detected, followed by a period of rapid mortality. Predicting longevity under extreme dry or cold conditions requires that we understand the interactions of temperature, moisture and cell constituents on the duration of the initial lag phase. Moreover, to elucidate why germplasm eventually succumbs, we need better assays to detect change under conditions where changes are presumed to not occur. The conceptual model we use is derived from investigations of movement and structure in visco-elastic materials and centers around measuring the properties of glasses formed in preserved germplasm. Orthodox seeds survive extreme drying and naturally form glasses at ambient temperatures. In contrast, cryopreservation treatments are needed to form glasses in desiccation-sensitive propagules, and strategies vary among labs and tissue types: vitrification by chemical dehydration, freeze desiccation during very slow cooling (~1oC/hr to -30oC), or partial air-drying and rapid cooling. The objective of our work consists of measuring the properties of glasses formed in an array of germplasm using different methods. Differential Scanning Calorimetry (DSC) can determine the propensity for ice and lipid crystallization and recrystallization events during storage. Dynamic Mechanical Analysis (DMA) measures structural properties within the formed glasses and temperatures associated with relaxation events that destabilize the glass. The combination of techniques provides unique insight into the mechanisms and kinetics of change as well as thermo-dynamically based estimates of longevity of cryopreserved materials.