Probing Life and Death in the Glassy State of Cryopreserved Seeds, Fern Spores and Pollen: Fundamental Research to Optimize Ex Situ Preservation

Dani Ballesteros, Christina Walters, Hugh W. Pritchard

Royal Botanic Gardens Kew, Wellcome Trust Millennium Building, United Kingdom, United States Department of Agriculture., United States

Often various plant propagules (e.g., seeds, pollen and fern spores) can be stored dry (or partially dry) at sub-zero temperatures for extended periods of time. Under such conditions the cell cytoplasm of these propagules solidifies and forms a glass, lethal water freezing does not occur, and metabolic reactions are inhibited. However, dry (or partially dry) plant propagules deteriorate and eventually die during sub- zero storage, including at liquid nitrogen temperatures (i.e. < -150°C). The rates of deterioration are diverse, depend on species and genotypes, and are often influenced by the dry architecture of the propagule’s cells. Probing life and death in the glassy state of cryopreserved materials is challenging, which makes the understanding of the ageing mechanisms of dry plant propagules difficult. However, new techniques in tomography, microscopy and biophysics have been adopted for the study of longevity of cryopreserved dry plant propagules from a structural point of view. Of importance among these tools is: Differential Scanning Calorimetry, Dynamic Mechanical Analysis, micro-Computer Tomography, and Cryo- Scanning Electron Microscopy. They are providing interesting clues about the deterioration mechanisms of seeds, fern spores and pollen during drying and sub-zero exposure and storage. Understanding these deterioration mechanisms will lead to the development of optimized storage protocols to maximize plant germplasm ex situ preservation.