|MICHALAK, MARCIN - Polish Academy Of Sciences
Submitted to: International Oaks
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2016
Publication Date: 10/1/2016
Citation: Walters, C.T., Crane, J., Hill, L.M., Michalak, M., Carstens, J.D., Conrad, K.P. 2016. Preserving oak (Quercus sp.) germplasm to promote ex situ conservation. International Oaks. 27:255-266..
Interpretive Summary: This paper describes progress preserving oak seeds. Oak seeds are recalcitrant, meaning they do not survive drying and hence the storage conditions typically used for seed banks. Often people assume that recalcitrant seeds can’t be stored at all. However, it is quite feasible to preserve these seeds. The basic principles are described in this paper and involve partial drying and cooling fast enough that ice crystals don’t form. Because rapid cooling is required, the smaller the tissue the better. We can achieve small specimen size by micro-dissecting the embryonic axis and growing it out in tissue culture. These technologies will help preserve the genetic diversity of a beloved tree species that is iconic in many American landscapes.
Technical Abstract: Germplasm banks are increasingly used as an ex situ conservation strategy. Germplasm banks are able to maintain extraordinary levels of diversity for long periods at relatively low cost. Studies using seeds – the preferred propagules in plant germplasm banks – have revealed the underlying reasons why some cells are able to survive extreme drying and low temperature. This ability is based on the non-lethal conversion of cytoplasm from fluid to solid, as described from materials sciences principles used in studies of thermoplastics and as applied to preserve dry foods and pharmaceuticals. These fluid to solid transitions are called glass formation or vitrification and the reverse process of solid to fluid is called plasticization. Cells of some seeds, such as acorns, do not survive the amount of drying it takes at room temperature to form a glass. To preserve these so-called recalcitrant seeds a combination of cooling and drying under tightly controlled conditions is required and, ultimately, embryos must be stored cryogenically. Feasibility of successful cryogenic treatments will vary among embryos and depend on sensitivity to desiccation, size of cell mass and a host of other factors. We have observed substantial variation in desiccation and freezing tolerance of embryonic cells from diverse oak species. This variation leads us to hypothesize that cryopreservation success is also likely to vary broadly among species or populations. To date, we have achieved high survival following cryogenic exposure of embryonic axes from cold-adapted trees.