Development of alternative loading solutions in droplet-vitrification procedures

In plant vitrification protocols, the loading treatment, which involves treating the explants with a moderately concentrated cryoprotectant solution, precedes dehydration of explants with highly concentrated vitrification solutions in order to reduce the toxicity which can be induced by their direct exposure to such highly concentrated solutions. This study aimed at developing alternative loading solutions composed of mixtures of glycerol and sucrose at various concentrations. Differential scanning calorimetry runs of loading solutions and of loaded and dehydrated explants were performed to assay thermal events occurring during cooling and warming. These loading solutions were applied to two model species, viz. garlic and chrysanthemum which were cryopreserved using a droplet-vitrification procedure. The loading treatment proved to be beneficial to both garlic and chrysanthemum and increased recovery of cryopreserved explants. However, response to the loading solutions tested varied between the two model species employed: with garlic, all the loading solutions had a similar effect, whereas survival of chrysanthemum shoot tips was significantly influenced by the composition of the loading solution employed. A loading solution comprising 1.9 M glycerol and 0.5 M sucrose was the most effective. The loading treatment may thus act as an osmotic stress neutralizer and/or induce the physiological adaptation of tissues and cells, including membranes, to both dehydration and freezing.