Glass transition and heat capacity behaviors of plant vitrification solutions

Abstract

Differential scanning calorimetry (DSC) was employed to investigate the vitrification and annealing behaviors of the most commonly used plant vitrification solutions (PVS). These solutions are employed to protect plant tissues toward ice formation and freeze injury, and help to the vitrification of these tissues, by globally reducing the intracellular fluids mobility. Glass transition temperatures (TG) and heat capacity increments (ΔCp) were determined for five solutions PVS1, PVS2, PVS2 mod, PVS3 and PVS3 mod, with different composition, and a range of cooling and warming rates was employed. Glass transitions showed clear and consistent temperature differences within vitrification solutions, which could be related to composition and water content. Roughly, two sets of TG values were obtained, those for PVS1 and PVS2, at −112°C and −114°C, respectively, and those for PVS3, at −90°C. The observed TG and ΔCp, unexpectedly, did not significantly change within a wide range of cooling rates (from 5°Cmin−1 to liquid nitrogen quenching) and warming rates (from 5 to 20°C). Garlic shoot tips cryopreserved after the droplet method produced a similar result to that of the vitrification solutions employed. After quench cooling to temperatures below TG, repeated excursions to higher temperatures were made and the cooling and warming TG were recorded. These treatments had little or no effect over the PVS solutions TG, which remained practically constant. A direct practical consequence is that the plant vitrification solutions glass transition temperature does not significantly change with cryopreservation methods based on either direct plunging of samples into liquid nitrogen or employing closed cryovials.