Glass transition determination in Allium shoot tips after dehydration

Glassy state is the objective status of cryopreservation methods named vitrification. Low water content minimizes the ice crystallization potentially dangerous for plant cells and to increase temperature of glass transition, supposing that changes of water status in certain range is not limiting for plants regeneration. Plant Vitrification Solutions (PVS) with numbering according to specific mixture of basic cryoprotectants and their concentrations are usually used for osmotic dehydration. Another method based on desiccation in air flow cabinet with defined flow rate, temperature and humidity or on desiccation over various saturated salt solution with steady-state activity of water was used. Experimental determination of glassy state in plants is complicated by overlapping endothermic reaction with glass transition. Ice crystallization as the first order reaction has discontinuous change in heat capacity in contrary to glass transition which is characterized by heat capacity change. The main problem is to distinguish the endothermic reaction and the glass transition during the measurement of the thermal events. Three methods for glass transition determination and melting ice crystals in plant tissue were compared: standard Differential Scanning Calorimetry (DSC) method, Temperature Modulated Differential Scanning Calorimetry (TMDSC) and Quasi-isothermal Temperature Modulated Differential Scanning Calorimeter (QITMDSC). The results can be summarized (i) both the TMDSC and the QITMDSC methods based on temperature modulation gave less ambiguous results (ii) from these methods the QITMDSC gave better results for distinguishing between heat capacity for endothermic ice melting and glass transition because the heat capacity is measured at quasi-isothermal temperature while thermal response of plant sample is under the temperature modulation. Knowledge of the glass transition temperature is useful not only for improving methods involving glassy state in plant shoots. It gives, furthermore, information essential for long term storage of shoot tips.