Temperature dependency of molecular mobility in preserved seeds.
Although cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual timescale for changes in viability has not been addressed theoretically or empirically. Molecular mobility within preserved biological materials provides a first approximation of the rate of deteriorative reactions that ultimately affect shelf-life. Here, temperature effects on molecular mobility in partially dried seeds are calculated from heat capacities, measured using differential scanning calorimetry, and models for relaxation of glasses based on configurational entropy. Based on these analyses, glassy behavior in seeds containing 0.07 g H(2)O/g dm followed strict Vogel-Tamman-Fulcher (VTF) behavior at temperatures above and just below the glass transition temperature (Tg) at 28 degrees C. Temperature dependency of relaxation times followed Arrhenius kinetics as temperatures decreased well below Tg. The transition from VTF to Arrhenius kinetics occurred between approximately 5 and -10 degrees C. Overall, relaxation times calculated for seeds containing 0.07 g H(2)O/g dm decreased by approximately eight orders of magnitude when seeds were cooled from 60 to -60 degrees C, comparable to the magnitude of change in aging kinetics reported for seeds and pollen stored at a similar temperature range. The Kauzmann temperature (T(K)), often considered the point at which molecular mobility of glasses is practically nil, was calculated as -42 degrees C. Calculated relaxation times, temperature coefficients lower than expected from VTF kinetics, and T(K) that is 70 degrees C below Tg suggest there is molecular mobility, albeit limited, at cryogenic temperatures.
Walters
Christina
C
United States Department of Agriculture, Agricultural Research Service, National Center for Genetic Resources Preservation, Fort Collins, Colorado 80521, USA. chrisv@lamar.colostate.edu
D002152:Calorimetry, Differential Scanning / Q000379:methods*; D005519:Food Preservation / Q000379:methods*; D005615:Freezing; D009038:Motion; D018532:Pisum sativum / Q000378:metabolism; D011058:Pollen / Q000378:metabolism*; D015203:Reproducibility of Results; D012639:Seeds / Q000502:physiology*; D012680:Sensitivity and Specificity; D013025:Glycine max / Q000378:metabolism; D013696:Temperature; D044366:Transition Temperature; D032401:Typhaceae / Q000378:metabolism; D014783:Viscosity; D014867:Water / Q000378:metabolism*
D014867:Water