Methods for Determination of Glass Transitions in Seeds

Abstract

When the non-freezeable (bound, amorphous, vitreous) water of pea or soybean embryos is examined by dielectric methods, three state transitions can be observed. The lowest occurs at the glass transition temperature (Tg) of pure amorphous water, is invariant with water content and is probably associated with crystal defects and other interfaces. The second, at about -60°C, is dependent upon water content and probably reflects rotational mobility of side chains (β-transition). It can be seen by dynamic mechanical analysis (DMA) as well, but the energies are too small to detect by differential scanning calorimetry (DSC). The third is the glass transition, which reflects the increase in diffusional mobility in the sample. The increase in fluidity can be measured by DMA and the increase in heat capacity of water is detectable by DSC. This is the transition that has been associated with changes in seed storage stability. Heretofore, DSC has been the method of choice because of its availability, familiarity and its ease of operation and interpretation. However, at low sample water contents, DSC signals become small and diffuse and the assignment of Tg becomes difficult. Because DMA is the most sensitive to local motion, it provides the most credible information on changes in sample viscosity that occur during glass transition. We have recently examined embryos in the Perkin-Elmer DMA-7 and found that DMA provides richer data for these studies than does DSC.