Characterization of molecular mobility within the glassy matrix of dried seeds: implications for storage and conservation

Authors: Walters, Christina; Ballesteros, Daniel

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 11/20/2011
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: The properties of aqueous glasses are fundamental to successful seed storage and conservation. Molecular mobility within a seed, once it forms a glass below Tg, is almost nil compared to mobility in seed cells above Tg. Instead of comparing mobilities below and above Tg, as is the usual approach for studying cytoplasmic glasses, we have considered cell glasses in the context of a viscoelastic “solid” material. Molecular mobility in a solid is quantified as the tendency for structure to deform, which is typically measured using mechanical analyses. We used Dynamic Mechanical Analysis (DMA) to evaluate contributions of elastic and viscous motion to biological glasses within seeds. Seeds are an ideal study material because they naturally form stable glasses during maturation drying, and variation in longevity of preserved seeds provides a valuable tool to assess the role of variation in glass properties. In visco-elastic solids, released constraint of molecules are considered “relaxations”. At least three molecular relaxations (a, ß and ') were regularly detected in seeds ranging in water content from 0.03 to 0.25 g H2O/ g dry mass. The highest temperature relaxation (a) is typically regarded as the Tg detected using other instrumentation. The ß and ' relaxations, observed at much lower temperatures, reflect temperature-water content relationships at which molecules within side chains can rotate or vibrate to effect molecular restructuring. As the water content within seeds increase, the temperature of the a, ß and ' relaxations decrease. The plasticization effect of water varies for each relaxation with a relaxations having the steepest slope. a and ß relaxations merge at 20oC in seeds containing about 0.17 g H2O/ g dry mass. Our data show considerable motion in biological glasses at temperatures below Tg (a relaxation) as well as a greater “fragility” of biological glasses in response to temperature as the water content within dry seeds increases. We provide a more detailed understanding of molecular motion and plasticization effects of water within the glassy matrix of cells than previously reported and this contributes important explanatory information about the physical stability and observed deterioration of germplasm during storage.