Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 15, 2011
Publication Date: September 1, 2011
Citation: Ballesteros, D., Walters, C.T. 2011. Detailed characterization of mechanical properties and molecular mobility within dry seed glasses: relevance to the physiology of dry biological systems. Plant Journal. 68(4): 607-619. Interpretive Summary: Problem: Seeds die during storage by an unknown aging mechanism and at an unpredictable rate. Different seed lots age at different rates. One explanation for differences in seed longevity is differences in motional and structural properties within seeds. These properties have not been measured within seeds before now. Accomplishments: We used dynamic mechanical analysis (DMA) to measure the motional and structural properties of pea seeds. We distinguished between elastic motion of a solid and viscous motion of a fluid. We detected discrete changes in motional properties of pea seeds as a function of water content and temperature. Impact: It is now possible to test hypotheses relating mobility and structure in seeds to their physiology, including longevity and dormancy. DMA may provide a non-invasive tool to explain and quantify seed responses to environmental conditions.
Technical Abstract: Molecular motion slows when seeds form a glassy matrix, conferring longevity during dry storage. However, seeds inevitably age and variation in aging rates suggests differences in mobility among seed glasses. Dynamic Mechanical Analysis (DMA) was used to distinguish elastic and viscous motion within dried pea cotyledons. Water plasticizes the pea glass in discrete relaxations that are more complex than current paradigms of a single glass transition (Tg). The Alpha relaxation is consistent with Tg but occurs outside the expected range of storage conditions, making it an unlikely explanatory factor for seed aging except under artificial experimental conditions. In contrast, the Beta relaxation occurs between -20 and +30 degrees C, depending on moisture, and confers increasingly greater mobility as cotyledon water content increases. Another relaxation, Gamma, occurs between -90 and -50 degrees C, and does not appear affected by moisture. The Beta relaxation, and its proximity to Alpha relaxation, is considered important in the physical aging of synthetic polymer glasses. This relaxation occurs when the void volume expands sufficiently to allow side chains of molecules forming the glassy matrix to rotate or additives to penetrate the glassy matrix. The structural properties of pea cotyledons change upon drying to less than 0.05 g H2O/g dry mass, and suggest that excessive drying reduces stability. We conclude that the highest stability of pea cotyledon structure occurs in cotyledons containing 0.05 to 0.07 g H2O/g dry mass and temperatures less than the Beta relaxation. This may also be a storage environment conferring maximum longevity to pea seeds.