Extreme Biology: Probing Life at Low Water Contents and Temperatures.

Seeds provide an interesting experimental material to probe life under extreme conditions that do not support growth and metabolism.   Under extreme cold or dry, seeds don’t seem to change; however, changes do occur with time and are manifested by a sudden loss or gain in germination ability.  Dry after-ripening promotes germination processes, and will be discussed by others in this symposium.   This paper focuses on the loss of germination ability when seeds deteriorate during storage.   The process occurs in two phases; the first has virtually no symptoms and the second is expressed by weak or abnormal seedlings and eventually by failure to germinate altogether.  The duration of the asymptomatic phase is called seed longevity.   Accurately predicting seed longevity is critical to seed conservation programs in genebanks, breeding programs and commercial seed markets that provide consistent quality to consumers.  Yet, predicting when lost vigor or viability will occur over an unknown time frame, when there are no other indicators of decline, is a challenge.  We have approached this question from a similar perspective as the materials sciences, foods and pharmaceuticals disciplines, asking how stable is the molecular structure within a seed and how do temperature, moisture, composition and cellular organization interact to change initial molecular structures and cellular function during aging.   Using  dynamic mechanical analysis (DMA), we have begun to document the complexity of seed visco-elastic properties, showing temperature-moisture relationships in which  seed glasses form and more subtle molecular movement allowable within the seed glass which likely contributes to aging.  Differential scanning calorimetry (DSC) is used to differentiate the propensity for lipid and ice crystallization and recrystallization among seeds having different longevities.  Other measurements, such as characterizing volatile emission and RNA integrity in stored seeds, are providing insights about the nature and kinetics of chemical changes that precede lost viability.  Collectively, these techniques contribute to broadened understanding of the mechanisms for changed physiology in dried seeds that will lead us to reliable phenotyping of the complex trait of seed longevity  and early or non-invasive assessments of seed deterioration.