49. Interactions between water and triacylglycerols may explain faster aging rates in stored germplasm at low temperatures

Abstract

Longevity of orthodox (desiccation tolerant) seeds, pollen and fern spores characteristically increases with decreasing storage temperature. We have discovered numerous seed and fern spore species that age faster when stored in the freezer near −20 °C compared to storage at higher or lower temperatures. Seeds exhibiting this temperature anomaly are considered to be intermediate between orthodox and recalcitrant physiologies. Intermediate traits are often associated with high proportions of saturated fatty acids within the triacylglycerols (TAG), accumulated into cells as food reserves. TAG high in saturated fatty acids crystallize and melt within the temperature range of the observed longevity anomalies and we have hypothesized a link between TAG phase change and increased damage to germplasm cells. Damage is exacerbated as water content within stored seeds and fern spores increases from near 5% to about 12%. These observations have led us to hypothesize that damage results from an interaction between TAG, water, temperature and time. The purpose of this research is to explore the extent and significance of possible interacting factors on the viability of stored germplasm. Our work begins with characterizing the kinetics of TAG and water phase changes in peanut ( Arachis hypogaea ) and papaya ( Carica papaya ) seeds equilibrated to different water contents and stored at temperatures between −5 and − 80°C. Water and TAG phase was measured using a Perkin Elmer Differential Scanning Calorimeter. Cytoplasm ultra-structure was visualized without chemical fixatives using low temperature scanning electron microscopy (cryo-SEM) performed with a Zeiss DSN 960 scanning microscope equipped with a Cryotrans CT-1500 cold plate (Oxford, UK). The time dependency of phase changes was assessed over a 3 week period. We report that TAG crystallization in seeds is independent of water content and occurs over several days. The amount of water that freezes in seeds increases with storage duration in a manner consistent with TAG crystallization kinetics.