Christina Walters, Jennifer Crane, Lisa Hill, United States Department of Agriculture., USDA-ARS
Seeds of papaya (Carica papaya) are tolerant to desiccation, but they are quickly damaged when stored at -5 to -20C. The seeds have a high lipid content that reflect “tropical oils,” which are high in saturated and mono-unsaturated fatty acids. Differential Scanning Calorimetry (DSC) experiments show that the lipids tend to crystallize into alpha and beta crystals and then re-crystallize to more stable crystalline beta prime structure as the seeds slowly pass through the -25 to +5C temperature range. A working hypothesis is that these transitions are associated with the poor storage response of papaya seeds. Cooling and warming sufficiently quickly (~ 50C/min) to liquid nitrogen temperatures allowed only the alpha-type crystals to form and slowed recrystallization. Seeds survived longer when stored at liquid nitrogen temperatures. However, after 15 years of storage, papaya seeds showed measurable degradation that may be associated with 1) multiple freeze-thaw cycles that occurred when retrieving samples for monitoring, 2) failure to warm seeds adequately before they were tested for viability, or 3) degradation processes that continue at liquid nitrogen temperatures. Papaya seeds serve as a convenient model to study a large class of seeds within the ‘intermediate’ storage category that originate from tropical or subtropical areas and cannot be stored using conventional seed banking methods. We show that storage at liquid nitrogen temperatures offers improved success, but may not provide long-term storage usually anticipated at such low temperatures.