Submitted to: Planta
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/1/2005
Publication Date: 11/24/2005
Citation: Crane, J., Kovach, D.A., Gardner, C.A., Walters, C.T. 2005. Triacylglycerol phase and 'intermediate' seed storage physiology: a study of cuphea carthagenensis. Planta 223:1081-1089. Interpretive Summary: Currently many species of seeds are not stored in genebanks because they do not survive the low relative humidity and temperature conditions used to store seeds for extended times. A specific class of seeds from these species, often said to have intermediate storage behavior, survive extreme drying but do not survive low temperatures. Species in this category often produce lipids with unusual fatty acids that are highly desired for foods, plastics and lubricants. Using seeds of Cuphea carthagenensis as a model, we have demonstrated that the sensitivity to low temperatures arises from the high crystallization and melting temperatures of the storage lipids. The lipids within the seeds crystallize during storage and remain crystallized when the seeds imbibe water at room temperature. For reasons currently under investigation, interaction of water and crystallized lipids is lethal. The damage can be prevented by melting the lipids (with a 45C pulse) before the seeds are imbibed. Because we can prevent the damage imposed by genebanking conditions, we can now store this valuable germplasm.
Technical Abstract: Seeds classified as having 'intermediate' storage physiology tolerate desiccation and low temperatures but store poorly under cold, dry conditions. Using seeds of Cuphea carthagenensis (Jacq.), we tested the hypothesis that the poor shelf life of intermediate seeds under dry, cool storage conditions can be attributed to phase changes in the storage lipids. While orthodox seeds survive storage for years, the viability of C. carthagenensis seeds stored at 5C decreased from 95 to 50% (P50) within 1 to 50 days depending on water content. Deterioration was slowest in seeds containing 0.04 g H2O /g dry mass (g/g) (RH = 12%). Longevity was greater for seeds stored at 25C compared to 5C (all water contents), but could be restored in the seeds stored at 5C and water contents less than 0.1 g/g if imbibition was preceded by a 45C treatment. The effects of water content and temperature on C. carthagenensis seed viability were related to the triacylglycerol phase behavior, specifically to the crystallization and melting temperatures and to the crystallization rate of triacylglycerols within the seed. In seeds of C. carthagenensis, storage lipids begin to crystallize at 6C during cooling, which contrasts with a crystalization temperature of less than -15C for seeds exhibiting orthodox storage behavior. At 5C, lipid crystallization within C. carthagenensis seeds progressed for 10 to over 200 days, depending on water content, and was slowest in seeds containing 0.04 g/ g. Germination of seeds decreased to 50% (P50) when between 16 and 38% of the triacylglycerols were crystallized. The restoration of germination in seeds stored at 5C by a 45C pulse is attributed to lipid melting transitions which are complete at 45C. Our results demonstrate interactions between water and triacylglycerols in seeds that have not been reported previously: 1) water content affects the propensity of triacylglycerols to crystallize and 2) hydration of seed containing crystallized triacylglycerols is lethal. We believe that these interactions form the basis of the syndrome of damage experienced when seeds with intermediate storage physiologies are placed in long-term storage.