Triacylglycerol phase and 'intermediate' seed storage physiology: a study of Cuphea carthagenensis

Seeds with 'intermediate' storage physiology store poorly under cold and dry conditions. We tested whether the poor shelf life can be attributed to triacylglycerol phase changes using Cuphea carthagenensis (Jacq.) seeds. Viability remained high when seeds were stored at 25 degrees C, but was lost quickly when seeds were stored at 5 degrees C. Deterioration was fastest in seeds with high (>or=0.10 g g(-1)) and low (0.01 g g(-1)) water contents (g H(2)O g dry mass(-1)), and slowest in seeds containing 0.04 g g(-1). A 45 degrees C treatment before imbibition restored germination of dry seeds by melting crystallized triacylglycerols. Here, we show that the rate of deterioration in C. carthagenensis seeds stored at 5 degrees C correlated with the rate that triacylglycerols crystallized within the seeds. Lipid crystallization, measured using differential scanning calorimetry, occurred at 6 degrees C for this species and was fastest for seeds stored at 5 degrees C that had high and very low water contents, and slowest for seeds containing 0.04 g g(-1). Germination decreased to 50% (P50) when between 16 and 38% of the triacylglycerols crystallized; complete crystallization took from 10 to over 200 days depending on water content. Our results demonstrate interactions between water and triacylglycerols in seeds: (1) water content affects the propensity of triacylglycerols to crystallize and (2) hydration of seed containing crystallized triacylglycerols is lethal. We suggest that these interactions form the basis of the syndrome of damage experienced when seeds with intermediate storage physiologies are placed in long-term storage.