Thermal behaviour of lipids in short-lived seeds of Australian rainforest species

Recent studies on desiccation-tolerant Australian rainforest seeds demonstrated that some were short-lived in storage. We sought to understand structural changes of storage lipids that might occur during storage at -20 °C that could contribute to a short lifespan.

We used differential scanning calorimetry (DSC) to examine exothermic and endothermic transitions during freezing and thawing in dry seed samples of 23 species. Seed samples and extracted triacylglycerols (TAGs) were cooled to -150 °C and rewarmed to 50 °C at 10 °C min-1; slower and faster rates of cooling/warming were used for a subset of species to examine lipid crystallization and melting kinetics. Thermograms were analysed for temperature and enthalpy of observed peaks, and these were compared with expected values to detect anomalies. Extracted lipids were further analysed using gas chromatography to characterize fatty acid composition. The thermal profiles of six species were used to design experiments comparing the impact of storage at -20 °C with storage at temperatures outside the range of thermal transitions.

Thermal activity was detected in 22 species within the narrow temperature range of -30 and -10 °C; activity at broader temperature ranges was also detected depending on species, cooling protocol and fatty acid composition. A profound interaction between DSC parameters and time at low temperature, as well as fatty acid composition, suggested that TAG crystallization rates contribute to low-temperature sensitivity. We confirmed that damage from TAG crystallization could be avoided by storing seeds at temperatures above TAG crystallization and melting events; storage at cryogenic temperatures improved survival over storage at -20 °C but requires further optimization to maintain pre-storage germination potential.

We conclude that the crystallization and melting of TAGs during storage may negatively impact seed longevity. Seed thermal profiles and rate of TAG crystallization may serve as predictive tools for sensitivity to storage at -20 °C.