Thermal analysis and cryopreservation of seeds of Australian wild Citrus species (rutaceae): Citrus australasica, C. inodora and C. garrawayi.

We investigated the relationship between the thermal transitions in seeds, cryopreservation and geographical origin for the rare and threatened northern Australian Citrus species, Citrus inodora and C. garrawayi, and southeastern Australian species C. australasica, which is cultivated as a 'bushfood'. Thermal analysis of phase transitions in cotyledon tissue revealed differences between species in the melt onset temperatures of in vivo seed oils, suggestive of differences in the proportion of saturated fatty acids. These differences appeared to be associated with geographic gradient, i.e. an increased mean onset temperature of lipid melt coincided with latitude (N NSW / SE QLD Australia to N QLD) and climatic zone (warm subtropical to hot tropical) of the natural distribution range. In addition, the thermal transitions of seed oils corresponded to the temperature limit for germination. Tolerance to cryopreservation was demonstrated in all three species after drying, with a mean germination of 75 +/- 2, 71 +/- 7 and 42 +/- 12 percent for C. australasica, C. inodora and C. garrawayi, respectively, when dried below the unfrozen water content (WCu) determined for each species. All three species have edible fruits and seed cryopreservation now offers an alternative strategy for the long-term ex situ conservation of this valuable germplasm.