WINTERFAT (EUROTIA-LANATA (PURSH) MOQ.) SEEDBED ECOLOGY - LOW-TEMPERATURE EXOTHERMS AND COLD-HARDINESS IN HYDRATED SEEDS AS INFLUENCED BY IMBIBITION TEMPERATURE

Thermal analyses of freezing events in hydrated lettuce (Lactuca sativ a L.) seeds show a correlation between low temperature exotherms (LTEs ) (evidence of ice crystal formation) and seed death. Yet, weather pat terns common to the Northern Great Plains of North America regularly c reate conditions where non-dormant seeds of native plants hydrate with snow melt and are subsequently exposed to -30 degrees C or colder con ditions. To determine if such weather patterns decimate dispersed seed s, we measured the effects of freezing on fully hydrated winterfat (Eu rotia lanata (Pursh) Moq.) seeds harvested from the Northern Plains at two USA and one Canadian location. Survival of hydrated seeds to -30 degrees C at a cooling rate of 2.5 degrees C h(-1) was similar to that of seeds not subjected to cooling, even though both a high temperatur e exotherm (HTE) and an LTE were observed. Although the LTE was not re lated to winterfat seed survival, freeze-stressed seeds had reduced ge rmination rates and reduced seedling vigour, particularly for the coll ection with the lightest seeds. The temperature of LTEs was similar am ong seed collections with a mean of -17.6 degrees C, but was warmer wh en the seeds were imbibed at 0 degrees C compared to 5, 10 or 20 degre es C. We found a significant correlation between the HTE and LTE tempe ratures. The difference and the correlation may be due to the higher m oisture content of seeds imbibed at 0 degrees C. After pericarp remova l, only one exotherm in the range of the LTE was observed. This was al so true for the naked embryo. We conclude that an LTE indicates ice fo rmation in the embryo, but that it does not signal the death of a wint erfat seed. (C) 1998 Annals of Botany Company.