SUPERCOOLING AND ICE NUCLEATION EVENTS IN THE CROWN OF WINTER WHEAT SEEDLINGS

Differential thermal analysis (DTA) was used to investigate the freezing of tissue water in crowns of two cultivars of winter wheat (Triticum aestivum L.), Frederick and Norstar, which differ in their winterhardiness. Crown tissue from 7-day-old nonacclimated seedlings had an LT 50 (lethal temperature at which survival was 50%) of −6 or −8 °C for Frederick and Norstar, respectively, and showed a single freezing exotherm. In contrast, crown tissue from seedlings, acclimated for 5 weeks at 2 °C, had an LT 50 of −14 or −18 °C, respectively, and exhibited a double freezing exotherm. The basal region of the crown has been shown to be more sensitive to freezing injury than the apical meristem and its ice nucleation point was at a higher temperature in all samples. The ability of crowns to supercool decreased slightly as the cooling rate was increased from 2 °C h −1 to 120 °C h −1 , but the water in the less hardy cultivar Frederick, consistently froze at a higher temperature than the more hardy Norstar. Slow cooling rates, such as 2° or 10 °C h −1 , improved the ability of DTA to discriminate the differences in freezing resistance among the cultivars. However, because of the large plant to plant variability this method has no practical value in the identification of more freezing tolerant plants.Key words: Differential Thermal Analysis, Triticum aestivum, freezing, winterhardiness