Freezing avoidance by deep supercooling in hydrated lettuce seeds

DRY seeds of several species survive exposure to extremely low temperatures, including immersion in liquid air for up to 60 d (refs 1–3), but most young seedlings are very susceptible4. Fully hardened twigs of several northern tree species tolerate freezing in liquid nitrogen (−196 °C) (ref. 5), and deep supercooling seems to be an important mechanism for the avoidance of freezing in both xylem ray parenchyma and flower buds of many other hardy but less cold resistant, deciduous, woody species6,7. Resistance to freezing stress in seeds has been suggested to be related to water content1,4,8, but little attention has been paid to either the relationship between stage of germination and degree of freezing resistance or the nature of the resistance. We report here that hydrated lettuce seeds (achenes) can avoid injury by deep supercooling and that the level of resistance can be determined precisely by differential thermal analysis (DTA). Furthermore, we show that the deep supercooling is dependent on the intact structure of the endosperm.