It is generally believed that low temperature storage is not suitable for recalcitrant seeds as they are sensitive to freezing temperatures. In the present study, the freezing sensitivity of Chinese cork oak and sawtooth oak acorns was determined by storing them at below freezing temperatures. The results showed that both species were sensitive to subzero temperatures, and the half-viability period (P50) was less than seven days when they were directly exposed to and kept at -10°C. When the seeds were gradually cooled to -10°C the survival rate significantly increased, and the relative electrolyte leakage and lipid peroxidation decreased significantly compared to control seeds. This indicated enhanced resistance to freezing damage of gradually cooling treated seeds. The P50 of gradually cooled sawtooth oak seeds at -10°C was prolonged to about 90 days. The cooling rate affected the freezing sensitivity of sawtooth oak seeds, and the optimum equilibration time to maximize survival rate was 14 days at intermediate temperatures. Therefore, gradual cooling could significantly reduce the sensitivity of seeds to freezing temperatures, and prolong seed P50 suggesting a potential way to store recalcitrant seeds at subzero temperatures.
