Abstract: [Objective] We compared the effects of CaCl₂, ABA and sucrose pretreatment on germination of Fraxinus mandshurica dormancy-released seeds dehydrated at low temperature, and explored the intrinsic physiological changes of seeds during dehydration under different pretreatment conditions to reveal the physiological mechanism of regulation and control of seed desiccation tolerance.[Method] The dormancy released seeds of F. mandshurica were used to investigate seed germination ability after dehydration at low temperature(5℃) pretreated by different concentrations(10^-2, 10^-3, 10^-4, 10^-5 mol·L^-1) of CaCl₂ solution, different concentrations(10^-3, 10^-4, 10^-5, 10^-6 mol·L^-1) of ABA solution or different mass concentrations(1, 10, 50, 100 g·L^-1) of sucrose solution for 24 hours,the seeds were soaked in distilled water for 24 hours as control. According to the above experimental result, the best treatment concentration to improve the seed germination ability, CaCl₂ 10^-3 mol·L^-1, ABA 10^-6 mol·L^-1 and sucrose 100 g·L^-1, were selected respectively. Then, the seeds dehydrated at low temperature after pretreating with the selected solution for 24 hours were used for this study, and the seeds soaked in distilled water for 24 hours were used as control, the effects of different pretreatment methods on seed cell membrane permeability, antioxidant enzyme activity and storage material metabolism were analyzed by measuring the relative electrical conductivity, the activities of antioxidant enzymes (SOD, POD and CAT), the malondialdehyde (MDA) content, soluble sucrose, soluble protein and starch content in the seeds.[Result] Pretreatment of 10^-3 mol·L^-1 CaCl₂or 10^-6 mol·L^-1 ABA could significantly improve the germination rate and germination index of the seeds after dehydration at low temperature (5℃), and shorten the seed germination time; pretreatment of 100 g·L^-1 sucrose could significantly improve the seed germination rate after dehydration at low temperature (5℃), but had no significant effect on the germination index and the germination time. For the seeds pretreated with 10^-3 mol·L^-1 CaCl₂, the SOD activity in embryos was significantly increased after dehydration at low temperature (5℃), and the relative electrical conductivity and MDA content in seeds were significantly decreased; For the seeds pretreated with 10^-6mol·L^-1 ABA, the POD activity in embryos was significantly increased after dehydration at low temperature (5℃), and the relative conductivity and MDA content in seeds decreased significantly; For the seeds pretreated with 100 g·L^-1 sucrose, the SOD and POD activity in embryos were significantly increased after dehydration at low temperature (5℃), the relative conductivity and MDA content in seeds decreased significantly. CaCl₂, ABA and sucrose pretreatment significantly reduced the soluble protein content, and increased the content of soluble sugar in seed embryo and endosperm.[Conclusion] The appropriate concentration of CaCl₂ (10^-3 mol·L^-1), ABA(10^-6 mol·L^-1)and sucrose (100 g·L^-1) pretreatment (especially CaCl₂ or ABA pretreatment) can improve the germination ability of Fraxinus mandshurica seeds after dehydration at low temperature. Under low temperature dehydration conditions, CaCl₂, ABA and sucrose pretreatment can significantly improve the antioxidant system enzyme activity in the seed embryo, better maintain the integrity of cell membrane structure, reduce ion leakage and MDA content of cells. But the antioxidant enzyme activities in different pretreated seeds were different. Pretreatment with CaCl₂ was more beneficial to improve the activity of SOD in embryos, and ABA pretreatment was more beneficial to improve the activity of POD in embryos, while sucrose pretreatment increased the activity of SOD and POD in embryos at the same time. The improvement of the protective function in the low temperature dehydration process by pretreatment of CaCl₂, ABA and sucrose was probably related to the increase of the soluble sugar in embryos.

Key words: CaCl₂, ABA, sucrose, dehydration tolerance, seed germination ability, cell membrane permeability, antioxidant enzyme activity