Abstract: Taking seedlings and 6year trees of variety Zihonglong (Hylocereus polyrhizus Briton & Rose) as materials, five artificial environmental control experiments and one basis experiment were conducted in artificial climate chamber in September, 2012. The control experiment temperature was 2, 0, -2, -4 and -6℃, respectively, and the basis experiment was 20℃. At each experiment, relative electrical conductivity (REC), MDA content, SOD activity and soluble protein content of pitaya branch were measured after treatment 12 hours, and morphological changes were observed at the same time, in order to investigate the relationship between the factors and temperature and find the indices of chilling injury for pitaya. The results showed that seedlings and 6year trees damaged obvious at 0℃ and -2℃ respectively. The lower temperature was, the more obvious injury symptoms came out. REC and MDA content increased with temperature decline. SOD activity and soluble protein content increased at beginning and then decreased. The Logistic Model was used to fit the sigmoid curve between REC from plant tissues and temperature of treatments. Then the semi lethal temperature, which was -1.7℃ and -2.2℃, was determined by the model. The results indicated that the semi lethal temperature of seedlings and 6year trees were about -1.7℃ and -2.2℃, respectively. Low temperature damaged both seedlings and 6year trees, and the lower temperature was, the more badly injury would get. 6year trees could tolerate lower temperature than seedling. The results could provide scientific basis for chilling injury prevention and planting division of pitaya.

Key words: Low temperature, Pitaya, Chilling injury, Temperature indices