Abstract: Camellia hainanica are frequently impaired by the seasonal high temperature weather in Hainan. In this study, we investigated the effects of high-temperature stress on the physiology of five grafted cultivars of C. hainanica (1.5 years old), which were exposed to high temperature (40±2 ℃) for nine days. A total of 13 physiological indices were measured in the leaves from each cultivar. The heat tolerance of the five cultivars was evaluated comprehensively using multivariate statistical analysis. The results showed that hightemperature exposure increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and the accumulation of soluble protein and free proline in the leaves of all the five C. hainanica cultivars. Under high-temperature stress for 3-6 days, the growth rate of malondialdehyde (MDA) content and relative conductivity in all the five cultivars was relatively smaller compared with that of the control group. Moreover, the total chlorophyll and carotenoid contents of the five cultivars increased to a certain extent, or showed a modest decline. However, on the 9th day of treatment, the MDA content and relative conductivity in the leaves of all the five cultivars significantly increased, while total chlorophyll and carotenoid content significantly decreased. There was difference in the physiological responses to high-temperature stress among cultivars. System clustering analysis indicated that the five cultivars were classified into three distinct heat-tolerance categories: heat-tolerant (Wanhai 1), moderately heat-tolerant (Wanhai 4), and heat sensitive (Wanhai 3, Haida 1, and Haida 4). A stepwise regression analysis was performed to evaluate heat tolerance in C. hainanica using the equation: Y=2.240-1.625x11 (P=0.014, R²=0.864), where Y represents the predicted value of comprehensive evaluation score, x11 denotes the heat resistance coefficient of SOD activity. This equation can serve as a reliable and accurate method for assessing heat tolerance in C. hainanica, and SOD activity can serve as a key index for screening heat-tolerance cultivars of C. hainanica. Our results revealed the physiological mechanisms underlying the response of different C. hainanica cultivars to high-temperatures stress, providing a valuable reference for identifying heat tolerance and optimizing cultivation of C. hainanica cultivars.

Key words: Camellia hainanica, high temperature stress, physiological characteristics, heat resistance, multivariate evaluation