Abstract: To provide scientific basis for the protection and utilization of Paranephelium hainanensis, a species with extremely small population in Hainan Province, we explored its photosynthetic physiological characteristics. The seasonal and daily dynamics of photosynthesis characteristics and chlorophyll fluorescence parameters of adult trees and understory seedlings of P. hainanensis were measured by Li-COR portable photosynthetic apparatus and FC1000-H portable plant fluorescence imaging system. The daily variation of net photosynthetic rate (P_n) and transpiration rate (T_r) of both adult trees and seedlings showed an obvious “double peak” curve in July and November. The maximum P_n and T_r of adult trees were 9.24 mol·m^-2·s^-1 and 3.34 mol·m^-2·s^-1, respectively, resulting in a stronger photosynthetic capacity than that of understory seedlings. The mean value of maximum initial fluorescence (F₀) of adult trees was 323.04 and 135.87 in July and November, respectively. The average maximum fluorescence (F_m) of adult trees was 502.78 in July and 307.76 in November. Both F₀ and F_m of seedlings were lower than that of adult trees. TheF_v/F_m curve of adult trees showed a “singlevalley” pattern in July, and displayed a daily fluctuation with a trend of decline first, then rise and decline again in November. Results of correlation analysis showed that P_n of adult trees was positively correlated with T_r and F_v/F_m, and that P_n of seedlings was positively correlated with F₀ in July. In November, P_n of adult trees was positively correlated with T_r and F_m, and negatively correlated with F_v/F_m. The P_n of seedlings was significantly positively correlated with F_m. Our results suggest that P. hainanensis is a lightdemanding tree species, with specific photosynthetic characteristics and chlorophyll fluorescence parameters in July and November to adapt to environments. Water may be the key factor causing seasonal changes of photosynthetic rate and chlorophyll fluorescence parameters inP. hainanensis.  

Key words: low temperature, salicylic acid, photosynthesis., brassinolide, cucumber (Cucumis sativa)