Abstract: In recent years, with the global climate change, high temperature and temporal drought during summer seasons have increasingly taken place in some subtropical regions. Consequently, the survival and growth of plants are restrained under the combined stress of high temperature and drought. In the present study, at different controlled temperature and soil moisture levels, the combined stress of high temperature and drought on the photosynthetic characteristics and chlorophyll fluorescence parameters of Broussonetia papyrifera seedlings, as well as the alleviation role of rewatering, were measured. The results showed that, under the combined stresses, the net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), maximal fluorescence (F_m), optimal/maximal photochemical efficiency of PSⅡ (F_v/F_m) of B. papyriferas seedlings presented a decreasing tendency, while minimal fluorescence (F₀) presented an increasing tendency. Combined stress showed synergistic effects on P_n, G_s, and T_r. The parameters such as intercellular CO₂ concentration (C_i), G_s, F_m and F_v/F_m showed significant differences among various temperature levels, and the parameters such as G_s, T_r, F₀ showed significant variations among different soil moisture levels, while P_n and F₀ were influenced by both temperature and soil moistures levels. Rewatering alleviated the stress at 25 and 33 ℃ through improving the photosynthesis performance and F_v/F_m to some degree, but no obvious variation in terms of F₀ and F_m after rewatering. Our results suggest that the combined stress has stronger effects on the photosynthetic characteristics of plants than the single stress, and rewatering can release the stress in a certain temperature range.

Key words: fragmentation, Xi’an City, ecosystem service value, landscape pattern