Effects of combined stress of elevated temperature and drought and of re-watering on the photosynthetic characteristics and chlorophyll fluorescence parameters of Broussonetia papyrifera seedlings
In recent years, with the global climate change, high temperature and temporal drought during summer seasons have increasingly taken place in some subtropical regions. Conse-quently, the survival and growth of plants are restrained under the combined stress of high tem-perature 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 alle-viation role of re-watering, were measured. The results showed that, under the combined stres-ses, 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 (F_0) pres-ented an increasing tendency. Combined stress showed synergistic effects on P_n, G_s, and T_r. The parameters such as intercellular CO_2 concentration (C_i), G_s, F_m and F_v/F_m showed signifi-cant differences among various temperature levels, and the parameters such as G_s, T_r, F_0 showed significant variations among different soil moisture levels, while P_n and F_0 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 var-iation in terms of F_0 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-wate-ring can release the stress in a certain temperature range.