Abstract: Taking the detached leaves of tree peony (Paeonia suffruticosa cv. ‘Roufurong’) as test materials, this paper studied the effects of high temperature (40 ℃) and low temperature (15 ℃) stresses on the PSⅡ functions and physiological characteristics of peony leaves under strong light intensity (1400 μmol·m^-2·s^-1), with 25 ℃ as the control. With the increasing time of high- and low temperature stress, the maximal photochemical efficiency (F_v/F_m), actual quantum yield of photosystemⅡ(Φ_PSⅡ), and efficiency of excitation capture of open PSⅡ center (F_v′/F_m′) all decreased continuously. After recovered in the dark for 4 hours, the F_v/F_m in treatments 15 ℃ and 25 ℃ quickly recovered, but that in treatment 40 ℃ only recovered to 75.5% of non-treatment, even if the leaves were treated in the dark for 15 hours. At 40 ℃, the balance of excited energy between PSⅠand PSⅡunder strong light intensity was perturbed seriously. Treatment 40 ℃ inhibited the superoxide dismutase (SOD) activity, enhanced the production of  O₂^-·, H₂O₂, and MDA, and reduced the contents of chlorophyll and soluble protein. This study revealed that strong light combined with high temperature impaired the photosynthetic apparatus of the tree peony irreversibly, whereas strong light plus low temperature had weaker impact.

Key words: tree peony, high temperature stress, low temperature stress, chlorophyll fluorescence, superoxide dismutase