Abstract: Using chlorophyll fluorescence instrument Mini PAM, authors measured the Quercus variabilis leaves maximal and actual chlorophyll fluorescence quantum yield of photo system Ⅱ(Fv/Fm and ΔF/Fm’) in order to study the temperature of photo system Ⅱ inactivation of Quercus variabilis leaves on high temperature stress.There were 10 high temperature treatments of 35～55℃ with treating time 5～85min.The results showed that (1) both Fv/Fm and ΔF/Fm’ decreased with increase of heat stress temperature and increase of high temperature treating time; and the process of that ΔF/Fm’decreased could be simulated with Logistic curves; (2) High temperature stress clearly influenced photosynthesis of Quercus variabilis leaves; the temperature of that ΔF/Fm’ decreased 50%(T50) was limit temperature index of photo system Ⅱ inactivation, and could be taken as characteristic point of high temperature stress; the high temperature tress index of Quercus variabilis was determined as 42℃. This paper is useful for analyzing the intensity and lasting time of high temperature stress of Quercus variabilis leaves, and providing methods of determining agricultural meteorology index of other plants.

Key words: High temperature stress, Quercus variabilis, PSⅡ, Chlorophyll fluorescence, Quantum yield