Objective  In this sutdy, we investigated the photosynthetic characteristic responses of Quercus variabilis and Quercus aliena var. acuteserrata seedlings to long-term high temperature stress in summer, as well as the combination between high-temperature levels and internal mechanism of photosynthesis to provide theoretical reference for seedling cultivation and selection of stress tolerant tree species under warming background.

  Method  QV and QA seedlings were exposed to three temperature levels (CK-27 ℃, HT1-35 ℃, HT2-42 ℃) for 8 consecutive days (6 h per day) in the artificial climate simulation chamber. The changes of chlorophyll fluorescence parameters (F_v/F_m, Fy, NPQ, YⅡ, qP), gas exchange parameters (P_n, G_s, C_i, T_r）, WUE and rapid light response curve were measured and analyzed.

  Result  (1) The two high temperature treatments did not affect the photosystem Ⅱ activity (F_v/F_m) and the light energy dissipation of antenna pigments in the leaves of two oak seedlings(NPQ). With the increase of the degree of high temperature stress, the two oak seedlings adapted to the high temperature stress environment by inhibiting the electron flow (qP) of PSⅡ reaction center, reducing the actual quantum yield (Fy, YⅡ), weakening the electron transfer rate (rETR). (2) High temperature stress could significantly affect P_n, G_s and C_i of two oak species. At 35 ℃, P_n, G_s and C_i of 2 tree species decreased significantly; at 42 ℃, G_s of the two tree species did not increase, but C_i showed an upward trend. After the high temperature stress being relieved, except for Quercus variabilis HT2 group, all gas exchange parameters basically returned to the initial values. Besides, the effects of high temperature stress on G_s and C_i of Quercus variabilis were more significant. (3) Under HT1 high temperature treatment, the main factor inhibiting the photosynthesis of the two oak seedlings was stomatal limitation, and under HT2 high temperature treatment, the main factor affecting the photosynthesis of the two oak seedlings was non stomatal factor.

  Conclusion  (1) The intrinsic regulation mechanism of photosynthesis of two deciduous oak seedlings is related to the degree of warming. (2) The photosynthetic capacity of two deciduous oak seedlings decreases under high temperature stress. However, Quercus aliena var. acuteserrata seedlings are more tolerant than Quercus variabilis as indicated by higher rETR and P_n.