Strawberry variety 'Benihoppe' was used as the experimental material. The temperature treatments were set at 32 ℃/22 ℃, 35 ℃/25 ℃, 38 ℃/28 ℃ and 41 ℃/31 ℃ (daily maximum temperature/daily minimum temperature), and the stress days lasted for 2, 5, 8 and 11 d, with 28 ℃/18 ℃ as the control. We measured the photosynthetic characteristics, chlorophyll fluorescence characteristics, reactive oxygen species, protective enzyme activity and membrane lipid peroxidation of strawberry under different high temperature treatments. The key indices were extracted by principal component analysis. The high temperature stress index (Z) was defined to divide the high temperature stress grade. The results showed that 1) with the aggravation of high temperature stress and the extension of stress time, chlorophyll a (Chl a), chlorophyll b (Chl b), carotenoid (Car), light saturation point (LSP), maximum net photosynthetic rate (P_max), apparent quantum efficiency (AQE) and maximum photochemical efficiency (F_v/F_m) decreased, while light compensation point (LCP) and dark respiration rate (R_d) increased. 2) High temperature hindered the energy transfer of thylakoid in PSⅡ center (ΔW_OK＞0), and accelerated the reduction rate of PSⅠ terminal electron receptor pool. On the 11th day of the stress, except that under 32 ℃, all other oxygen evolution complexes (OEC) were inactivated. 3) The content of reactive oxygen species (H₂O₂ Content and O₂^-· production rate) and malondialdehyde (MDA) increased with the stress days under different high temperature treatments. 4) The protective enzyme activities and soluble protein content increased first and then decreased with stress duration. 5) Based on principal component analysis (PCA) and combined with the difficulty of index acquisition, Chl a, P_max, F_v/F_m and MDA were extracted as the key indices, and Z value was calculated. Five high temperature stress grades were divided which were normal (0<Z≤1), mild (1<Z≤2), moderate (2<Z≤3), severe (3<Z≤4) and extra severe (4<Z). The results would be helpful for the prevention of strawberry high temperature disaster and the optimization of microclimate environment.

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