Abstract: The artificial environment control experiment was conducted at Nanjing University of Information Science and Technology from November to December 2022 using strawberry "Selva" (Fragaria ananassa) as the test material, and the strawberry seedlings were selected and placed in the artificial climate chamber for low-temperature treatment in mid-November when they reached the flowering stage. Three levels of temperature were set: 5℃(daily minimum)/15℃ (daily maximum), 10℃/20℃ and 15℃/25℃ (CK). The treatment duration was 3d, 6d and 9d. The kinetic parameters of photosystem fluorescence induction and protease activity of strawberry leaves were measured under different days of low-temperature to understand the effect of low-temperature on photosystem fluorescence and protease activity of strawberry leaves. The effects of low-temperature on the fluorescence properties of strawberry leaves and the activity of protease were measured. The results showed that (1) the light energy absorbed per reaction center (ABS/RC), the energy dissipated per reaction center (DIo/RC) and the energy captured per reaction center for QA reduction (TRo/RC) of strawberry leaves increased with decreasing temperature and increasing duration in the range of daily minimum temperature from 5℃ to 15℃, but the energy captured per reaction center for QA reduction (ETo/RC) increased with increasing duration. The energy captured per reaction center for electron transfer (ETo/RC) decreased. ABS/RC and DIo/RC were 1.367 and 1.718 times higher than those of CK for the treatment with daily minimum temperature of 5℃ for 9d, respectively. TRo/RC increased by 7.42%, 14.66%, 25.27%, and 30.48% for daily minimum temperature of 10°C for 3d, 6d, 9d and daily minimum temperature of 5°C for 9d, respectively, compared to CK, while ETo/RC decreased by 1.71%, 20.39%, 26.65%, and 28.32%, respectively, compared to CK. The quantum ratio for thermal dissipation (φDo) increased significantly with the duration of low-temperature, 12.34% and 25.32% higher than CK at a daily minimum temperature of 5℃ for 3d and 9d, respectively. However, the quantum yield for electron transfer (φEo) decreased with the prolongation of low-temperature, and decreased by 27.92% and 43.02% compared with CK for 3d and 9d, respectively. (2) The activities of superoxide dismutase (SOD),catalase (CAT), peroxidase (POD), and malondialdehyde (MDA) of strawberry leaves gradually increased with the decrease of temperature and the extension of the duration of low-temperature. The activities of SOD and POD reached the maximum at the daily minimum temperature of 5℃ for 6d, which were 1.39 and 2.03 times higher than those of CK, respectively, while CAT was less affected by low temperature. The MDA content was the highest at the minimum daily temperature of 5°C for 9d, which was 2.10 times higher than that of CK. The study showed that low temperatures will damage the structure and function of the photosystem of strawberry leaves, the photochemical efficiency decreases, the energy dissipated by heat increases, resulting in a decrease in the ability of PSⅡ donor side to provide electrons, and the electron transfer efficiency decreases, strawberry leaves will stimulate the antioxidant system in the body, and produce a certain resistance to the environment, which shows that the activities of SOD, POD and CAT have different degrees of increase compared with CK. However, the cell membrane structure was still damaged and the membrane permeability increased.

Key words: Strawberry, Low-temperature, Chlorophyll fluorescence, Protective enzymes, MDA content