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In order to elucidate the effects of low-temperature stress on the photosynthetic and stomatal properties of strawberry leaves during the flowering phase, the perennial strawberry cultivar 'Selva' was selected as the experimental material. The environmental control experiment was designed and conducted at Nanjing University of Information Science and Technology (NUIST) with Venlo greenhouse and artificial climate chamber in 2022. Two low temperature levels (15°C/5°C, 20°C /10°C) were set, and each of them represented day temperature with night temperature. Treatment T1 and T2 were used to distinguish them. The duration of low temperature was set at 3d, 6d, and 9d. The plants grown in the greenhouse with temperature set at 25°C/15°C were used as the control group (CK). Photosynthetic parameters and stomatal indices were determined for the leaves. The results showed that: (1) low temperature stress significantly inhibited the photosynthesis of strawberry, with the intensification of low temperature, the maximum net photosynthetic rate (Pnmax), light saturation point (LSP), stomatal conductivity (Gs), transpiration rate (Tr) gradually decreased, while light compensation point (LCP), dark respiration rate (Rd), intercellular CO2 concentration (Ci), stomatal limit (Ls) and water use efficiency (WUE) tended to increase. The main cause of the reduced photosynthetic rate was stomatal restriction. (2) Low temperature stress reduced the content of chlorophyll a, chlorophyll b, total chlorophyll content (a+b) and carotenoids in leaves. However, as the duration lengthened, the photosynthetic pigment content increased slightly, although it remained significantly lower than CK. (3) Low temperature stress had a particularly significant effect on leaf stomatal characteristics, i.e. the stomatal length, stomatal width, stomatal open degree, stomatal perimeter, stomatal area and stomatal opening ratio, which all decreased significantly with the decrease of temperature and the increase of the duration. Stomatal density tended to decrease but not significantly. This study showed that low temperatures reduced the stomatal size and the chlorophyll content, inhibiting the photosynthetic capacity of the leaves during the flowering stage. [ABSTRACT FROM AUTHOR]

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°C (daily minimum)/15°C (daily maximum), 10°C/20°C and 15°C/25°C (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°C to 15 °C, 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°C 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 C 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 5C 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 PS II 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. [ABSTRACT FROM AUTHOR]