Abstract: In order to study the effects of high temperature and high humidity on the chlorophyll fluorescence characteristics in the center of photosystem II of cucumber leaves at the seedling stage, a cucumber variety "Jinyou 101" was used as the test material to conduct an artificial environment control experiment. A three-factor orthogonal test was designed, and a high temperature conditions was designed with 4 levels: 32℃(the highest daily temperature)/22℃(the lowest daily temperature), 35℃/25℃, 38℃/28℃ and 41℃/31℃.The air relative humidity was designed with three gradient levels: 50%±5pp (percent point), 70%±5pp and 90%±5pp. The treatment duration was designed with 4 levels: 3d, 6d, 9d and 12d, and 28℃/18℃, 50%±5% was used as the control (CK). The kinetic parameters of chlorophyll fluorescence induction in cucumber leaves under different treatments were determined to understand the effect of high temperature and high humidity on the chlorophyll fluorescence characteristics of cucumber photosystem II center. The results showed that: In the daily maximum range of 32-41℃, with the increase of temperature, the normalized curve ΔV_t of fast fluorescence kinetics of cucumber leaves at seedling stage showed obvious convexity in J and K phases. The curve of ΔV_t at 41℃ was the most obvious (ΔV_J=0.24848, ΔV_K=0.09116), and both ΔK and ΔJ were >0. The normalized curve ΔWt exhibits a distinct K peak. However, the normalized curve ΔVt of 70% relative air humidity treatment had no obvious bulge (ΔV_J=0.00421, ΔV_K=-0.0031). With the increase of temperature, the energy dissipated per unit reaction center (DI_o/RC) and the quantum ratio used for heat dissipation (φD_o) both increased, reaching the highest at 41℃. Compared with CK, DI_o/RC and φD_o increased by 85.24% and 18.96% respectively at 41℃. The quantum yield for electron transfer (φE_o) and the energy for electron transfer captured by unit reaction center (ET_o/RC) were reduced, reaching the lowest at 41℃, which were 49.64% and 38.05% less than CK, respectively. Under the high temperature stress, ET_o/RC and φE_o were higher than 50% and 90% in the treatment of 70% relative air humidity. Compared with the 50% treatment, the ET_o/RC and φE_o increased by 28.69% and 47.04% under the 70% relative air humidity treatment, and increased by 18.36% and 12.02% compared with the 90% treatment. Research suggests that high temperature destroys the structure and function of the photosystem of cucumber leaves, resulting in a decrease in photochemical efficiency. The relative air humidity of 70% can reduce the damage of high temperature on the donor side and acceptor side of photosystem II, and relieve the damage of high temperature on the photosynthetic mechanism of cucumber leaves.

Key words: High temperature and humidity disaster, Cucumber, Photosystem II, Chlorophyll fluorescence