Abstract: To investigate the effects of high temperature and high humidity compound disasters on leaf photosynthesis and flower sex differentiation of cucumber plants, the cucumber variety "Jinyou101" was used as the test materials. The three factors orthogonal test of air temperature, air relative humidity and treatment days was conducted in the artificial climate chamber of Nanjing University of Information Science and Technology from May to September 2020. Four levels of day/night temperature were set to 32℃/22℃, 35℃/25℃, 38℃/28℃ and 41℃/31℃. The air relative humidity was set at three levels: 50%, 70% and 90%（±5 percent point）, and the durations of treatment time were designed for 3, 6, 9 and 12 days. The treatments of day temperature 28℃/ night temperature 18℃, air relative humidity 50%±5pp were used as control (CK). The photosynthetic parameters, chlorophyll content and endogenous hormone content of terminal bud were measured systematically, and the floral differentiation of each node and final yield were observed. The results showed that: （1）the total amount of chlorophyll in cucumber leaves decreased with the increase of temperature and the extension of stress time. After 12 days of stress, chlorophyll a, chlorophyll b and carotenoid contents in cucumber leaves reached the lowest value, which decreased by 31.51%, 22.62% and 37.10% compared with CK, respectively. （2）The Pmax (maximum net photosynthetic rate), LSP (light saturation point) and AQE (apparent quantum efficiency) values of cucumber leaves decreased significantly with the deepening of high temperature and humidity stress. The LSP, AQE and Pmax values of cucumber leaves at 41℃ were the lowest, which were 48.78%, 40.00% and 43.04% lower than those of CK, respectively. In the range of 32−41℃, the Pmax values of cucumber leaves under 70% air relative humidity were higher than those under other humidity treatments, and the differences were small compared with CK. （3）In the process of flower bud differentiation, the contents of ZT (anti-zeaxin), ABA (abscisic acid) and GA3 (gibberellin) in cucumber terminal buds increased firstly and then decreased, while the contents of IAA decreased gradually with the growth of cucumber plant. The contents of ZT, IAA and GA3 in cucumber terminal buds decreased gradually with the increase of temperature and days of stress, while the changes of ABA content were on the contrary. At the end of 41℃, the contents of ZT, IAA and GA3 decreased by 47.37%, 26.38% and 83.65%, respectively, compared with CK, while the ABA content increased by 27.27% compared with CK. After 12 days of stress, the contents of ZT, IAA and GA3 reached the lowest values in the same period (0.07µg·g⁻¹, 1.11µg·g⁻¹, 0.75µg·g⁻¹), while the ABA content reached the highest values in the same period (1.89µg·g⁻¹).（4）High temperature and high humidity stress increased the node position of each female flower, but decreased the node rate of female flower and the number of flower buds. When treated with 70% air relative humidity, the nodes of female flowers were decreased, and the proportion of female flower nodes and the number of flower buds were increased. （5）High temperature and high humidity stress significantly reduced cucumber yield, and the yield of cucumber treated at 41℃ was 38.50% lower than that of CK. The study found that high temperature inhibited photosynthesis in cucumber leaves, disrupted the endogenous hormone balance in terminal buds, and reduced the number of female flower differentiation, resulting in lower final yield.

Key words: Cucumber, High temperature and humidity, Photosynthesis, Endogenous hormone, Flower sex differentiation, Yield