Abstract: In this experiment, summer squash variety' Nongyuan No.1' was used as experimental material, and seven treatments were set up: suitable temperature treatment (day/night 25℃/16℃, CK), watering 1/3 times Yamazaki cucumber formula; Suboptimal temperature treatment (day/night 16℃/8℃, TCK), watering 1/3 times Yamazaki cucumber formula. Under sub-optimal temperature conditions, 1/3 times Yamazaki cucumber formula was used to prepare nutrient solution with HIDS concentration of 100, 400, 700, 1000, 1300mg·L−1(T1, T2, T3, T4 and T5 treatments, respectively). After 18 days of indoor culture in artificial climate, the effects of HIDS on the growth index, osmotic adjustment substances in leaves and MDA of summer squash seedlings were analyzed. The results showed that the growth status, antioxidant enzyme activity and photosynthetic capacity of summer squash treated with TCK were worse than those treated with CK. Under suboptimal temperature, HIDS treatment with different concentrations can alleviate the inhibition of suboptimal temperature on the growth of summer squash seedlings, and the optimum concentration is 700mg·L−1 (T3). Compared with TCK treatment, the leaf area per plant, fresh and dry weight of the whole plant, soluble protein, proline content, POD, SOD and CAT activities are significantly increased, and MDA content is significantly decreased. The total amount of chlorophyll, photosynthetic rate, transpiration rate, stomatal conductance increased significantly; The Fv/Fm, ΦPSⅡ, Fv'/Fm', ETR, qP and NPQ decreased, alleviate the damage caused by suboptimal temperature to photosynthetic organs of summer squash. Therefore, adding suitable concentration of HIDS can improve the adaptability of summer squash seedlings to suboptimal temperature by promoting the accumulation of biomass and osmotic adjustment substances in leaves, reducing the degree of cell membrane lipid peroxidation, and improving the activity of antioxidant enzymes and photosynthetic capacity, and when the concentration of HIDS is 700mg·L−1, each index is closest to CK treatment.

Key words: Zucchini, Suboptimal temperature, HIDS, Antioxidant, Photosynthetic