（1College of Horticulture Science and Engineering，Shandong Agricultural University，Huang-Huai-Hai Region Scientific Observation and Experimental Station of Environment-Controlled Agricultural Engineering，Ministry of Agriculture，Tai’an 271018，Shandong，China；2State Key Laboratory of Crop Biology，Tai’an 271018，Shandong，China）
Abstract：Taking‘ Jingxin No.1’ watermelon as experimental material，this paper investigated the effects of different humic substances（HS） concentration（ 0，0.1，0.3，0.6，1.0 g·L-1） in Hoagland on photosynthesis，chlorophyll content，biomass，electrolyte leakage percentage，antioxidant enzyme，proline and Na+，K+contents of watermelon seedling under salt stress（100 mmol·L-1 NaCl）．The results showed that adding HS
could improve the plant growth by different degrees under salt stress，especially the 0.3 g·L-1 treatment had the best alleviation effect，in which the chlorophyll contents，photosynthesis，Fv/Fm and ΦPSⅡ were significantly increased，and the distribution of Na+ and K+ was improved. At the same time，the activities of SOD and CAT were enhanced，and the electrolyte leakage percentage and MDA accumulation were reduced. Thus，watermelon plant
growth was promoted.
