Physiological effects of foliar pre spray of salicylic acid on alleviating high temperature stress in Citrullus lanatus seedlings

[Objective] This study aims to investigate the mitigation effect and physiological mechanism of salicylic acid (SA) on high temperature stress injury of watermelon seedlings, and to provide a theoretical basis for the application of SA in watermelon cultivation. [Methods] Potted ‘K53’ watermelon seedlings in greenhouse were used as experimental materials. After spraying different concentrations (0, 0.5, 1.0, 1.5, and 2.0 mmol/L) of SA on the leaves, the seedlings were stressed at 42 ℃ for 36 h. The growth characteristics, anatomical structure, physiological and biochemical indexes of watermelon seedlings were observed, and the mitigation effects of each treatment were comprehensively evaluated by membership function method. [Results] After high temperature stress, the leaves of watermelon seedlings were wilted seriously, the growth of root length was inhibited, the leaves became thinner, and the mesophyll cell structure was damaged. After SA application, the leaves became thicker, photosynthetic pigment content, photosynthetic gas exchange parameters, antioxidant enzyme (POD, SOD, CAT) activity and osmotic adjustment substances (proline, soluble sugar, soluble protein) content were increased significantly, while MDA content and relative conductivity were decreased significantly. Compared with the high temperature stress group, the MDA content and relative conductivity of leaves under 1.0 mmol/L SA treatment were reduced by 33.70% and 58.58%, while the contents of chlorophyll a and chlorophyll b, net photosynthetic rate, and the activities of SOD and CAT were increased by 22.71%, 21.63%, 98.34%, 23.98% and 19.71%, respectively. [Conclusion] Pre-spraying SA could alleviate the damage of watermelon seedlings under high temperature stress, with 1.0 mmol/L SA had the best effect.