【Objective】Chitosan oligosaccharides (COS) can induce plant resistance to biotic stress and abiotic stress, and the effects of COS on physiological characteristics of watermelon seedlings with different salt tolerance under NaCl stress were investigated, in order to provide a theoretical basis for watermelon resistance breeding and watermelon cultivation in saline-alkali land. 【Methods】Two watermelon materials, 'HQ-2'and'HQ-3', were used to study the effects of COS on the dry and fresh mass, photosynthetic pigment content, SOD, CAT, POD act...