Abstract: With the watermelon ( Citrullus vulgarris ) Zhengkang No.1 as the scion and  C. moschata, C. ficifolia , and  Lagenaria siceraia as the rootstocks, this paper studied the chilling-tolerance of grafted and self-rooted watermelon seedlings under (7.5±0.5) ℃ stress. Under this low temperature stress, the malondialdehyde (MDA) content and electrolyte effusive rate of grafted watermelon seedlings were significantly lower, while their chlorophyll, proline (Pro) and soluble sugar contents and superoxid dismutase (SOD), peroxidase (POD) and catalase (CAT) activities were significantly higher, compared with those of self-rooted watermelon seedlings. On the twelfth day after low temperature treatment, the total chlorophyll content in the leaves of  C. moschata -Zhengkang No.1,  C. ficifolia-Zhengkang No.1, and  L. siceraia-Zhengkang No.1 decreased by 30.48%, 28.48% and 52.69%, respectively. The MDA content of  L. siceraia-Zhengkang No.1 changed obviously, with an increase of 313.16% from 0 to 6 days, and the relative electrolyte effusive rate was significantly higher than that of the other two grafted groups. Under the low temperature treatment, the soluble sugar content of  C. moschata-Zhengkang No.1 was 24.71% and 31.17% higher than that of  C. ficifolia-Zhengkang No.1 and  L. siceraia-Zhengkang No.1, respectively, and the SOD, POD and CAT activities of  C. moschata-Zhengkang No.1 were significantly higher than those of the two latter grafted groups. The survival rate of  C. moschata-Zhengkang No.1,  C. ficifolia-Zhengkang No.1,  L. siceraia-Zhengkang No.1, and Zhengkang No.1 was 84;29%, 79;%, 65;59%, and 45;12%, respectively. It was suggested that the chilling-tolerance of watermelon seedlings could be improved via grafting with suitable rootstocks.  C. moschata-Zhengkang No.1 had the best chilling-tolerance, followed by  C. ficifolia-Zhengkang No.1, and L. siceraia-Zhengkang No.1.

Key words: Biomass, Nodule number, Infection rate, ALP, SDH