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Grafting with pumpkin rootstock could improve chilling tolerance in watermelon, and salicylic acid (SA) as a signal molecule is involved in regulating plant tolerance to chilling and other abiotic stresses. To clarify the mechanism in pumpkin rootstock-induced systemic acquired acclimation in grafted watermelon under chilling stress, we used self-grafted (<i>Cl</i>/<i>Cl</i>) and pumpkin rootstock-grafted (<i>Cl</i>/<i>Cm</i>) watermelon seedlings to study the changes in lipid peroxidation, photosystem II (PSII) activity and antioxidant metabolism, the spatio–temporal response of SA biosynthesis and H₂O₂ accumulation to chilling, and the role of H₂O₂ signal in SA-induced chilling tolerance in grafted watermelon. The results showed that pumpkin rootstock grafting promoted SA biosynthesis in the watermelon scions. Chilling induced hydrolysis of conjugated SA into free SA in the roots and accumulation of free SA in the leaves in <i>Cl</i>/<i>Cm</i> plants. Further, pumpkin rootstock grafting induced early response of antioxidant enzyme system in the roots and increased activities of ascorbate peroxidase and glutathione reductase in the leaves, thus maintaining cellular redox homeostasis. Exogenous SA improved while the inhibition of SA biosynthesis reduced chilling tolerance in <i>Cl</i>/<i>Cl</i> seedlings. The application of diphenyleneiodonium (DPI, inhibitor of NADPH oxidase) and dimethylthiourea (DMTU, H₂O₂ scavenger) decreased, while exogenous H₂O₂ improved the PSII activity in <i>Cl/Cl</i> plants under chilling stress. Additionally, the decrease of the net photosynthetic rate in DMTU- and DPI-pretreated <i>Cl</i>/<i>Cl</i> plants under chilling conditions could be alleviated by subsequent application of H₂O₂ but not SA. In conclusion, pumpkin rootstock grafting induces SA biosynthesis and redistribution in the leaves and roots and participates in the regulation of antioxidant metabolism probably through interaction with the H₂O₂ signal, thus improving chilling tolerance in watermelon.