The effects of pumpkin rootstock (‘Qingyan No.1’) grafting on growth,chlorophyll fluorescence,photosynthetic gas exchange,expression of Calvin cycle and stresstolerance genes of watermelon (‘97103’) seedlings under chilling stress (10℃/5℃) were studied to clarify the mechanism of photosynthetic adaptation involved in chilling tolerance in pumpkin rootstockgrafted watermelon seedlings.The results showed that the chlorophyll fluorescence parameter such as maximum quantum yield of PSⅡ (Fv/Fm) in selfgrafted watermelon decreased significantly under chilling stress in comparison with control.After 5 days of chilling stress,the effective quantum yield of PSⅡ (ΦPSII) in selfgrafted watermelon seedlings and pumpkin rootstockgrafted seedlings decreased by 92.9% and 59.2% compared with the control.Pumpkin rootstock grafting effectively delayed the damage of PSⅡ induced by chilling and promoted the expression of key genes including Rubisco large subunit (rbcL),glycerate 3phosphate kinase (PGK),phosphoribulose kinase (PRK), and Fructose1,6bisphosphatase (FBPase) involved in Calvin cycle.A higher CO2 assimilation rate (Asat) in pumpkin rootstockgrafted seedlings under chilling stress was observed.which was attributed to the enhanced expression of 9 stresstolerance genes including GPX, tAPX, GR, and ERD10.After 5 days of chilling treatment,the chilling injury index of selfgrafted watermelon seedlings and pumpkin rootstockgrafted seedlings were 61.1% and 22.2%,respectively.The whole plant dry weight was decreased by 61.0% and 33.2%,respectively compared with the control.It is indicated that pumpkin rootstock grafting reduces the water loss by regulating transpiration,upregulates the expression of key genes involved in Calvin cycle and stresstolerance,thus alleviates photoinhibition of PSⅡand reduces the damage of membrane lipid peroxidation to enhance the adaptability of watermelon under chilling stress.