Abstract:The effects of pumpkin rootstock (‘Qingyan No.1’) grafting on growth,chlorophyll fluorescence,photosynthetic gas exchange,expression of Calvin cycle and stresstolerance 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 rootstockgrafted watermelon seedlings.The results showed that the chlorophyll fluorescence parameter such as maximum quantum yield of PSⅡ (Fv/Fm) in selfgrafted watermelon decreased significantly under chilling stress in comparison with control.After 5 days of chilling stress,the effective quantum yield of PSⅡ (ΦPSII) in selfgrafted watermelon seedlings and pumpkin rootstockgrafted 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 3phosphate kinase (PGK),phosphoribulose kinase (PRK), and Fructose1,6bisphosphatase (FBPase) involved in Calvin cycle.A higher CO2 assimilation rate (Asat) in pumpkin rootstockgrafted seedlings under chilling stress was observed.which was attributed to the enhanced expression of 9 stresstolerance genes including GPX, tAPX, GR, and ERD10.After 5 days of chilling treatment,the chilling injury index of selfgrafted watermelon seedlings and pumpkin rootstockgrafted 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,upregulates the expression of key genes involved in Calvin cycle and stresstolerance,thus alleviates photoinhibition of PSⅡand reduces the damage of membrane lipid peroxidation to enhance the adaptability of watermelon under chilling stress.