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The aimwas to explore the effects of calcium (Ca2+) on heat tolerance of cucumber (Cucumis sativus L.).Ca2 + is a second messenger that plays a role in regulating responses to environmental stresses.Grafted cucumber seedlings were pretreated with CaCl2, a Ca2 + chelator [ethylene glycol bis (oxyethylenenitrilo) tetraacetic acid], a Ca2 + channel blocker (LaCl3), or a CaM antagonist (trifluoperazine), and assessed their heat tolerance during a 10-day heat stress treatment.The seedlings pretreated with CaCl2 showed greater heat tolerance than that of controls or seedlings treated with the other chemicals.The increased heat tolerance was characterized by normal growth, lower membrane permeability, lower levels of lipid peroxidation and osmotic substances, and higher activities of antioxidant enzymes under heat stress, compared with those of controls and those of plants treated with Ca2 +-inhibitors.These results suggest that the Ca2 + signaling system was involved in regulating antioxidant system activity and heat tolerance of grafted cucumber seedlings under high temperature stress.