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High temperatures (HT) are a significant threat for crop production, and strategies for maintaining high crop yields and quality under HT stress are crucial agricultural objectives. The changes in sugar metabolism during HT stress were examined in watermelon cv. Crimson Tide leaves. The leaves obtained from plants were subjected to 35, 40, 45, 50, 55 and 60°C. Heat-stress tolerance (HST; LT50), hydrogen peroxide (H2O2), sucrose (Suc), reducing sugars and starch contents and some sucrolytic enzyme activities in leaf samples held at each temperature for 30 minutes were determined. The HST and H2O2 content, rose with increasing temperatures, most noticeably between 50-55°C. As a result, the LT50 value was determined to be 53.84°C. The Suc content increased almost 2-fold between 50-55°C. The reducing sugars and starch content sharply decreased with HT up to 50°C comparing to the control, however both increased almost 2-fold between 50-55°C. The H2O2 may act as a signal molecule at 40-45°C and triggers sucrose metabolism. It was determined that alkaline-INV and SuSy activities were at the maximum level at 40°C. The increase in enzyme activities has been associated with increased energy needs under stress conditions. The findings revealed that sugar metabolism contributes significantly to HST.