Abstract: 【Objective】 To investigate the dynamic changes in sugar content and sucrose-metabolizing enzyme activities during watermelon fruit development at different sowing times,and to provide a reference for high-quality cultivation and quality improvement of watermelon. 【Methods】 The watermelon variety Qianli No. 4 was used as the experimental material. A spring experiment（sowing time Ⅰ and sowing time Ⅱ）and autumn experiment（sowing time Ⅲ and sowing time Ⅳ）were set. Fruit sugar content and sucrose-metabolizing enzyme activities were measured at different days after pollination. 【Results】 Glucose content was highest at 20 d after pollination in sowing time Ⅰ,26 d after pollination in sowing time Ⅱ,35 d after pollination in sowing time Ⅲ,and 26 d after pollination in sowing time Ⅳ （39.67,46.94,33.12,and 29.01 mg/g,respectively）. Fructose content was highest at 23 d after pollination in sowing time Ⅰ,29 d after pollination in sowing time Ⅱ,44 d after pollination in sowing time Ⅲ,and 44 d after pollination in sowing time Ⅳ（76.37,66.33,65.83,and 70.00 mg/g,respectively）. Sucrose content was highest at 29 d after pollination in sowing time Ⅰ,29 d after pollination in sowing time Ⅱ,44 d after pollination in sowing time Ⅲ,and 41 d after pollination in sowing time Ⅳ（64.57,43.61,38.63,and 38.54 mg/g,respectively）. Total sugar content was highest at 29 d after pollination in sowing time Ⅰ,29 d after pollination in sowing time Ⅱ,44 d after pollination in sowing time Ⅲ,and 41 d after pollination in sowing time Ⅳ（153.20,141.93,118.25,and 119.15 mg/g,respectively）. The activity of neutral invertase（NI） was highest at 26 d after pollination in sowing time Ⅰ,14 d after pollination in sowing time Ⅱ,44 d after pollination in sowing time Ⅲ,and 23 d after pollination in sowing time Ⅳ [1 757.33,1 837.93,1 607.89,and 1 890.57 μg/（min·g）,respectively]. The activity of acid invertase（AI）was highest at 23 d after pollination in sowing time Ⅰ,7 d after pollination in sowing time Ⅱ,38 d after pollination in sowing time Ⅲ,and 23 d after pollination in sowing time Ⅳ [1 645.44,1 541.99,1 404.88,and 1 604.70 μg/（min·g）,respectively]. In both spring and autumn fruits,the contents of sucrose,glucose,fructose,and total sugar were significantly or extremely significantly correlated with the activity of sucrose phosphate synthase（SPS）,sucrose synthase[（synthetic direction）,SSs],sucrose synthase[（cleavage direction）,SSc）],and the net activity of sucrose-metabolizing enzymes（P<0.05 or P<0.01）. 【Conclusion】 For watermelon fruits at different sowing times,fructose content is higher during the development stages,glucose content is higher at the early development stages,sucrose content is higher at the late development stages,and total sugar and glucose contents are higher in spring than in autumn;the activities of sucrose-metabolizing enzymes show fluctuating changes,and all invertase activities were higher. The differences in fruit sugar accumulation are related to the activities of sucrose-metabolizing enzymes.

Key words: Watermelon, Sowing time, Sugar content, Sucrose-metabolizing enzyme, Correlation analysis