Day and night variations of net photosynthetic rate,sucrose content,reducing sugar content,starch content and the activities of related enzymes in carbohydrate metabolism in source leaves in response to sinksource manipulation by defruiting and retaining fruits were studied when fruits removed 7 d during fruit expansion period (Cucumis melo L.cv.Hetao).The results indicated that:(1)Day and night variation of net photosynthetic rate was typical single peak curve and showed no evidence of midday depression.There was no significant difference between defruiting and retaining fruits in net photosynthetic rate;(2)Single peak curve variations in contents of sucrose and reducing sugar in leaves were found in day and night changes.Day and night variations of activities of sucrose phosphate synthase (SPS) and sucrose synthase on synthesis (SS⁺) in leaves showed bimodal curves.There were no significant curve variations in activities of sucrose synthase on cleavage (SS^-),neutral invertase (NI) and acid invertase (AI) in leaves.All of those physiological indices showed no significant differences between defruiting and retaining fruits.Content of sucrose in leaves increased was related to the activities of SPS and SS⁺,but which decreased regulated by many factors;(3)Single peak curve variations in content of starch and activity of adenosine diphosphateglucose pyrophosphorylase (ADPGPPase) in leaves were found in day and night changes.Content of starch and activity of ADPGPPase in leaves of defruiting treatment were higher than those of retaining fruits.Content of starch in leaves increased was related to the activity of ADPGPPase.