Abstract: The effects of summer shoot growth on sugar metabolism and abscission of fruitlet in ‘Shatangju’mandarin（Citrus reticulata Blanco）were investigated. The results showed that，summer shoot obviously increased fruitlet abscission and suppressed fruitlet growth. During 18 days of summer shoot growth，the relative abscission rate（RAR）tended to increase and subsequently decrease，with the peak of 7.63% · d^-1 at the 9th day. At the 18th day，the accumulative abscission rate（AAR）in shoot-retained treatment was 94.05%，which was 1.58 times higher than that in shoot-removed treatment. Moreover，summer shoot growth decreased the contents of sucrose and reducing sugar in fruitlet at 6th day，but no significant difference was observed for those between shoot-retained and shoot-removed treatments at 18thday. Furthermore，it was found that summer shoot increased the activities of acid invertase（AI）and neutral invertase（NI），and inhibited those of the amylase and sucrose phosphate synthase（SPS）in fruitlet. However，sucrose synthase（SS）activity was not influenced by summer shoot. Overall，these findings suggest that the accelerated decomposition of sucrose by the increased activities of sucrose degradation enzymes during summer shoot growth contributed to fruitlet abscission in‘Shatangju’mandarin.

Key words: citrus, summer shoot, sugar metabolism, abscission