Sugar composition difference between white- and red-fleshed loquat fruits and its relation with activities of sucrose-metabolizing enzymes

White-fleshed loquat cultivars Tianzhong,Biqizhong and Bingtangzhong as well as red-fleshed loquat cultivars Baozhu, Dayeyangdun and Dameiguihongpao were used to study the sugar composition difference between white- and red-fleshed loquat flesh and its relation with activities of sucrose-metabolizing enzymes. The results showed that total soluble sugar content of white-fleshed loquats was higher than that of red-fleshed ones, with the highest found in Tianzhong and Bingtangzhong; sucrose was the major sugar in white-fleshed loquats (60% to 70%), followed by fructose (20% to 30%) and glucose (9% to 11%), while the content of these sugars were similar in red-fleshed fruits. Percentage of sucrose among total soluble sugars was significantly higher in white-fleshed fruits than in red-fleshed ones, while percentage of glucose was on the contrary, and that of fructose was similar. Activities of NI, AI, SUS-synthetic and SUS-cleavage were higher in red-fleshed loquat fruits, and that of SPS was similar in two fruit groups. Net activity of sucrose-metabolizing enzymes was negative in both loquat groups, but was lower in red-fleshed ones. The data suggested that the activities of sucrose-metabolizing enzymes were closely related to the difference in soluble sugar accumulation and composition between two loquat groups.