Sugar accumulation and related enzyme activities in sugar metabolism of processing apple cultivars

Three processing apple cultivars (Malus domestica Borkh cv. Judeline, Judaine and Judestar) were used for studying the changes of sugar accumulation and enzyme activities involved in sucrose metabolism during fruit development. Results showed that more fructose was accumulated in Judeline fruit, accounted for 53.58% of the total sugar content in maturation while the amount of glucose was 24.85%. Judaine accumulated more fructose, accounted for 45.52% of the total sugar and next was sucrose, 35.39%. Judestar was sucrose-fructose-accumulating cultivar, containing 47.94% of sucrose and 46.65% of fructose. Judestar accumulated sucrose earlier than other cultivars, and sucrose rose continually during fruit development. While sucrose-accumulating-cultivars Judeline accumulated more fructose at the early stage of growth and sucrose was accumulated slowly. The sucrose content in all these cultivars had the negative correlation with the activities of acid invertase. The activity of acid invertase in Judestar was significantly lower than that in other cultivars, while Judeline had higher acid invertase activities. The increase in sucrose had correlation with the increase of net activities of sucrose-metabolizing enzymes.