Changes in Soluble Sugar Accumulation and Activities of Sucrose-Metabolizing Enzymes during Fruit Ripening of Jackfruit

Jackfruit (Artocarpus heterophyllus Lam.) is an important food crop widely grown in the tropical region. However, little is known about sugar metabolism during fruit ripening of jackfruit. Here we examined sugar profiles (sucrose, glucose and fructose) and corresponding enzyme activities (SPS, E.C.2.4.1.14; SuSy, EC 2.4.1.13; IV, EC 3.2.1.26) of four soft type and four firm type varieties of jackfruit during four stages of fruit ripening. We found that during fruit ripening, there was a rapid increase in contents of total soluble sugar and sucrose, whereas increases in glucose and fructose contents were much slower. Ratios of glucose versus fructose varied among different varieties and ripening stages but within the range of 0.9 to 1.2 in the ripe fruits. Five of these varieties exhibited markedly high levels of SuSy activity for sucrose synthesis at early ripening stage, and then decreased towards fully ripe stage. All soft type varieties exhibited a conspicuous peak of AIV activity and had overall higher AIV activities than NIV during ripening. The changing patterns for other enzymes varied among varieties. Our studies support the notion that sucrose was the major sugar species contributing to the fruit sweetness, followed by fructose and glucose. We also demonstrated that AIV and NIV were probably the primary enzymes responsible for sucrose hydrolysis during ripening, while SPS and SuSy were responsible for sucrose synthesis. We propose that during fruit ripening of jackfruit, glucose is released from starch hydrolysis, followed by sucrose hydrolysis leading to increase in both glucose and fructose contents.