RELATIONSHIP BETWEEN ACID INVERTASE ACTIVITY AND SUGAR CONTENT IN GRAPE SPECIES

Among grape species muscadines are the important grape crops of the southeastern United States, where cultivation of Vitis vinifera is limited due to Pierce's disease. Muscadine grapes are less desirable because of their low sugar content and leathery skin. Sugar accumulation in berry depends on the activity of sugar metabolizing enzymes during ripening. The present study was carried out to investigate differences in invertase activity among Muscadinia rotundifolia (Muscadine), Vitis spp. (Florida hybrid bunch) and V. vinifera (Bunch) grape genotypes and to characterize muscadine invertase/s. Bunch grape showed highest invertase activity followed by Florida hybrid bunch and muscadine. Muscadine invertase showed a pH optimum between 2.5 and 3.5 and substrate optimum at 150 mM. These data suggest that low sugar content of muscadine berry may be related to its low invertase activity, and enhancement of invertase activity may be one way to increase the sugar content of muscadine grape. PRACTICAL APPLICATIONS Sugar accumulation is an important event in the berry-ripening physiology of grapevine, and is an important characteristic essential for superior enological characteristics and product value. Sugars are synthesized in leaves as a result of photosynthesis and imported into the berry in the form of sucrose. Any limitations or restrictions in sucrose breakdown can adversely affect the sugar content and composition of grape berry as well as its quality. Our studies has revealed that one of the key enzyme acid invertase required for sucrose breakdown is low in muscadine grape compared to bunch and Florida hybrid bunch grapes indicating a positive correlation between invertase activity and berry sugar content. The data also suggest that increasing acid invertase activity may be a way to increase sugar content of muscadine grape to improve its enological characteristics.