Stage of maturation, crop load, and shoot density affect the fruit quality of cold-hardy grape cultivars
Wine grape production in the Upper Midwest and other cold-climate regions is increasing due to the release of cold-hardy grape cultivars that are interspecific hybrids. Grape production practices for Vitis vinifera L. cultivars in regions with long growing seasons, such as California, are standardized more than in other regions due to the extensive amount of research on cultivars of V. vinifera. In spite of that, few researchers document changes in grape composition as fruits mature, and those that do primarily only report soluble solids, pH, and titratable acidity.
Two experiments were designed to examine the changes in soluble solids, pH, titratable acidity, acid profile, and sugar profile that occur during the maturation of fruits of commercially important cold-hardy grape cultivars. Fruits from Edelweiss, Frontenac, La Crescent, Marquette, and St. Croix were harvested from vines at the typical commercial harvest time, and one to two weeks before and after. Glucose:fructose ratio ranged from 0.95 to 1.24 and generally decreased with stage of maturation. Malic acid ranged from 5.3 to 15.2 g/L and decreased with stage of maturation. Tartaric:malic acid ratio was smaller than is reported for other interspecific hybrids, and was never larger than 0.55 for Edelweiss and La Crescent. Cold-hardy grape cultivars have unique fruit chemistry as compared to cultivars of V. vinifera, V. labruscana Bailey, and French hybrids. Harvesting fruits later in maturation can be effective in reducing malic acid in cold-hardy grape cultivars, however they still have large amounts of malic acid and a small tartaric:malic acid ratios. Soluble solids content is commonly used as an indicator of fruit maturity to determine when grapes should be harvested. However when soluble solids did not change with stage of maturation, there were changes in other fruit parameters such as pH, titratable acidity, and acid profile, indicating that soluble solids should not be overemphasized as the deciding factor when to harvest grapes.
Yield is often used as a predictor of fruit quality; however grapevine balance is dependent on both fruit yield and the amount of vegetative growth, which is not factored into yield alone. Crop load (grape yield/pruning weight) is increasingly being used as an indicator of vine balance for cultivars of V. vinifera and French hybrids. An experiment was designed to determine what changes occur in the fruit chemistry and grapevine canopies of Frontenac and St. Croix, cold-hardy grape cultivars, at crop loads ranging from 2 to 14. Frontenac was less responsive to crop load than St. Croix. Leaf area/grape weight (m2/kg) and fruit malic acid concentration generally decreased with crop load, while tartaric:malic acid ratio increased. Increasing crop load within the examined ranges can be an effective approach to increase yield and decrease the large amounts of malic acid found in fruits of cold-hardy grape cultivars without negative consequences on other fruit quality parameters and vine growth.
Shoot and cluster quantity are commonly managed on grapevines, however their effects are not separated in most studies. An experiment was designed to impose four shoot levels (15, 30, 45, and 60 shoot/vine) and three cluster levels (15, 30, and 60 clusters/vine) to Marquette grapevines. Grape pH and malic acid concentration increased as the quantity of shoots per vine increased, while the tartaric:malic acid ratio decreased. Cluster quantity did not have an impact on fruit chemistry. Leaf area per vine decreased as shoots per vine decreased, which is the likely mechanism for malic acid decreasing as shoot quantity decreased. Leaf area per kg of fruit decreased as shoot quantity decreased and increased as cluster quantity decreased. For vigorous vines it is effective to decrease the amount of shoots, within a set cluster quantity, to decrease leaf area per vine and fruit malic acid. Increasing the amount of shoots, within a set cluster quantity, can be useful to increase leaf area per vine and kg of fruit to balance fruit and vegetative growth on grapevines with low vigor.