Solute Distribution and Malic Enzyme Activity in Developing Grape Berries

Abstract

Patterns of distribution of malic acid, malic enzyme activity, tartaric acid, potassium, and tissue pH were studied during ripening in Chardonnay grapes. Ripening-related changes in each of these variables were tissue- specific. The concentration of malic acid was high in the fleshy tissues of the fruit prior to veraison, then declined abruptly during ripening. In contrast, the malic acid concentration in the skin was initially low, but increased during the week following veraison. There was no clear correlation between the change in malic acid concentration and the activity of malic enzyme in a specific tissue. Malic enzyme activity was low in the mesocarp, where malic acid declined rapidly during ripening. Enzyme activity was high both in the skin, where malic acid accumulated during ripening and in the outer mesocarp, which had the lowest concentration of malic acid at harvest of any of the tissues assayed. The concentration of tartaric acid was higher in the outer mesocarp than in other parts of the berry before veraison. However, tartrate concentration declined rapidly in the outer mesocarp during ripening, while remaining essentially constant in the other tissues. These results suggest a different pattern of tartaric acid metabolism in the outer mesocarp than in the other tissues of the grape berry. The concentration of potassium was significantly higher in the skin than in the other tissues of the berry and was correlated with a significantly higher pH in the skin than in the fruit flesh.