Metabolic effects of elevated temperature on organic acid degradation in ripening Vitis vinifera fruit

Berries of the cultivated grapevine Vitis vinifera are notably responsive to temperature, which can influence fruit quality and hence the future compatibility of varieties with their current growing regions. Organic acids represent a key component of fruit organoleptic quality and their content is significantly influenced by temperature. The objectives of this study were to (i) manipulate thermal regimes to realistically capture warming-driven reduction of malate content in Shiraz berries, and (ii) investigate the mechanisms behind temperature-sensitive malate loss and the potential downstream effects on berry metabolism. In the field we compared untreated controls at ambient temperature with longer and milder warming (2-4 degrees C differential for three weeks; Experiment 1) or shorter and more severe warming (4-6 degrees C differential for 11 days; Experiment 2). We complemented field trials with control (25/15 degrees C) and elevated (35/20 degrees C) day/night temperature controlled-environment trials using potted vines (Experiment 3). Elevating maximum temperatures (4-10 degrees C above controls) during pre-veraison stages led to higher malate content, particularly with warmer nights. Heating at veraison and ripening stages reduced malate content, consistent with effects typically seen in warm vintages. However, when minimum temperatures were also raised by 4-6 degrees C, malate content was not reduced, suggesting that the regulation of malate metabolism differs during the day and night. Increased NAD-dependent malic enzyme activity and decreased phosphoenolpyruvate carboxylase and pyruvate kinase activities, as well as the accumulation of various amino acids and gamma-aminobutyric acid, suggest enhanced anaplerotic capacity of the TCA cycle and a need for coping with decreased cytosolic pH in heated fruit.