The incidence of solar radiation on red-skinned grapes can promote the synthesis of flavonoids desirable for wine production, but elevated temperature may impair their accumulation. We performed a shade cloth trial covering the fruit zone (from pepper-corn size to maturity) with four polyethylene 1 m curtains with different optical properties (20% shading factor Pearl colored and 40% shading factor Aluminet, Blue, and Black colored) and a Control with no cover. Cluster temperature was 3.7 °C lower on the Southwest side in Black-40% clusters during the warmest part of the day compared to Control. Results indicated a lower berry weight under the Aluminet-40%. Berries under the nets often had significantly lower pH and higher TA than Control, but only the Black-40% were significant at harvest. Black-40% had higher values of anthocyanins than Control toward the last weeks of development. Berry skin flavonol and anthocyanin composition and concentration were measured by C18 reversed-phased HPLC; and proanthocyanidin isolates were characterized by acid catalysis in the presence of excess phloroglucinol followed by reversed-phase HPLC. Proanthocyanidins and flavonol contents were lower in Black-40% before veraison and the first part of ripening, respectively. However, their contents in Control decreased toward the end of ripening to a point where any net was different from Control. Anthocyanin and flavonol profiles were richer in 3', 4', 5' hydroxylated forms. Proanthocyanidin chain length was not affected while small changes were observed in the proportion of terminal catechin/epicatechin and in seed galloylation in response to treatments. Results show that shade cloths may efficiently palliate temperature spikes, especially the last weeks before harvest, while transmitting enough radiation into the fruit zone to achieve a better grape composition compared to uncovered grapes.
Keywords: abiotic stress; anthocyanins; climate change; flavonols; light; tannins; temperature.