Evolution over the growing season of volatile organic compounds in Viognier (Vitis vinifera L.) grapes under three irrigation regimes.

Recent studies revealed that terpenes present in grapes determine the 'stone fruit' aroma of Viognier wines. This study analysed the evolution of free and glycosylated terpenes along with other volatile organic compounds (VOCs) during berry development in Viognier berries exposed to three irrigation regimes. These regimes (standard irrigation over the season, early deficit irrigation from 25 days after anthesis to veraison, prolonged deficit irrigation from 25 days after anthesis to harvest) were imposed to Viognier grapevines grown in the Okanagan Valley (British Columbia, Canada). Berry samples were collected over the season and analysed for determining the chemical composition, including free and glycosylated VOCs, and the expression of genes related to VOC biosynthesis. Among free VOCs, aldehydes peaked at veraison, alcohols did not vary across berry development, and terpenes increased during ripening. Among glycosylated VOCs, aldehydes and terpenes mirrored the free fractions while alcohols peaked before veraison. At harvest, a larger concentration of linalool oxides was detected in the glycosylated than in the free fraction. Increases of terpenes during ripening were modulated at the transcriptional level, particularly by two terpene synthases VviCSbOci and VviPNLinNer1. Early deficit irrigation increased the concentration of free α-terpineol and linalool, while prolonged deficit irrigation reduced the concentration of glycosylated α-terpineol. These results provide a basis for further studies on managing Viognier grape aroma in vineyards.