Effect of temperature and water availability on grape phenolic compounds and their extractability in Merlot grown in a warm area

•Grape total anthocyanins decrease with increasing Tmin before veraison.•Grape extractable anthocyanins decrease with increasing Tmin and Tmax during ripening.•Increasing Tmin and Tmax during ripening decrease flavonols and total polyphenols.•Water availability decrease total anthocyanins but favour their extractability.•Flavonol content is not significantly affected by water availability. In view of the increase in temperature observed in recent years and its projection, there is a great concern about its effects on grape quality, especially in warm areas. This research aims to contribute to this knowledge by assessing the effect of temperature and water availability along the growing cycle on grape quality and in particular on the phenolic compounds and their extractability. The research was carried out in a commercial vineyard in La Mancha Designation of Origen (Spain) planted with Merlot. Edaphoclimatic characteristics and the grape composition at maturity were evaluated for several years in the period 2007–2017. An increase in temperature decreased the content of phenolic compounds, while higher water availability contributed to increase their levels in the grapes. Total anthocyanin content was mainly affected by minimum temperature before veraison while extractable anthocyanin, flavonols and total polyphenols were affected by both maximum and minimum temperature during ripening. Higher water availability decreased the concentration of anthocyanins but favoured their extractability, while flavonols were not significantly affected by water availability. Thus, phenolic ripening can be affected by changes in temperature and water distribution throughout the growing cycle, but due to the opposite sign of their effect they may be partially balanced. The projected warmer conditions can affect phenolic composition of Merlot and therefore quality of the wines made with such variety. In order to maintain grape quality, strategies to reduce heat stress should be adopted, and irrigation doses and timing should be adapted not only in the ripening period but also in the period prior to veraison.