Rootstock-Dependent Responses of Monastrell Grape Ripening and Phenolic Traits to Moderate Cluster-Zone Warming Under Semi-Arid Vineyard Conditions

We evaluated moderate cluster-zone warming in field-grown Monastrell grafted onto 110R and 140Ru over two seasons (2020–2021) in a semi-arid vineyard in south-eastern Spain. From pea-size to harvest, an open warming system increased cluster-zone air temperature by 1.15 °C on average. Warming increased cluster-zone mean and maximum temperatures, but berry temperature responses depended on year and rootstock because berry–air temperature differences and also time above thresholds changed. Berry weight increased in harvest in both years, and technological ripening advanced, with higher total soluble solids and maturity index, especially during ripening. Acid composition responded differently between rootstocks: under warming, 140Ru maintained higher malic acid and lower tartaric acid than 110R, resulting in a lower tartaric/malic ratio. Phenolic and colour responses were clearest at mid-ripening. Warming reduced must total phenolic index (TPI), skin-related phenolics, anthocyanins, and must colour more strongly in 140Ru, while 110R retained a more stable phenolic and colour profile. Exposure–response analyses showed that time in the 25–35 °C range and berry thermal time were more closely related to technological and phenolic variation than time above 35 °C. These results suggest that, under the warming conditions imposed in this study, 110R offered a better compromise than 140Ru between ripening advancement and the preservation of phenolic and colour-related traits.