Xylem Sap Bleeding as a Physiological Indicator in Grapevine: Genotype and Climate Influence

The aim of the research was to investigate several xylem sap parameters (onset, sap bleeding duration and intensity, and main chemical components) in four cultivars (‘Cabernet Sauvignon’, ‘Merlot’, ‘Muscat Ottonel’, and ‘Pinot Noir’) grown in the climate of western Romania over three consecutive growing seasons (2022–2024). Understanding early-season sap characteristics is relevant for optimizing vineyard management and improving grape output in fluctuating environmental conditions. Sap onset and duration differed significantly among cultivars and years (p < 0.05), with warmer springs resulting in earlier and longer sap bleeding. ‘Pinot Noir’ consistently exhibited the highest and earliest xylem sap flow (8.2–10.8 mL/vine/day). Chemical profiling revealed cultivar- and year-dependent variation in soluble solids, macro- and micronutrients, proteins, organic acids, and phenolic compounds. ‘Muscat Ottonel’ and ‘Pinot Noir’ had higher phenolic contents, while ‘Merlot’ displayed lower metabolic activity inferred from sap composition. Years with higher precipitation showed reduced phenolic acid and resveratrol concentrations. Principal component analysis highlighted strong effects of both cultivar and season on sap chemistry, with warmer years favoring nutrient- and metabolite-rich profiles, particularly in ‘Pinot Noir’ and ‘Muscat Ottonel’. These findings confirm that xylem sap bleeding is a sensitive indicator of grapevine reactivation, shaped by genotype and climate. Monitoring sap traits at dormancy release can serve as an early diagnostic tool to guide pruning, irrigation, and fertilization and supports the selection of climate-resilient cultivars and rootstock–scion combinations.