Polyphenol profiling unveils time-dependent responses of grape berries to high temperature

• High temperature triggers stage-dependent shifts in berry polyphenol dynamics. • Heat treatment at ripening causes a rise then fall in berry polyphenol contents. • Cabernet-Sauvignon shows earlier and longer polyphenol responses than Merlot. • Phenolic acids and flavanols show distinct cultivar- and time-specific responses. • Fine-scale sampling captures transient heat-induced changes in berry composition. High temperature is a major abiotic stress that profoundly affects grape berry development and secondary metabolism. This study investigated the temporal dynamics of polyphenolic compounds in Vitis vinifera cv. Cabernet Sauvignon (CS) and Merlot berries subjected to berry‑scale heat treatments (HT) under greenhouse conditions. HT was applied at both the herbaceous and ripening stages. Polyphenolic profiles were only marginally altered during the herbaceous stage, whereas a markedly broader response was observed during the ripening stage. In both cultivars, HT induced a characteristic “increase-then-decrease” pattern in polyphenol accumulation, with CS exhibiting an earlier onset of the induction and a more delayed decline phase than Merlot. Furthermore, transient variations in specific phenolic classes, particularly phenolic acids and flavanols, displayed distinct cultivar‑ and time‑dependent signatures under HT. Taken together, these results reveal both conserved and cultivar‑specific responses to heat stress and highlight the critical importance of high temporal resolution for capturing transient yet biologically meaningful changes in grape berry composition.