Effects of Different Application Rates of a β-Carotene-Based New Bio-Protector (NBP) on Flavonol Accumulation in cv. ‘Sangiovese’ Grapevines

Excessive flavonol accumulation in berry skin under high solar radiation may increase the risk of quercetin-related instability in red wines. In 2024, a field trial was conducted on ‘Sangiovese’ grapevines grown in a non-irrigated vineyard in central Italy to evaluate the effects of a novel β-carotene-based bio-protector (NBP). The product was applied once at veraison at 1% or 2% (v/v). At harvest, yield components and technological maturity parameters were assessed; berry-skin phenolics and carotenoids were quantified by HPLC, and the expression of CHS, CHI, FLS, and a MYB regulator was quantified by RT-qPCR. NBP application did not significantly affect yield components or must composition. In contrast, both NBP concentrations reduced quercetin-3-O-glucoside and quercetin-3-O-glucuronide, as well as kaempferol derivatives, while flavan-3-ols were unaffected. Lutein content increased in treated berries, reaching the highest levels in 2% NBP vines, whereas β-carotene remained unchanged. Consistent with these findings, gene expression analysis showed downregulation of flavonol-biosynthetic transcripts, with FLS exhibiting the strongest repression. Overall, a single NBP application at veraison effectively limited flavonol accumulation under warm, high-radiation conditions without compromising yield or ripening. Differences between the two NBP concentrations were more evident for lutein accumulation than for flavonol reduction, suggesting distinct metabolic responsiveness of these pathways to the treatment.