Harvest-Time–Dependent variations in phytochemistry and in vitro bioactivities of Vitis vinifera Leaves

Abstract Background Vitis vinifera L. leaves are rich in bioactive phytochemicals, but their composition and biological activities may vary considerably with harvest time. This study investigated harvest-dependent changes in the phytochemical profile and in vitro biological activities of grape leaves. Methods Methanolic extracts of V. vinifera leaves collected in March, May, and September were evaluated for photosynthetic pigments, total polyphenols, flavonoids, and tannins. HPLC analysis was used to characterize individual phenolic and flavonoid constituents, while major compounds were isolated and structurally identified. The extracts were further assessed for antioxidant and radical-scavenging, antidiabetic, anti-Alzheimer’s, anti-inflammatory, anti-arthritic, and cytotoxic activities. Results Harvest time markedly affected the phytochemical composition and biological activities of the leaves. The May-harvested leaves showed the highest levels of total chlorophyll, polyphenols, flavonoids, and tannins, whereas carotenoids were comparatively higher in the September and March samples. HPLC analysis revealed greater accumulation of phenolic and flavonoid constituents in the May extract, with chlorogenic acid, rosmarinic acid, gallic acid, and naringenin among the predominant compounds. Consistently, the May extract exhibited the strongest antioxidant and radical-scavenging activities and showed superior inhibitory effects in the antidiabetic, anti-Alzheimer’s, anti-inflammatory, anti-arthritic, and cytotoxic assays compared with the other harvest periods. Conclusion Harvest time substantially influences the accumulation of bioactive constituents and the in vitro biological potential of V. vinifera leaves. Among the investigated periods, May appeared to be the most favorable harvest time for obtaining grape leaves enriched in bioactive phytochemicals with promising biological activities, supporting their potential valorization as a value-added viticultural by-product.