Abstract
Polyphenols are plant secondary metabolites widely recognized for their antioxidant properties and potential health benefits. Grapevine leaves are a rich yet underexplored source of these bioactive compounds. In this study, caftaric acid, and predominant flavonol glycosides were characterized in leaves of seventeen grapevine genotypes by high-performance liquid chromatography, while total phenolic content (TPC) was determined using the Folin–Ciocalteu assay. In addition, the growth-promoting effect of a polyphenol-rich Vitis amurensis leaf extract on Lactiplantibacillus plantarum and its protective effect under H2O2-induced oxidative stress were evaluated. Both TPC and phenolic composition were strongly genotype-dependent, with TPC ranging from approximately 20 to 100 mg gallic acid equivalents (GAE) g−1 dry weight (DW). Quercetin and kaempferol glycosides were the predominant flavonols, and quercetin-3-O-glucuronide, one of the most abundant derivatives, varied nearly tenfold among genotypes (approximately 2100–20,000 mg kg−1 DW). The leaf extract of V. amurensis, the genotype with the highest TPC, significantly promoted the in vitro growth of L. plantarum in a dose-dependent manner, resulting in higher viable cell counts after 24 h. The extract also markedly enhanced bacterial survival under H2O2-induced oxidative stress. At 2 mM H2O2, no bacterial growth was detected in the absence of the extract, whereas extract-supplemented cultures maintained substantial growth and reached a viable cell count of 1.8 × 108 colony-forming units (CFU) mL−1 after 24 h. The phenolic patterns identified across the investigated genotypes provide a basis for selecting Vitis genotypes as sources of specific flavonol glycosides. Furthermore, our microbiological results provide preliminary evidence of the beneficial effects of V. amurensis leaf extract on L. plantarum and warrant further investigation of its potential functional applications.