UV‑LED waveband and dose optimization to enhance polyphenols in postharvest baby‑leaf lettuce

Baby‑leaf lettuce for ready‑to‑eat (RTE) salads is typically low in antioxidant polyphenols because it is cultivated under UV‑blocking tunnels. Postharvest UV treatments can enhance phytochemical contents, but optimal spectral bands, intensities, and energy doses remain undefined. Here, bagged RTE lettuce was exposed postharvest to narrow‑band UV‑LED radiation during six days of refrigerated storage (5 °C). The effects of UV‑B (306 nm), UV‑A (366 nm), and their combination were assessed across multiple radiation intensities, exposure times, and corresponding daily energy doses. Epidermal phenols (EPhen) and chlorophyll were monitored non‑destructively. Individual polyphenols were profiled by HPLC‑DAD‑ESI‑QToF‑MS. Transcriptomic analysis was performed on samples subjected to the most effective UV‑B treatment. UV‑B increased the EPhen Index by 22% in cv. Falstaff and by 27% in cv. Luna Bionda, whereas UV‑A had no effect and UV‑A + UV‑B was less effective than UV‑B alone. In Falstaff, EPhen accumulation was linear at low (3.5 kJ m⁻²) daily UV‑B doses and became sigmoidal at higher ones (>10 kJ m⁻²), enabling identification of a minimal UV‑B energy threshold (∼16 kJ m⁻²) beyond which EPhen saturated. No macroscopic damage or chlorophyll loss occurred under UV‑B. Metabolomic analysis confirmed hydroxycinnamic acids and flavonols as the predominant polyphenolic classes, with UV‑B increasing total phenolic content by 28–42% depending on cultivar and dose, consistent with EPhen estimates. Post-treatment transcriptomics revealed up‑regulation of stress‑response and protein‑ubiquitination pathways, indicating activation of adaptive mechanisms that preserve tissue integrity under prolonged UV‑B exposure. Overall, this work defines a practical, damage‑free UV‑B LED treatment as a promising postharvest strategy to enhance the nutraceutical quality of RTE salads.