Controlled-Environment Blue LED Lighting Enhances Rosmarinic Acid Accumulation and Physiological Performance in Hydroponic Basil

Light spectrum and intensity shape primary and secondary metabolism in culinary herbs, yet how monochromatic high-energy wavelengths regulate phenylpropanoid biosynthesis remains unsettled. Here we examined two Ocimum basilicum L. cultivars (‘Genovese’ and ‘Amethyst’) grown in a recirculating soilless system under full-spectrum white or monochromatic 450-nm illumination delivered at low, medium, or high photon flux densities, and quantified growth, leaf ultrastructure, gas-exchange and chlorophyll-fluorescence traits, as well as phytochemical profiles by HPLC-DAD and antioxidant assays. We show a clear dose-response: high-intensity treatments elicited compact architecture, greater stomatal density, and photosynthetic acclimation without chronic photoinhibition (Fv/Fm ≈ 0.82–0.84), while markedly elevating phenylpropanoid output. Compared with white light, the most intense treatment increased leaf rosmarinic-derived phenolics to 46.5 mg/g DW in ‘Genovese’ and 41.8 mg/g DW in ‘Amethyst’ (≈2.1–2.2-fold), paralleled by higher total phenolics and stronger reducing power; DPPH IC50 decreased threefold and ferric reducing antioxidant power (FRAP) rose ≈3×, indicating superior antioxidant capacity. Although biomass responses were cultivar-specific—recovery at high intensity in ‘Genovese” but progressive reduction in ‘Amethyst’—carbon assimilation converged toward control values, evidencing robust acclimation of electron transport efficiency. By integrating morpho-anatomical imaging, gas-exchange kinetics, and targeted quantification across an intensity gradient, the study disentangles spectral from dose effects that confounded prior assessments in controlled systems. These findings resolve contradictory reports by identifying an intensity threshold above which photoreceptor signaling (consistent with cryptochrome–HY5 regulation) initiates a eustress program that upgrades phenylpropanoid metabolism. The results provide a practical, chemical-free lever for quality-driven indoor production, enabling targeted enhancement of nutraceutical value while maintaining photosynthetic function.