New Insights into Midday Supplemental Lighting in Greenhouse-Grown Sweet Basil (Ocimum basilicum L.): Photosynthetic and Sensory Quality Traits

Supplemental light spectrum strongly influences crop physiology, quality, and marketability in controlled-environment horticulture. This study evaluated greenhouse-grown basil (Ocimum basilicum L.) under natural daylight supplemented with 250 µmol m−2 s−1 of multi-band WHITE or narrow-band RED, GREEN, and BLUE LEDs, compared with non-supplemented controls (CNT). Diurnal gas exchange, cumulative photosynthetic activity, quality, sensory traits, and productive responses were assessed to identify spectrum-specific effects. WHITE supplementation produced the highest net photosynthesis (Pn) throughout the photoperiod, increasing carbon assimilation by up to 90% during active supplementation compared with CNT, while improving stomatal–photosynthetic coordination and titratable acidity. BLUE light promoted the highest stomatal conductance and transpiration, whereas RED induced faster stomatal closure without affecting the plant’s final biomass. GREEN supplementation showed intermediate physiological responses while enhancing fresh biomass, leaf brightness, and greenness. Spectral treatments significantly affected chlorophyll content, colorimetric parameters, sensory descriptors, aroma typicity, and texture, although antioxidant activity and overall consumer acceptability remained largely stable. WHITE and CNT samples achieved the highest sensory appreciation, while BLUE supplementation increased bitterness and reduced aromatic typicality. Overall, no single spectrum optimized all production objectives. WHITE provided the most balanced strategy for maximizing photosynthetic efficiency and premium-quality traits, whereas RED, GREEN, and BLUE offered more specialized benefits related to biomass increase, visual quality, or pigmentation. These findings highlight precision spectral management as an effective tool for tailoring basil production to specific agronomic and commercial targets in protected cultivation systems.