Blue:red spectral gradients modulate basil phytochemical composition while maintaining yield stability in indoor vertical farming

Light spectrum is a controllable factor in indoor vertical farming, yet responses of basil (Ocimum basilicum L.) phytochemicals to continuous blue:red spectral gradients remain poorly characterized. This study evaluated six blue:red ratios (100% blue, 80:20, 60:40, 40:60, 20:80, and 100% red) to determine their effects on basil morphology, yield, and selected phytochemicals under controlled-environment conditions. Three cultivars were assessed for growth traits across three independent trials, and two cultivars were analyzed for flavonoids (apigenin, quercetin, and rutin) and volatile organic compounds (eugenol and linalool) using liquid chromatography-tandem mass spectrometry. Plants were grown at 100 ± 10 µmol m⁻² s⁻¹ with a 16 h photoperiod. Spectral responses were evaluated using mixed-effects models and continuous trend analysis of blue-light fraction. Morphological traits and shoot fresh weight showed limited variation across the spectral gradient, indicating that biomass production was relatively stable under moderate light intensity. In contrast, phytochemical responses were compound- and cultivar-dependent. Quercetin increased with blue fraction in green basil, whereas apigenin declined in lemon basil. Rutin in lemon basil and eugenol in green basil exhibited nonlinear responses with higher concentrations at intermediate spectra, while linalool in lemon basil showed a pronounced quadratic response across the gradient. These responses occurred despite minimal growth differences, indicating that spectral redistribution primarily affected phytochemical composition. Overall, spectral redistribution between blue and red photons altered basil phytochemical composition without substantially affecting growth. These findings demonstrate that continuous spectral tuning can be used to adjust aroma- and nutraceutical-related compounds in indoor basil production while maintaining biomass stability.