Blue:red spectral gradients decouple phytochemical composition from biomass production in indoor basil

Light spectrum is a controllable factor in indoor vertical farming, yet basil ( Ocimum basilicum L.) phytochemical responses to continuous blue:red spectral gradients remain poorly characterized. This study evaluated six blue:red ratios (100:0, 80:20, 60:40, 40:60, 20:80, and 0:100) to determine their effects on basil morphology, yield, and selected phytochemicals under controlled-environment conditions. Three cultivars, O. basilicum ‘MIA Prospera Active DMR OG’ (green basil), O. basilicum ‘Prospera Red DMR F1 OG’ (red basil), and ‘Mrs. Burns Lemon’ (lemon basil), were assessed for growth across three independent trials, while green and lemon basil 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. Morphological traits and shoot fresh weight showed little variation across the spectral gradient, indicating stable biomass production. In contrast, phytochemical responses were compound- and cultivar-dependent. Increasing the blue-light fraction enhanced quercetin accumulation in green basil but reduced apigenin in lemon basil. Rutin in lemon basil and eugenol in green basil were highest under intermediate blue:red ratios, while linalool in lemon basil showed a pronounced quadratic response. These findings demonstrate that continuous spectral tuning can selectively modify aroma- and nutraceutical-related compounds while maintaining biomass production, providing a practical strategy for improving basil quality in indoor production systems.