Optimizing Red–Blue LED Light Recipes for Improved Biomass Production and Nutritional Quality of Purple Celery in Plant Factory Cultivation

Purple celery (Apium graveolens L.) is a high-value functional vegetable rich in anthocyanins, but achieving a balanced optimization between biomass accumulation and nutritional quality in plant factories remains challenging due to species-specific spectral trade-offs, and current research lacks integrated light recipes that simultaneously optimize photosynthetic efficiency, antioxidant defenses, and functional quality while suppressing nitrate accumulation. This study evaluated five LED treatments over 30 days: white light (CK), monochromatic red (R), and red-to-blue photon flux density ratios of 5:5, 7:3, and 8:2. Red light induced stem elongation but suppressed root development. Conversely, 8R2B delivered the best performance, significantly elevating leaf, petiole, root, and total fresh biomass and increasing total dry weight by 68.8% over CK. It also markedly enhanced chlorophyll a, b, total chlorophyll, and carotenoids, as well as improved PSII quantum yield (YII), photochemical quenching (qP), and electron transport rate (ETR), while reducing non-photochemical heat dissipation (NPQ) by 61.0%, indicating superior light-use efficiency. Furthermore, 8R2B upregulated CAT and POD activities promoted soluble sugars (up 688.4% in leaves), soluble proteins, and ascorbic acid accumulation, yet reduced leaf nitrate by 28.5%. Principal component analysis confirmed 8R2B as the most effective formulation across all growth, photochemical, defense, and quality traits. These findings provide a novel, evidence-based 8:2 red-to-blue LED light recipe for high-yield, high-quality, and commercially viable purple celery production in controlled environment agriculture.