Synergistic Regulation of Growth and Quality in Substrate-Grown Spinach by LED Light Quality and Intensity in PFALs

In this study, a full factorial experiment was conducted to investigate the interactive effects of different red-to-blue light ratios (with R–B ratios of 0.6, 1.2, and 2.4) and photosynthetic photon flux densities (PPFDs of 200, 250, 300, and 350 μmol·m−2·s−1) on the growth, biomass accumulation, and nutritional quality of spinach (Spinacia oleracea L.) in a plant factory using substrate cultivation. The results demonstrated that both LED light quality and light intensity had significant regulatory effects on spinach’s morphological development, pigment biosynthesis, photosynthetic activity, and nutritional quality. The treatment combining an R–B ratio of 1.2 with a PPFD of 300 μmol·m−2·s−1 produced the most favorable outcomes, resulting in the largest leaf area (98.3 cm2), the highest net photosynthetic rate (16.4 μmol·m−2·s−1), and the greatest shoot fresh mass (48.7 g·plant−1). Moreover, this treatment also led to the highest vitamin C content in the leaves and a notable reduction in nitrate accumulation. Correlation analysis revealed significant positive relationships (r ≥ 0.70) between leaf number and shoot fresh mass, chlorophyll content, and vitamin C content. Principal component analysis further indicated that PC1 and PC2 jointly accounted for 83.4% of the total variance, with growth-related and quality-related traits contributing primarily to PC1 and PC2, respectively. Among all treatment combinations, the R–B ratio of 1.2 and 300 μmol·m−2·s−1 condition achieved the highest comprehensive performance score. These findings underscore the critical role of finely tuned LED light environments in optimizing spinach productivity and nutritional quality in a controlled environment. Based on the results, an R–B ratio of 1.2 combined with a PPFD of 300 μmol·m−2·s−1 is recommended as the optimal lighting strategy for spinach cultivation in plant factories.