Effects of stage-specific dynamic LED lighting on the growth and photosynthetic system of sweet pepper seedlings

Previous studies have demonstrated that the physiological responses of seedlings to light quality vary among growth stages. In this experiment, 'Trirosso RZ F1′ sweet pepper seedlings were grown in a plant factory with artificial lighting. Six light treatments were established to investigate the effects of stage-specific light quality regulation on seedling growth and physiological characteristics, and to identify optimal dynamic light quality combinations. The treatments included constant LED lighting (constant red and blue light RB, constant white light W) and stage-specific lighting (early white and blue light-late red and blue light WB-RB, early white and red light-late red and blue light WR-RB, early red and blue light-late white and red light RB-WR, early red and blue light-late white and blue light RB-WB). Morphological indices, biomass, photosynthetic parameters, chlorophyll fluorescence, antioxidant enzyme activities, and leaf color parameters were measured, and principal component analysis was conducted. The results showed: 1) Compared with W, the biomass, light use efficiency (LUE) and electrical energy use efficiency (EUE) of sweet pepper seedlings were significantly increased by all stage-specific light modes except RB-WB. The highest LUE and EUE were both observed under RB-WR treatment, increases of 44.4% and 93.1%, respectively, relative to W (p < 0.05). After 30 days of light treatment, compared with W, the plant height, stem diameter, leaf area, and leaf number of sweet pepper seedlings under treatments containing red and blue light were all increased, with ranges of 6.7%-26.4%, 2.6%-16.5%, 15.8%-76.1%, and 8.3%-30.6%, respectively. Among them, the largest stem diameter and leaf area were under WB-RB, while the highest plant height and leaf number were under RB-WR. 2) Compared with W, reactive oxygen species content in seedlings was significantly reduced by 11.1%-32.9% under all other treatments. Except for RB-WB, the net photosynthetic rate (Pₙ) of sweet pepper leaves was increased under the other light treatments relative to W. The highest Pₙ and the water use efficiency (WUE) were under WB-RB, with significant increases of 54.3% and 94.0% compared with W (p < 0.05). RB-WR treatment exhibited the highest photosystem II performance indices (PI abs , PI total ), maximum photochemical efficiency (Fv/Fm) and PSI terminal electron acceptor reduction flux (REo/RC), along with the lowest dissipated energy per reaction center (DIo/RC). 3)Morphological indices, photosynthetic parameters (Pₙ, WUE), and peroxidase (POD) activity exhibited extremely significant positive correlations with one another. Principal component analysis indicated that the highest composite scores were observed under the RB-WR and WB-RB treatments, suggesting that these two stage-specific light quality strategies best promote robust seedling growth, optimize photosynthesis, and maintain redox balance. In conclusion, stage-specific lighting treatments, particularly RB-WR and WB-RB, were recommended as optimal dynamic light quality regulation strategies for sweet pepper seedling cultivation in plant factories. These treatments enhanced growth and energy utilization by optimizing morphogenesis, improving photosynthetic performance, and alleviating oxidative stress compared with constant lighting modes.