Introduction

Bell pepper (Capsicum annuum L.) is a high-value greenhouse crop, crucial for its year-round stable production and rich Vitamin C and antioxidant content. The primary hurdle in controlled environments is light deficiency, which impedes growth and photosynthesis (400-730 nm). Traditional lighting (e.g., sodium lamps) was inefficient and caused adverse effects, leading to the recommendation of LED supplementary lighting. LEDs are highly efficient and can produce specific spectra (Red: 600-800 nm, Blue: 380-480 nm) that maximize photosynthesis by enhancing chlorophyll biosynthesis and stomatal regulation via blue light. Furthermore, the direction of light, especially intra-canopy lighting, is vital for boosting both quantitative and qualitative traits (like total soluble solids). Therefore, this research aims to meticulously investigate the effect of different LED supplementary light spectra and their irradiation direction on the physiological, qualitative, and yield characteristics of bell pepper to optimize cultivation in light-deficient areas.

Materials and methods

The experiment, employing a factorial arrangement in a completely randomized design with three replications, was conducted at the hydroponic greenhouse. on bell pepper cultivar Westland 2090. Treatments included three light environments Control, 100% Red (R100), and 70% Red + 30% Blue (R70:B30) and three irradiation directions Top-Middle (TM), Top (T), and Middle/Intra-canopy (M). Seedlings were raised in a peat-based substrate and then transplanted into a coco-coir hydroponic system, where they received the LED supplementary light treatments for 60 days with a 14-hour photoperiod. The study measured various morphological, physiological (e.g., photosynthesis, chlorophyll), and qualitative traits (e.g., Vitamin C, TSS). Data analysis was performed using ANOVA in SAS software (version 9.4), and mean comparisons were conducted via the LSD test (p≤0.05).

Results and Discussion

The study demonstrated that both LED spectral composition and irradiation direction significantly modulate the physiological and yield traits of bell pepper. The combined light spectrum of 70% Red + 30% Blue (R70:B30) proved superior for vegetative traits, maximizing node number and chlorophyll content when combined with Top (T) irradiation. However, Leaf area was highest under 100% Red (100R-T) light from the top. Crucially, the R70:B30 spectrum combined with Intra-canopy (M) irradiation maximized leaf photosynthesis. For reproductive parameters, the 100R spectrum generally led to the highest total fruit weight (100R-M) and plant yield (100R-T), where the specific directions Middle (M) and Top (T) optimized carbon allocation to fruits and overall biomass, respectively. Finally, fruit quality indicators like Total Soluble Solids (TSS) and Ascorbic Acid (Vitamin C) were maximized by the R70:B30 treatment, with Top (T) irradiation being optimal for Vitamin C preservation.

Conclusion

The research concludes that maximizing bell pepper growth and quality in greenhouses requires precise light management. The R70:B30 spectrum is superior for quality metrics (TSS, Vitamin C) and vegetative development (node number) due to blue light's role in metabolite biosynthesis. Conversely, 100% Red (100R) light applied from the Top (T) or Middle (M) is best for maximum quantitative yield (total fruit weight, plant yield). The optimal strategy for the highest economic return is implementing a combined lighting strategy with an R70:B30 spectrum applied through both top-lighting and intra-canopy lighting can establish an optimal balance among plant growth, yield, and fruit quality, ultimately maximizing economic efficiency.