Beyond peak wavelength: spectral bandwidth of blue and red-blue laser diodes modulates photosynthesis, canopy architecture, chlorophyll maintenance, and whole-plant growth

High planting densities in indoor horticulture often accelerate lower-leaf chlorophyll degradation, increasing trimming frequency, reducing effective photosynthetic area, and raising labor costs. While spectral composition has been widely studied, the role of spectral bandwidth, particularly when peak wavelength is identical, remains poorly understood. Here, we used laser diodes (LDs) with an extremely narrow full width at half maximum of light spectrum (FWHM) to examine how spectral bandwidth influences photosynthesis, canopy architecture, chlorophyll degradation in leaves, and plant growth under monochromatic blue light and combined red and blue (R+B) light in tobacco ( Nicotiana tabacum L. ‘Wisconsin-38’), lettuce ( Lactuca sativa L. ‘Red Fire’), and Arabidopsis thaliana (L.) Heynh. ‘Col-0’. Under monochromatic blue light, narrow-band LD blue (LD B ; FWHM = 1.6 nm) reduced CO 2 assimilation rates and shoot dry weight compared with broad-band LED blue (LED B ; FWHM = 20.1 nm) across species. However, LED B was accompanied by accelerated the chlorophyll degradation in lower leaves, whereas LD B promoted more upright canopy architecture, which was associated with higher chlorophyll maintenance in lower leaves. Under combined red and blue light, LD lighting (LD R+B ) mitigated stress induced by 24-hour continuous illumination and promoted coordinated improvements in photosynthetic performance, leaf expansion, and canopy architecture. Compared with LED R+B , these integrated responses resulted in higher shoot fresh weight and a healthier physiological state indicated by higher chlorophyll content and lower anthocyanin accumulation. However, there was no significant difference in shoot dry weight, suggesting that this growth enhancement was primarily driven by cellular water accumulation and leaf expansion rather than an absolute increase in dry matter carbon accumulation. Together, our results suggest that spectral bandwidth is an important factor associated with altered plant growth responses, even when peak wavelengths remain constant. The use of LD lighting offers the potential to modulate physiological traits traditionally linked in the classic ‘sun’ or ‘shade’ leaf syndromes, providing a localized approach to influence canopy architecture, chlorophyll maintenance, and overall plant growth. This study highlights LD lighting as powerful yet underutilized light system for optimizing canopy health and yield in indoor horticulture.