Beyond Peak Wavelength: Spectral Bandwidth of Blue and Red-Blue Laser Diodes (LDs) Reprograms Photosynthesis, Canopy Architecture, Senescence, and Whole-Plant Growth

In indoor horticulture high planting densities often accelerate the senescence of lower leaves, 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) as narrow-band light sources to examine how spectral bandwidth influences photosynthesis, canopy architecture, leaf senescence, 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 (LDB) reduced CO{square} assimilation rates and shoot dry weight compared with broad-band LED blue (LEDB) across species. However, LEDB cultivation was accompanied by accelerated the senescence of lower leaves, a major limitation under dense planting. In contrast, plants grown under LDB developed a more upright canopy architecture, characterized by larger leaf angles, which was associated with improved light penetration and delayed the senescence of lower leaves. When red and blue light were combined, narrow-band LD lighting (LDR+B) mitigated stress induced by 24-hour continuous illumination and promoted coordinated improvements in photosynthetic performance, leaf expansion, and canopy architecture. These integrated responses resulted in consistently higher shoot fresh weight and a healthier physiological state, as indicated by higher chlorophyll content and lower anthocyanin accumulation, compared with broad-band LEDR+B. Together, our results demonstrate that spectral bandwidth, even when peak wavelength is held constant, is a critical parameter shaping plant growth strategies. Precise control of bandwidth enables partial decoupling of traits traditionally linked in the classic sun or shade leaf syndromes, allowing independent regulation of canopy architecture, leaf senescence, and plant growth. This study highlights spectral bandwidth as a powerful yet underutilized lever for optimizing canopy health and yield in indoor horticulture.