Discrete blue and green light wavebands alter biomass, morphology, and color in lettuce (Lactuca sativa L.)

Previous research demonstrated that blue (B) light decreased internode elongation and leaf surface area (LSA) while increasing anthocyanin concentration as the red (R) to B ratio decreased, but the effects of specific B wavebands on lettuce remain underexplored. The first experiment objective was to evaluate the impacts of three different wavebands of B light (peaks and full width at half max [FWHM]: 412 nm, FWHM 16 nm; 425 nm, FWHM 15 nm; 454 nm, FWHM 20 nm) and green (G) light (523 nm, FWHM 34 nm) on 'Rex' and 'Red Oak' lettuce biomass and morphology. Among B light treatments, 412 and 425 nm produced similar results, with significantly greater growth (width, fresh weight (FW), leaf area (LA) for both cultivars, height and dry weight (DW) for 'Rex') compared to the 454 nm treatment. 454 nm treatment showed more intense characteristics associated with B light exposure (decreased height, width, LA, FW, and DW) but had the highest chlorophyll content. 'Red Oak' anthocyanin concentration was unaffected by B wavelengths. Plants under G light had similar biomass to 412 and 425 nm B light for 'Rex' and 412 nm for 'Red Oak' but lower chlorophyll and anthocyanin concentration. The second objective of this study was to assess whether G light's biomass could be complemented by B light's pigmentation effects when G was replaced by B late in the crop cycle. The control treatment used 80% R and 20% G (523 nm), with G substituted by 454 nm B light 2, 4, or 8 days before harvest. Two days of B light at the end of the cycle resulted in similar FW as the control in 'Red Oak' but significantly higher anthocyanin. In 'Rex', FW and DW were statistically unchanged, but longer B light exposure significantly increased chlorophyll content. This indicated that two days of B light at the cycle's end preserved biomass and improved crop quality through enhanced leaf coloration in both cultivars.