Optimizing Blue, Red, and Far-Red Light to Enhance Growth and Coloration of Indoor Red-Leaf Lettuce Seedlings

• Terri-Ellen Becker, Wildlife Ecology and Conservation, University of Delaware

• Qingwu Meng, Plant and Soil Science, University of Delaware

Title: Optimizing Blue, Red, and Far-Red Light to Enhance Growth and Coloration of Indoor Red-Leaf Lettuce Seedlings

Terri-Ellen Becker and Qingwu Meng
Department of Plant and Soil Sciences
University of Delaware

Abstract

Optimizing lighting conditions is crucial for controlled environment agriculture, particularly for space-based food crop cultivation where efficient resource use is essential. This research was needed to improve our understanding of how different light wavelengths affect plant growth and development. This study investigated the influence of supplemental red (R, 600-700 nm) and far-red (FR, 700-750 nm) light on the growth, morphology, and coloration of indoor-grown red-leaf lettuce (Lactuca sativa) at varying blue (B, 400-500 nm) photon flux densities (PFDs). From seed to the end of the seedling phase, lettuce ‘Cherokee’ and ‘Rouxai’ were grown under nine sole-source lighting treatments from light-emitting diodes (LEDs): 1) B₂₅R₁₇₅; 2) B₂₅R₁₇₅FR₁₀₀; 3) B₂₅R₂₇₅; 4) B₁₀₀R₁₀₀; 5) B₁₀₀R₁₀₀FR₁₀₀; 6) B₁₀₀R₂₀₀; 7) B₁₇₅R₂₅; 8) B₁₇₅R₂₅FR₁₀₀; and 9) B₁₇₅R₁₂₅, where each subscript denotes the waveband PFD in μmol∙m⁻²∙s⁻¹. Preliminary results indicated interactions between the B PFD and supplemental R or FR light affecting plant growth, morphology, and coloration. Increasing the B PFD increased chlorophyll concentration and enhanced red leaf coloration, while supplemental R light increased biomass accumulation and chlorophyll concentration. A low B-to-R ratio (B₂₅R₁₇₅) resulted in the highest shoot fresh and dry mass at a total light intensity of 200 μmol∙m⁻²∙s⁻¹. The addition of FR light (B₂₅R₁₇₅FR₁₀₀ and B₁₇₅R₂₅FR₁₀₀) further increased biomass, with the highest biomass under B₁₇₅R₂₅FR₁₀₀. Supplemental FR light promoted stem elongation and increased biomass but reduced chlorophyll concentration and red leaf coloration, which were enhanced by increasing the B PFD. Additionally, higher R PFDs produced larger leaves, while supplemental FR light increased hypocotyl length and leaf size. These findings suggest that higher B light intensifies leaf coloration, whereas higher R or FR light promotes growth. Light quality can be optimized accordingly to improve plant yield or the aesthetic quality of red-leaf lettuce.