Blue, Red, and Far-red Radiation Interact to Influence Growth, Morphology, and Pigmentation of Lettuce and Basil

Horticultural lighting has primarily focused on photosynthetically active radiation, especially blue (B, 400 to 500 nm) and red (R, 600 to 700 nm), and its effects on plant growth. However, far-red (FR, 700 to 800 nm) radiation regulates numerous pathways crucial to photomorphogenesis, thus meriting consideration in sole-source plant lighting applications. We investigated how interactions among B (peak=450 nm), R (peak=660 nm), and FR (peak=730 nm) radiation from light-emitting diodes influenced growth of green butterhead lettuce (Lactuca sativa) ‘Rex’, red oakleaf lettuce ‘Cherokee’, and basil (Ocimum basilicum) ‘Genovese’. Seedlings were grown in a growth chamber at 22 °C and were continuously irradiated by 180 µmol∙m⁻²∙s⁻¹ of B and/or R radiation at various B-to-R ratios (R₁₈₀, B₃₀R₁₅₀, B₉₀R₉₀, and B₁₈₀, where subscripts indicate respective photon flux densities), with or without 30 µmol∙m⁻²∙s⁻¹ of FR radiation (FR₃₀). Growth, morphology, and pigmentation were quantified 12 and 16 days after sowing lettuce and basil, respectively. FR₃₀ increased leaf length and shoot weight of all crops, although the effects were more pronounced when FR₃₀ was added to B₉₀R₉₀ than to B₃₀R₁₅₀. For example, the addition of FR₃₀ increased shoot dry weight of all crops by 34–43% at B₉₀R₉₀, in contrast to 8–17% at B₃₀R₁₅₀. Adding FR₃₀ increased root dry weight of basil by 18–26% at B₃₀R₁₅₀ or B₉₀R₉₀ and that of lettuce ‘Cherokee’ by 25% at B₃₀R₁₅₀, but did not influence that of lettuce ‘Rex’. Adding FR₃₀ to B+R radiation reduced chlorophyll content in lettuce by 10–20%, but not in basil. In the absence of B radiation, R₁₈₀FR₃₀ produced spindly lettuce seedlings with elongated, curling, chlorotic, and high-moisture leaves. Partially substituting B radiation for R radiation in R₁₈₀FR₃₀ suppressed hypocotyl and leaf length of lettuce and increased chlorophyll content in both lettuce and basil. A color space analysis revealed that more B radiation or lack of FR radiation corresponded with darker and redder leaves of lettuce ‘Cherokee’, possibly linked to a greater anthocyanin concentration. We conclude that both B and R radiation are required for desirable plant traits, and that B and FR radiation exert antagonistic effects on extension growth and pigmentation. FR radiation can be added to sole-source B+R lighting to increase leaf expansion, radiation capture and thus, growth.