2019 ASHS Annual Conference

Green (G, 500 to 600 nm) radiation is often considered less effective at driving photosynthesis than red (R, 600 to 700 nm) radiation. In addition, far-red (FR, 700 to 800 nm) radiation is regarded as inefficient at directly promoting photosynthesis. However, G and FR radiation induce shade-avoidance responses including promotion of extension growth that increases radiation capture. Here we investigated how substituting G with R radiation influenced plant growth with and without FR radiation. We grew young plants of lettuce (Lactuca sativa) and tomato (Solanum lycopersicum) indoors under lighting from light-emitting diodes in a controlled-environment growth room at 22 ºC. All lighting treatments provided a photosynthetic photon flux density of 180 µmol∙m⁻²∙s⁻¹ for 18h∙d⁻¹ with the following photon flux densities (subscript in µmol∙m⁻²∙s⁻¹) of blue (B, 400 to 500 nm, peak=448 nm), G (peak=527 nm), R (peak=662 nm), or/and FR (peak=734 nm) radiation: B₆₀G₆₀R₆₀FR₆₀, B₆₀G₃₀R₉₀FR₆₀, B₆₀G₁₅R₁₀₅FR₆₀, B₆₀G₆R₁₁₄FR₆₀, B₆₀R₁₂₀FR₆₀, B₆₀G₆₀R₆₀, B₆₀G₃₀R₉₀, B₆₀G₁₅R₁₀₅, and B₆₀R₁₂₀. The substitution of G with R radiation had little or no effect on leaf number and leaf size of either crop, or fresh and dry weight of tomato. However, as more G radiation was substituted with R radiation in the presence of FR, fresh and dry weight of lettuce increased by up to 34% and 29%, respectively. Regardless of the portion of G and R radiation, the addition of FR promoted plant growth of lettuce and tomato. In lettuce, the addition of FR increased plant diameter by 18-30%, fresh weight by 29-74%, and dry weight by 26-92%. In tomato, FR radiation increased plant height by 151-249% and fresh weight by 56-130%. These results show value in including of FR in an indoor radiation spectrum, while from a growth standpoint, there is little to no value of including green radiation.