Quantifying Growth Responses of High-value Ornamental Seedlings to Red and Far-red Light

Light-emitting diodes (LEDs) are a developing technology that has promising potential for use as sole-source plant-growth lighting in enclosed environments. Far-red light (FR, 700 to 800 nm), associated with the red (R; 600 to 700 nm) to FR ratio, can regulate photomorphogenesis in plants, including stem elongation and leaf expansion, but little research has been published on how FR LEDs can be used to improve plant growth and quality attributes during seedling production. We grew seedlings of geranium (Pelargonium ×hortorum), petunia (Petunia ×hybrida), snapdragon (Antirrhinum majus), and impatiens (Impatiens walleriana) at 20 °C under six sole-source LED lighting treatments with an 18-h photoperiod. The following light quality treatments all included 32 µmol∙m^-2∙s^-1 blue light (peak=451 nm) with R (peak=660 nm) and/or FR (peak = 729 nm) light (values after each waveband indicate intensity, in µmol∙m^–2∙s^–1): R₁₂₈+FR₀, R₁₂₈+FR₁₆, R₁₂₈+FR₃₂, R₁₂₈+FR₆₄, R₉₆+FR₃₂, and R₆₄+FR₆₄. Preliminary results indicate that plant height generally increased as the R:FR of each FR treatment decreased. Leaf area of geranium and petunia under an R:FR ≤3 was 6% to 47 % greater than plants under an R:FR ≥4, whereas impatiens and snapdragon under an R:FR =1 had 13 to 51% greater leaf area than plants under an R:FR ≥2. Shoot dry weight was similar to, or slightly increased, under an R:FR ≤8 compared to plants grown under less FR light. The calculated yield photon flux was not an accurate predictor of shoot biomass; in some instances, additional FR light (considered non-photosynthetic) increased growth. Photosynthetic efficiency (PE), which is calculated as shoot dry weight per unit leaf area, was generally similar among treatments. We conclude that FR light can increase biomass accumulation by increasing light interception (from greater leaf area) without reducing PE or excessive stem elongation.