2018 ASHS Annual Conference

A mixture of blue (B, 400-500 nm) and red (R, 600-700 nm) radiation used for horticultural applications makes plants appear purplish, causing difficulties in detecting nutritional deficiencies, disease, and physiological disorders compared to a broad (white) spectrum. White light-emitting diodes (LEDs) have a high efficacy, but little research has been published about their use in sole-source lighting of plants. We grew seedlings of begonia (Begonia ×semperflorens), geranium (Pelargonium ×horturum), petunia (Petunia ×hybrida), and snapdragon (Antirrhinum majus) at 20 °C under six sole-source LED lighting treatments with an 18-h photoperiod. Six treatments delivered a photosynthetic photon flux density (PPFD) of 160 µmol∙m^–2∙s^–1 using B (peak= 447 nm), green (G, peak= 531 nm), R (peak= 660 nm), and/or mint white (MW, peak=558 nm) LEDs, which emitted 15% B, 59% G, and 26% R plus 6 µmol∙m⁻²∙s⁻¹ of far-red radiation. The lighting treatments (with percentage from each LED in subscript) were MW₁₀₀, MW₇₅R₂₅, MW₄₅R₅₅, MW₂₅R₇₅, B₂₀G₄₀R₄₀, and B₁₅R₈₅. At the transplant stage, seedling height, total leaf area, and fresh and dry weight were similar among treatments in all species except for seedling height of snapdragon. In snapdragon, seedlings grown under MW₁₀₀, MW₇₅R₂₅, and MW₂₅R₇₅ were 26-33% taller than those grown under B₁₅R₈₅. Unexpectedly, when petunia seedlings were grown longer (beyond the transplant stage) under the sole-source lighting treatments, the primary stem elongated and had flower buds earlier under MW₁₀₀ and MW₇₅R₂₅ compared to under B₁₅R₈₅. We conclude that B+R and MW LEDs have generally similar effects on seedling growth and morphology at the same PPFD when they emit a similar portion of B radiation, but MW LEDs can cause stem elongation and promote flowering in some species.