Spectral effects of light-emitting diodes on plant growth, visual color quality, and photosynthetic photon efficacy: White versus blue plus red radiation

Arrays of blue (B, 400−500 nm) and red (R, 600−700 nm) light-emitting diodes (LEDs) used for plant growth applications make visual assessment of plants difficult compared to a broad (white, W) spectrum. Although W LEDs are sometimes used in horticultural lighting fixtures, little research has been published using them for sole-source lighting. 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 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 W (MW, peak = 558 nm) LEDs that 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 MW(100), MW(75)R(25), MW(45)R(55), MW(25)R(75), B(15)R(85), and B(20)G(40)R(40). At the transplant stage, total leaf area, and fresh and dry weight were similar among treatments in all species. Surprisingly, when petunia seedlings were grown longer (beyond the transplant stage) under sole-source lighting treatments, the primary stem elongated and had flower buds earlier under MW(100) and MW(75)R(25) compared to under B(15)R(85). The color rendering index of MW(75)R(25) and MW(45)R(55) were 72, and 77, respectively, which was higher than those of other treatments, which were ≤64. While photosynthetic photon efficacy of B(15)R(85) (2.25 μmol∙J(–1)) was higher than the W light treatments (1.51−2.13 μmol∙J(–1)), the dry weight gain per unit electric energy consumption (in g∙kWh(–1)) of B(15)R(85) was similar to those of MW(25)R(75), MW(45)R(55), and MW(75)R(25) in three species. We conclude that compared to B+R radiation, W radiation had generally similar effects on seedling growth at the same PPFD with similar electric energy consumption, and improved the visual color quality of sole-source lighting.