Regulation of extension growth and flowering of seedlings by blue radiation and the red to far-red ratio of sole-source lighting

Manipulating the radiation spectrum of sole-source lighting (SSL) with light-emitting diodes (LEDs) enables the regulation of growth, development, and quality attributes of ornamental transplants. We quantified growth responses and subsequent flowering of annual bedding plant seedlings grown at 20 °C under eight SSL combinations of blue (B; 400−500 nm), red (R; 600−700 nm), and far-red (FR; 700−800 nm) radiation (subscript values indicate the photon flux density of each waveband, in μmol m^–2 s^–1): B₂₀R₁₆₀, B₂₀R₁₆₀FR₁₀, B₂₀R₁₆₀FR₂₀, B₂₀R₁₆₀FR₄₀, B₆₀R₁₂₀, B₆₀R₁₂₀FR₁₀, B₆₀R₁₂₀FR₂₀, and B₆₀R₁₂₀FR₄₀. Seedlings were also grown indoors under warm-white (WW₁₈₀) LEDs or in a greenhouse for comparison. Among all nine species tested, the addition of FR at 40 μmol m^–2 s^–1 increased the seedling height of only snapdragon (Antirrhinum majus, by 64–134%) and zinnia (Zinnia elegans, by 52–96%), regardless of the proportion of B, compared with SSL treatments without FR or in the greenhouse. Similarly, WW₁₈₀ promoted seedling stem elongation in snapdragon (by 75–139%) compared with SSL without FR or the greenhouse control, but not in the other species. Relative chlorophyll content of zinnia and petunia (Petunia × hybrida) generally decreased as the photon flux density of FR increased, and in both species was 13–14% greater under B at 60 μmol m^–2 s^–1 than at 20 μmol m^–2 s^–1. Radiation treatments did not influence leaf area and dry shoot weight in any species. The subsequent flowering of snapdragon was accelerated by 7–11 d with the additional 20 or 40 μmol m^–2 s^–1 of FR, or under WW₁₈₀, compared with SSL without FR or the greenhouse control. We conclude that when ≥20 μmol m^–2 s^–1 of B is delivered to crops, the addition of FR at ≥20 μmol m^–2 s^–1 can accelerate flowering of at least some long-day plants, with little to no effect on extension growth, but has no effect on flowering of day-neutral or short-day plants.