Application of sole-source light-emitting diode (LED) lighting in floricultural crop production is of increasing interest because of their high energy efficiency, longevity, and ability to deliver a customized radiation spectrum. One of the most important aspects of implementing LED lighting is to identify the radiation spectrum that elicits the desired plant characteristics. The objectives of this research were to investigate how sole-source lighting, with different spectral and intensity combinations, of blue (B, 400-500 nm), red (R, 600-700 nm), far-red (FR, 700-800 nm), and white (W) LEDs regulates photomorphogenic traits and flowering responses on a range of floriculture crops in highly controlled environments. During the first experiment, we investigated how the addition of FR radiation or partial substitution of R with FR radiation in B+R sole-source lighting influences seedling growth and subsequent flowering of geranium (Pelargonium ×hortorum), petunia (Petunia ×hybrida), snapdragon (Antirrhinum majus), and impatiens (Impatiens walleriana). As the R:FR (or estimated phytochrome photoequilibrium) of radiation treatments increased, seedling height in all species and total leaf area of geranium and snapdragon linearly decreased. In geranium and snapdragon, the increased total leaf area (by 7 %) with the addition of FR at the same photosynthetic photon flux density (PPFD) was accompanied by an increase in shoot dry weight (by 28-50%) while the increase of total leaf area (by 30-40%) with partial substitution of R with FR produced similar shoot dry weight compensating the reduced PPFD (by 40%). In addition, inclusion of FR (≥ 16 μmol∙m–2∙s–1) during seedling stage accelerated flowering of long-day plant snapdragon at finishing stage. Two additional experiments were performed with geranium, petunia, and coleus (Solenostemon scutellariodes) to determine how different PPFDs and B photon flux densities regulate seedling growth and subsequent flowering responses to FR radiation. When B photon flux density was kept constant, decreasing R:FR with the addition of FR promoted stem elongation and leaf expansion, and subsequent dry mass accumulation, independently of PPFD. However, the promotive effect of low R:FR on flowering of long-day plant petunia was greater under the lower PPFD. Under the same PPFD, including a moderately high B photon flux density (80 μmol∙m–2∙s–1) diminished the effects of FR radiation on extension growth but had little effect on FR-mediated subsequent flowering promotion in long-day petunia. In a final study, the utility of different shades of W radiation was evaluated, compared to a typical mixture of B+R radiation, considering their effects on human vision, photosynthetic photon efficacy, and plant growth and subsequent development in begonia (Begonia ×semperflorens), geranium, petunia, and snapdragon. While using W radiation generally increased the visual quality, seedling growth in all species was similar under B+R and W radiation treatments at the same PPFD. In addition, when W radiation treatments were created with the combination of mint W and R LEDs, they were energy efficient as much as B+R mixture in terms of producing plant dry mass per electrical energy input. Collectively, the results of this research generate new information on how the use of FR and W radiation can regulate plant growth and flowering responses with other wavebands to optimize the spectrum of sole-source lighting to increase product quality of floriculture crops and potentially decrease crop production times.
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