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Photoperiodic lighting from lamps with a moderate ratio of red [R (600–700 nm)] to far-red [FR (700–800 nm)] light effectively promotes flowering of long-day plants (LDPs). Because of spectral controllability, long life span, and energy efficiency, light-emitting diodes (LEDs) have emerged as an alternative to conventional light sources, such as incandescent (INC) and high-pressure sodium (HPS) lamps. We conducted a coordinated trial with five commercial greenhouse growers to investigate the efficacy of R + white (W) + FR LEDs, with an R:FR of 0.82, to regulate flowering of daylength-sensitive ornamental crops. The trial was also performed in two replicate greenhouses at Michigan State University (MSU). Ageratum (Ageratum houstonianum), calibrachoa (Calibrachoa ×hybrida), dahlia (Dahlia ×hybrida), dianthus (Dianthus chinensis), petunia (Petunia ×hybrida), snapdragon (Antirrhinum majus), and verbena (Verbena ×hybrida) were grown under natural short days (SDs) with 4-hour night-interruption (NI) lighting provided by the R + W + FR LEDs or conventional lamps typically used by each grower. Two companies used HPS lamps, whereas the other sites used INC lamps. In addition, a natural SD treatment, a truncated 9-hour SD treatment, or a compact fluorescent lamp (CFL) NI treatment was provided at three different sites. With few exceptions, time to flower and flowering percentage of the bedding plant crops tested were similar under the R + W + FR LEDs to that under the conventional lamps at all sites. At MSU, ageratum, dianthus, petunia, snapdragon, and verbena flowered earlier under NI lighting treatments than under 9-hour SDs. In addition, plant height and visible flower bud or inflorescence number at flowering were similar under the R + W + FR LEDs and INC lamps for most crops. Therefore, we conclude that the R + W + FR LEDs are as effective as lamps traditionally used in greenhouses at controlling flowering of photoperiodic plants.

Adding green [G (500–600 nm)] radiation to blue [B (400–500 nm)] and red [R (600–700 nm)] radiation creates white radiation and improves crop inspection at indoor farms. Although G radiation can drive photosynthesis and elicit the shade-avoidance response, its effects on plant growth and morphology have been inconsistent. We postulated G radiation would counter the suppression of crop growth and promotion of secondary metabolism by B radiation depending on the B photon flux density (PFD). Lettuce (Lactuca sativa ‘Rouxai’) was grown in a growth room under nine sole-source light-emitting diode (LED) treatments with a 20-hour photoperiod or in a greenhouse. At the same photosynthetic photon flux density [PPFD (400–700 nm)] of 180 μmol·m⁻²·s⁻¹, plants were grown under warm-white LEDs or increasing B PFDs at 0, 20, 60, and 100 μmol·m⁻²·s⁻¹ with or without substituting the remaining R radiation with 60 μmol·m⁻²·s⁻¹ of G radiation. Biomass and leaf expansion were negatively correlated with the B PFD with or without G radiation. For example, increasing the B PFD decreased fresh and dry mass by up to 63% and 54%, respectively. The inclusion of G radiation did not affect shoot dry mass at 0 or 20 μmol·m⁻²·s⁻¹ of B radiation, but it decreased it at 60 or 100 μmol·m⁻²·s⁻¹ of B radiation. Results suggest that the shade-avoidance response is strongly elicited by low B radiation and repressed by high B radiation, rendering G radiation ineffective at controlling morphology. Moreover, substituting R radiation with G radiation likely reduced the quantum yield. Otherwise, G radiation barely influenced morphology, foliage coloration, essential nutrients, or sensory attributes regardless of the B PFD. Increasing the B PFD increased red foliage coloration and the concentrations of several macronutrients (e.g., nitrogen and magnesium) and micronutrients (e.g., zinc and copper). Consumers preferred plants grown under sole-source lighting over those grown in the greenhouse, which were more bitter and less acceptable, flavorful, and sweet. We concluded that lettuce phenotypes are primarily controlled by B radiation and that G radiation maintains or suppresses lettuce growth depending on the B PFD.

Under natural short days, growers can use photoperiodic lighting to promote flowering of long-day plants and inhibit flowering of short-day plants. Unlike traditional lamps used for photoperiodic lighting, low-intensity light-emitting diode (LED) lamps allow for a wide array of adjustable spectral distributions relevant to regulation of flowering, including red (R) and white (W) radiation with or without far-red (FR) radiation. Our objective was to quantify how day-extension (DE) photoperiodic lighting from two commercially available low-intensity LED lamps emitting R + W or R + W + FR radiation interacted with daily light integral (DLI) to influence stem elongation and flowering of several ornamental species. Long-day plants [petunia (Petunia ×hybrida Vilm.-Andr. ‘Dreams Midnight’) and snapdragon (Antirrhinum majus L. ‘Oh Snap Pink’)], short-day plants [african marigold (Tagetes erecta L. ‘Moonsong Deep Orange’) and potted sunflower (Helianthus annuus L. ‘Pacino Gold’)], and day-neutral plants [pansy (Viola ×wittrockiana Gams. ‘Matrix Yellow’) and zinnia (Zinnia elegans Jacq. ‘Magellan Cherry’)] were grown at 20/18 °C day/night air temperatures and under low (6–9 mol·m⁻²·d⁻¹) or high (16–19 mol·m⁻²·d⁻¹) seasonal photosynthetic DLIs from ambient solar radiation combined with supplemental high-pressure sodium lighting and DE LED lighting. Photoperiods consisted of a truncated 9-hour day (0800–1700 hr) with additional 1-hour (1700–1800 hr, 10 hours total), 4-hour (1700–2100 hr, 13 hours total), or 7-hour (1700–2400 hr, 16 hours total) R + W or R + W + FR LED lighting at 2 μmol·m⁻²·s⁻¹. Days to visible bud, plant height at first open flower, and time to first open flower (TTF) of each species were influenced by DLI, lamp type, and photoperiod though to different magnitudes. For example, plant height of african marigold and potted sunflower at first open flower was greatest under R + W + FR lamps, high DLIs, and 16-hour photoperiods. Petunia grown under R + W lamps, high DLI, and 10- and 13-hour photoperiods were the most compact. For all species, TTF was generally reduced under high DLIs. For example, regardless of the lamp type, flowering of african marigold occurred fastest under a high DLI and 10-hour photoperiod. Flowering of petunia and snapdragon occurred fastest under a high DLI, R + W + FR lamps, and a 16-hour photoperiod. However, only under high DLIs, R + W or R + W + FR lamps were equally effective at promoting flowering when used to provide DE lighting. Our data suggest that under low DLIs, flowering of long-day plants (petunia and snapdragon) occurs more rapidly under lamps providing R + W + FR, whereas under high DLIs, flowering is promoted similarly under either R + W or R + W + FR lamps.

Chrysanthemum (Chrysanthemum ×morifolium) is a common ornamental crop with a qualitative short-day flowering response. Extending a short day with moderate blue [B (400–500 nm)] light inhibits flowering in greenhouse conditions with sunlight but does not indoors (without sunlight) under B + red [R (600–700 nm)] light or white light. We postulated that the contrasting responses to B light as a day extension depended on far-red [FR (700–800 nm)] light during the day, which is plentiful under sunlight but lacking indoors under B+R or white light-emitting diodes. To study this response in three chrysanthemum cultivars, we delivered indoor lighting treatments at two locations with an 11-hour main photoperiod of B, green [G (500–600 nm)], R, and FR light, where subscript values indicate the photon flux density (in µmol·m⁻²·s⁻¹) of each waveband: B₆₀R₁₂₀, B₆₀G₆₀R₆₀, and B₆₀R₆₀FR₆₀. After each short main photoperiod, plants received 0 or 4 hours of day-extension lighting of 60 µmol·m⁻²·s⁻¹ of B light (B₆₀). Under all treatments except B₆₀R₆₀FR₆₀ with day-extension B₆₀, it took ‘Chelsey Pink’, ‘Gigi Gold’, and ‘Gigi Yellow’ 13 to 17 days to reach the first visible inflorescence and 42 to 51 days to the first open flower. In contrast, plants grown under B₆₀R₆₀FR₆₀ with day-extension B₆₀ took 41 to 67 days to reach the first visible inflorescence with few plants developing open flowers. Plants were tallest at the first open flower and after 9 weeks of treatments when grown under B₆₀R₆₀FR₆₀ with day-extension B₆₀. These results indicate that the inclusion of FR light, but not G light, in the main photoperiod is necessary for day-extension B light to inhibit flowering in chrysanthemum. On the basis of these results and those of other studies, we postulate that the spectral dependence of flowering in chrysanthemum depends on whether and how the phytochrome photoequilibrium changes during the day. In particular, a sufficiently high daytime phytochrome photoequilibrium (e.g., under B+R and B+G+R light) could establish a predominant mode of floral signaling that prevents perception of subsequent B light as a long day.