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Plant growth is plastic and adaptive to the light environment; characteristics such as extension growth, architecture, and leaf morphology change, depending on the light spectrum. Although blue (B; 400–500 nm) and red (R; 600–700 nm) light are generally considered the most efficient wavelengths for eliciting photosynthesis, both are often required for relatively normal growth. Our objective was to quantify how the B:R influenced plant seedling growth andmorphology and understand how plants acclimated to these light environments. We grew seedlings of three ornamental annuals and tomato under six sole-source light-emitting diode (LED) lighting treatments or one cool-white fluorescent treatment that each delivered a photosynthetic photon flux (PPF) of 160 mmol·m·s for 18 h·d. The following treatments were provided with B (peak = 446 nm) and R (peaks = 634 and 664 nm) LEDs: B160 (160 mmol·m ·s of B light only), B80+R80, B40+R120, B20+R140, B10+R150, and R160. Seedlings of impatiens (Impatiens walleriana), salvia (Salvia splendens), petunia (Petunia 3hybrida), and tomato (Solanum lycopersicum) were grown for 31 to 37 days at a constant 20 8C. Plants with as little as 10mmol·m·s of B light were 23% to 50% shorter and had 17% to 50% smaller leaves than plants under only R light. Impatiens and salvia had 53% to 98% greater fresh shoot weight under treatments without B light than with ‡80 mmol·m·s. Plants grown under fluorescent lamps had the greatest chlorophyll content but also had among the thinnest leaves of treatments. Blue-rich light increased flowering in impatiens and reduced incidence of intumescences on tomato. We conclude that, in sole-source lighting of propagules, B light inhibits leaf and stem expansion, which subsequently limits photon capture and constrains biomass accumulation. As little as 10 mmol·m·s of B light in an R-dominant background can elicit desirable growth responses for the production of young plants and for other situations in which compact growth is desired. Blue (B; 400–500 nm) and red (R; 600– 700 nm) light are generally considered the most efficient wavelengths for eliciting photosynthesis in plants (McCree, 1972; Sager et al., 1988). Therefore, B and R LEDs with peak light emission that coincides with peaks of the relative quantum efficiency curve (McCree, 1972) make a logical choice for sole-source commercial plant production (Mitchell et al., 2012). Previous results with tomato (Solanum lycopersicum), salvia (Salvia splendens), impatiens (Impatiens walleriana), and petunia (Petunia ·hybrida) (Wollaeger and Runkle, 2014) and those for lettuce (Lactuca sativa) (Johkan et al., 2010), cherry tomato (Solanum lycopersicum var. cerasiforme) (Liu et al., 2011b), rice (Oryza sativa) (Ohashi-Kaneko et al., 2006), and strawberry (Fragaria ·ananassa) (Nhut et al., 2003) showed that plants grown under a combination of wavebands, particularly including B light, have growth characteristics more similar to those grown under sunlight than those grown under a single waveband of light. However, the addition of B to R light can decrease shoot biomass, such as in lettuce, which had 25% or 17% less fresh shoot biomass when grown under B light alone compared with R or R+B light, respectively (Johkan et al., 2010). Similarly, salvia, petunia, and tomato seedlings grown under 50% green (G; 500–600 nm) + 50% R light from LEDs or those with $25% B light at the same PPF had 35% to 57% less fresh shoot weight than plants under only R light (Wollaeger and Runkle, 2014). Marketable characteristics of young ornamental plants include, but are not limited to, compact growth, presence of a well-developed root system, and adequate branching. Plant growth retardants and limited watering and fertility are methods commercial growers use to suppress extension growth (Hendriks and Ueber, 1995). Extension growth can also be inhibited by modifying the light spectrum, especially by B light and the R:far-red (FR, 700–800 nm) light ratio (Liu et al., 2011a; Smith, 2000). Blue-light-stimulated cryptochrome receptors suppress gibberellic acid biosynthesis, which in turn inhibits cell elongation and stem extension of plants (Ahmad et al., 2002; Cashmore et al., 1999; Liu et al., 2011a; Sellaro et al., 2010). For example, sweetpotato (Ipomoea batatas) stems were 17% shorter when the B:R was 1:10 compared with that of plants grown under a B:R of 1:4 at a PPF of 35 mmol·m·s (Yang et al., 2011). In a separate study, cherry tomato plants grown under B or R+B LEDs were 33% or 49% shorter than those grown under only R LEDs at a PPF of 320 mmol·m·s (Liu et al., 2011b). Phytochromes, with absorption peaks at 660 and 735 nm, are a family of photoreceptors that mediate stem elongation, as well as leaf expansion, chloroplast development, and flowering (Folta and Childers, 2008; Horwitz et al., 1988; Parks et al., 2001; Valverde et al., 2004). In addition to extension growth, plants acclimate to a high B:R by increasing chlorophyll concentration (Lichtenthaler et al., 1981). Blue light stimulates cryptochrome (CRY1), which upregulates the transcription of genes for chlorophyll synthesis (Li et al., 2009). High chlorophyll content in plants, which causes a dark green coloration of leaves, is also desirable in commercial production of young plants such as microgreens, herbs, and ornamental propagules. Growing plants under sole-source solid-state lighting that includes B light in an R-dominant background can yield this characteristic (Goins et al., 1997; Li et al., 2011; OhashiKaneko et al., 2006; Saebo et al., 1995; Tennessen et al., 1994). For example, lettuce grown under B or B+R (1:1) LEDs had 11% greater chlorophyll per unit of dry mass than plants grown under R LEDs at the same intensity (Johkan et al., 2010). In a separate study, cucumber (Cucumis sativus) had increasing chlorophyll content per unit leaf area with increasing ratios of B:R light at the same intensity (Hogewoning et al., 2010). Plants also acclimate to a low R:FR or B-deficient environment by increasing leaf thickness (Fan et al., 2013; Fukuda et al., 2008; Schuerger et al., 1997). Thicker leaves have not been directly attributed to cryptochrome or phototropin photoreceptors (OhashiKaneko et al., 2006). However, the CRY1 cryptochrome receptor downregulates gibberellic acid biosynthesis and therefore suppresses leaf expansion, which in turn results in thicker leaves (Ahmad et al., 2002; Liu et al., 2011a; Sellaro et al., 2010). Therefore, plants grown under solely R light typically have larger, thinner leaves than those of plants grown under light that includes B. For example, pepper plants (Capsicum annuum) grown at a PPF of 330 mmol·m·s had 24%, 37%, or 29% greater overall leaf thickness, palisade parenchyma, or spongy parenchyma layers, respectively, under R LEDs and B fluorescent Received for publication 18 Nov. 2014. Accepted for publication 10 Feb. 2015. We gratefully acknowledge support from Osram Opto Semiconductors, the USDA-ARS Floriculture and Nursery Research Initiative, andMichigan State University’s AgBioResearch. We thank Bert Cregg for statistical advice, C. Raker & Sons for donation of plant material, and Mike Olrich for technical assistance. This work was supported by the USDA National Institute of Food and Agriculture, Hatch project 192266. Former graduate research assistant and currently commercial horticulture educator. Present address: Michigan State University Extension, 3299 Gull Road, P.O. Box 42, Nazareth, MI 49074. Professor and floriculture extension specialist. To whom reprint requests should be addressed; e-mail runkleer@msu.edu. 522 HORTSCIENCE VOL. 50(4) APRIL 2015 lamps than plants grown under R LEDs alone (Schuerger et al., 1997). Similarly, geranium (Pelargonium zonale) irradiated with 100 mmol·m·s of B LED light had 16% thicker leaves compared with plants under R LED light at the same intensity (Fukuda et al., 2008). Our objective was to quantify how plants acclimate to light environments with different B:R ratios, but the same PPF to facilitate the commercial production of young plants with desirable morphological characteristics. We postulated that as the proportion of B light increased, extension growth would decrease, whereas chlorophyll concentration and leaf thickness would increase. Materials and Methods Plant materials. Seeds of tomato ‘Early Girl’, salvia ‘Vista Red’, impatiens ‘SuperElfin XP Red’, and petunia ‘Wave Pink’ were sown in 128-cell (2.7 · 2.7 cm; 12.0-mL volume) seedling trays by a commercial young plant producer (C. Raker and Sons, Inc., Litchfield, MI). Trays were moved to Michigan State University (East Lansing, MI) within 2 d, and all the seedling trays were then cut into sections that each contained $20 seedlings, thinned to one plant per cell, and immediately placed in the lighting treatments. Light treatments and environment. Six modules that were described by Wollaeger and Runkle (2013) contained dimmable B (peak = 446 nm) and R (peaks = 634 and 664 nm) LEDs. The intensities of these three LED types were adjusted to create six light-quality treatments based on an average of six measurements from a spectroradiometer (PS-200; Apogee Instruments, Inc., Logan, UT) made at seedling-tray level at different horizontal positions inside each module. Each module delivered a PPF of 160 mmol·m·s that consisted of the following light treatments: B160 (160 mmol·m·s of B and no R light; 100%R), B80+R80 (50%B, 50%R), B40+R120 (25% B, 75% R), B20+R140 (12.5% B, 87.5% R), B10+R150 (6.2% B, 93.8% R), and R160 (100% R) (Fig. 1). All treatments that delivered R light were delivered equally by the two types of R LEDs. The LED modules were placed in a refrigerated growth chamber and the experiment was performed three times. In a separate growth chamber, plants were grown under cool-white fluorescent lamps (F96T12; Philips, Amsterdam, Netherlands) with the same PPF and temperature set points, which served as a control. The flux of pho

Growth and Acclimation of Impatiens, Salvia, Petunia, and Tomato Seedlings to Blue and Red Light