Growth of Impatiens, Petunia, Salvia, and Tomato Seedlings under Blue, Green, and Red Light-emitting Diodes

By H. Wollaeger, Erik S. Runkle. 2014

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Plant growth and architecture are regulated in part by light quality. We performed experiments to better understand how young plants acclimate to blue (B), green (G), and red (R) light and how those responses can be used to produce plants with desirable morphological characteristics. We grew seedlings of impatiens (Impatiens walleriana), salvia (Salvia splendens), petunia (Petunia 3hybrida), and tomato (Solanum lycopersicum) under six sole-source light-emitting diode (LED) treatments or one coolwhite fluorescent treatment that each delivered a photosynthetic photon flux (PPF) of 160 mmol·m·s for 18 h·d. Leaf number was similar among treatments, but plants grown under 25% or greater B light were 41% to 57% shorter than those under only R light. Plants under R light had 47% to 130% greater leaf area and 48% to 112% greater fresh shoot weight than plants grown under treatments with 25% or greater B. Plants grown under only R had a fresh shoot weight similar to that of those grown under fluorescent light for all species except tomato. In impatiens, flower bud number at harvest generally increased with B light, whereas in tomato, the number of leaflets with intumescences decreased with B light. This research discusses how light quality can be manipulated for desired growth characteristics of young plants, which is important in the production of specialty crops such as ornamentals, herbs, and microgreens. Light-emitting diodes are well suited for commercial plant production because of their high energy efficiency and spectral specificity (Mitchell et al., 2012). When operated at favorable temperatures, well-constructed LEDs have an operating lifetime of 50,000 h or more, which is at least two times longer than conventional high-pressure sodium or fluorescent lamps (Bourget, 2008; Morrow, 2008). LEDs also have potential to be used as intercanopy lighting, to increase nutrient concentration in edible crops and to decrease pesticide and plant growth regulator applications (Cosgrove, 1981; Doukas and Payne, 2007; Hemming, 2011; Kumar and Poehling, 2006; Li and Kubota, 2009; Rapisarda et al., 2006; van Ieperen and Trouwborst, 2008; Watanabe, 2011; Weinig et al., 2004). Using LEDs for sole-source photosynthetic lighting is of commercial interest because high-density shelving systems could allow multiple-tiered growing, which can increase land use efficiency (Watanabe, 2011). In addition, the light spectrum can be customized for specific growth responses such as to inhibit or promote extension growth or deliver the most photosynthetically efficient light to increase biomass (van Ieperen, 2012). However, although LEDs enable the light spectrum to be tailored for desirable plant responses, few studies have been published on light manipulation on nonagronomic crops grown in highly controlled environments. Sole-source lighting with B (400 to 500 nm) or R (600 to 700 nm) LEDs or fluorescent lamps has been studied primarily on agronomic crops (Massa et al., 2008). For example, wheat (Triticum aestivum) grown under a PPF of 350 mmol·m·s from R LEDs (peak = 660 nm) had 50% less dry weight than under B or white fluorescent lamps, and the instantaneous photosynthetic rate was 45% less than that of plants under white light (Goins et al., 1997). In contrast, when four cultivars of spinach (Spinacea oleracea) received a PPF of 300 mmol·m·s, those grown under R or white fluorescent light had 10% to 80% more biomass than plants grown under only B fluorescent light (Li et al., 2011). Spinach and several other species had thinner leaves with less chlorophyll and thinner, longer stems when irradiated with only R light, whereas plants grown under only B light had reduced biomass accumulation (Eskins, 1992; Fukuda et al., 1993, 2008; Li et al., 2011; Saebo et al., 1995). For example, Arabidopsis thaliana grown at a PPF of 50 mmol·m·s had less biomass under B fluorescent light and up to one-third shorter petioles compared with plants grown under R fluorescent light (Eskins, 1992). In the ornamental geranium (Pelargonium zonale), leaves were 16% thicker, particularly the palisade layers, when irradiated with 100 mmol·m·s of B LED (peak = 460 nm) light compared with those of plants irradiated with the same intensity of R (peak = 660 nm) LED light (Fukuda et al., 2008). Plants grown under a mixture of wavelengths could possess more desirable, marketable traits (e.g., more compact seedlings for transplanting or larger leaves for harvesting) than those grown under a single light waveband. Multiple waveband research with LEDs has focused on R and B light, and specific R:B responses have varied among studies (Goins et al., 1997; Kato et al., 2011; Yang et al., 2011). An R (peak = 660 nm) to B (peak = 450 nm) ratio of 8:1 produced sweetpotato (Ipomoea batatas) plants with the greatest root-to-shoot ratio, whereas plants grown under the highest ratio of R light (10:1) had the least (Yang et al., 2011). Providing B light in an R-dominant environment can also increase seed yield, increase chlorophyll content, and promote flowering in some crops (Goins et al., 1997; Imaizumi et al., 2003; Li et al., 2011; Ohashi-Kaneko et al., 2006; Saebo et al., 1995; Tennessen et al., 1994). For example, wheat plants grown under R LEDs (peak = 660 nm) had approximately half of the number of tillers as plants grown under 90% R LED light and 10% B fluorescent light or white fluorescent light (Goins et al., 1997). Plant growth under G (500 to 600 nm) light alone can be poor: the net photosynthetic rate of cherry tomato (Lycopersicon esculentum var. cerasiforme) was approximately one-third as much when plants were grown under only G light (peak =520 nm; PPF of 320 mmol·m·s) than under only B (peak = 450 nm), only R (peak = 650 nm), or B + R light (Liu et al., 2011b). However, the addition of G light to R and B light can increase biomass accumulation in some crops. For example, lettuce (Lactuca sativa) was grown at 150 mmol·m·s under R and B LED light with and without G fluorescent lamps (Kim et al., 2004). Lettuce had lower rates of photosynthesis when the percentage of G light exceeded 50%, but lettuce had 89% more biomass when the light spectrum included up to 24% G light. Similarly, when lady slipper orchid was grown in tissue culture, plants under R, G, and B LEDs (ratio of 8:1:1; peak = 660, 525, and 450 nm, respectively) had 66% to 84% greater shoot dry weight than plants irradiated with only B, R, or a combination of R and B (9:1) at the same PPF (Lee et al., 2011). Therefore, in at least some crops, including wavelengths other than B and R, can elicit a desired growth response. We grew seedlings of four common annual crops under ratios of B, G, and R light to Received for publication 2 Mar. 2014. Accepted for publication 22 Apr. 2014. We gratefully acknowledge support from Osram Opto Semiconductors, the USDA-ARS Floriculture and Nursery Research Initiative, and Michigan State University’s AgBioResearch. We thank Bert Cregg for statistical advice, Rajendra Swamy for development of the LED modules, C. Raker & Sons for donation of plant material, and Randy Beaudry and Mike Olrich for technical assistance. Former graduate research assistant and currently commercial horticulture educator. Current address: Michigan State University Extension, 3299 Gull Road, P.O. Box 42, Nazareth, MI 49074. Associate Professor and Floriculture Extension Specialist. To whom reprint requests should be addressed. e-mail runkleer@msu.edu. 734 HORTSCIENCE VOL. 49(6) JUNE 2014 better understand how the light spectrum regulated plant growth attributes such as leaf size and shoot biomass. Specialty-crop species were chosen for this study based on economic value and to include variation in plant family, plant habit, and shade adaptation. Our objective was to quantify how seedlings grew and acclimated to light quality treatments to help facilitate the production of young specialty crops with desirable morphological characteristics. Materials and Methods Plant materials. Seeds of tomato (Solanum lycopersicum ‘Early Girl’), salvia (Salvia splendens ‘Vista Red’), impatiens (Impatiens walleriana ‘SuperElfin XP Red’), and petunia (Petunia ·hybrida ‘Wave Pink’) were sown in 128-cell (2.7 · 2.7 cm; 12.0-mL volume) plug trays at a commercial greenhouse (C. Raker and Sons, Inc., Litchfield, MI). On arrival at Michigan State University, East Lansing, MI, a seedling tray of each species was then cut into sections, each with 20 or more seedlings, thinned to one plant per cell, and immediately placed in the lighting treatments. Light treatments and environment. Six white rigid plastic LED light modules, custom-designed and constructed for experimentation, had four sides and were 80 cm deep, 27 cm wide, and 52 cm tall, as described by Wollaeger and Runkle (2013). The top of each module contained B (peak = 446 nm), G (peak = 516 nm), and two R LED types (peaks = 634 and 664 nm) that were facing downward inside the module. Eighty LEDs of each type were mounted on fan-cooled driver boards that were open to the environment to allow for adequate cooling. The light output of each color of LED could be adjusted manually by a dimmer switch. The LED modules were placed on open, metal mesh benches inside a refrigerated walk-in growth chamber. Six light-quality treatments that delivered a PPF of 160 mmol·m·s emitted the following percentages: B100 (100% light from B LEDs), B50 + G50, B50 + R50, B25 + G25 + R50, G50 + R50, and R100. All treatments that delivered R light were delivered equally by the two types of R LEDs. The intensity of each LED type was adjusted according to an average of six measurements from a spectroradiometer (PS-200; StellarNet, Inc., Tampa, FL) made at the seedling tray level at different horizontal positions inside each module. To provide a more uniform light intensity within each module, wire mesh was placed in the middle two-thirds of the chamber, just below the LEDs. In a separate growth chamber

Subfields: Plant Science

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