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Qingwu Meng and Erik S. Runkle

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.

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Mengzi Zhang and Erik S. Runkle

Manipulating light quality is a potential alternative method of regulating plant height in the commercial production of ornamental crops. In particular, end-of-day (EOD) lighting with a high red (R; 600–700 nm) to far-red (FR; 700–800 nm) ratio (R:FR) can suppress extension growth, whereas a low R:FR can promote it. We investigated the effects of the R:FR and duration of EOD lighting in regulating extension growth and flowering of two poinsettia cultivars, White Glitter and Marble Star. Plants were grown at 20 °C under 9-hour days with or without EOD lighting provided by two types of light-emitting diode bulbs: R+white+FR (subsequently referred to as R+FR) and FR only. The R:FR ratios were 0.73 and 0.04, respectively, and the photon flux density between 400 and 800 nm was adjusted to 2 to 3 μmol·m−2·s–1 at plant canopy. The six EOD lighting treatments were R+FR or FR for 2 or 4 hours, 2 hours of R+FR followed by 2 hours of FR, and 4 hours of R+FR followed by 2 hours of FR. We also investigated the impact of a 4-hour moderate-intensity (13 μmol·m−2·s–1) EOD FR treatment in the second replication. EOD lighting generally increased poinsettia extension growth, with the greatest promotion under the longest lighting periods. There were no differences in days to first bract color and days to anthesis when the 9-hour day was extended by 2 hours, but flowering was delayed under 4- or 6-hour EOD treatments, except for the 2-hour R+FR + 2-hour FR and 4-hour FR treatments. Four hours of moderate-intensity EOD FR greatly promoted extension growth and delayed or prevented bract coloration in both cultivars. We conclude that EOD lighting promotes extension growth of poinsettia, and specifically, EOD FR at a low intensity (2–3 μmol·m−2·s–1) is not perceived as long-day signal, whereas a higher intensity (13 μmol·m−2·s–1) of FR delays flowering.

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Daedre S. Craig and Erik S. Runkle

In protected cultivation of short-day (SD) plants, flowering can be inhibited by lighting from incandescent (INC) lamps during the night. INC lamps are being phased out of production and replaced by light-emitting diodes (LEDs), but an effective spectrum to control flowering has not been thoroughly examined. We quantified how the red [R (600 to 700 nm)] to far red [FR (700 to 800 nm)] ratio (R:FR) of photoperiodic lighting from LEDs influenced flowering and extension growth of SD plants. Chrysanthemum (Chrysanthemum ×morifolium), dahlia (Dahlia hortensis), and african marigold (Tagetes erecta) were grown at 20 °C under a 9-hour day with or without a 4-hour night interruption (NI) treatment by INC lamps or LEDs with seven different R:FR ranging from all R to all FR. Flowering in the most sensitive species, chrysanthemum, was not inhibited by an R:FR of 0.28 or lower, whereas an R:FR of 0.66 or above reduced flowering percentage. Flowering in dahlia was incomplete under the FR-only NI and under SDs, but time to flower was similar under the remaining NI treatments. The least sensitive species, african marigold, flowered under all treatments, but flowering was most rapid under the FR-only NI and under SDs. For all species, stem length increased quadratically as the R:FR of the NI increased, reaching a maximum at R:FR of ≈0.66. We conclude that in these SD plants, a moderate to high R:FR (0.66 or greater) is most effective at interrupting the long night, blue light is not needed to interrupt the night, and FR light alone does not regulate flowering.

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Brian R. Poel and Erik S. Runkle

Light-emitting diodes (LEDs) have the potential to replace high-pressure sodium (HPS) lamps as the main delivery method of supplemental lighting (SL) in greenhouses. However, few studies have compared growth under the different lamp types. We grew seedlings of geranium (Pelargonium ×hortorum), pepper (Capsicum annuum), petunia (Petunia ×hybrida), snapdragon (Antirrhinum majus), and tomato (Solanum lycopersicum) at 20 °C under six lighting treatments: five that delivered a photosynthetic photon flux density (PPFD) of 90 μmol·m−2·s−1 from HPS lamps (HPS90) or LEDs [four treatments composed of blue (B, 400–500 nm), red (R, 600–700 nm), or white LEDs] and one that delivered 10 μmol·m−2·s−1 from HPS lamps (HPS10), which served as a control with matching photoperiod. Lamps operated for 16 h·d−1 for 14 to 40 days, depending on cultivar and season. The LED treatments defined by their percentages of B, green (G, 500–600 nm), and R light were B10R90, B20R80, B10G5R85, and B15G5R80, whereas the HPS treatments emitted B6G61R33. Seedlings of each cultivar grown under the 90 μmol·m−2·s−1 SL treatments had similar dry shoot weights and all except pepper had a similar plant height, leaf area, and leaf number. After transplant to a common environment, geranium ‘Ringo Deep Scarlet’ and petunia ‘Single Dreams White’ grown under HPS90 flowered 3 days earlier than those grown under HPS10, but flowering time was not different from that in LED treatments. There were no consistent differences in morphology or subsequent flowering among seedlings grown under HPS90 and LED SL treatments. The inclusion of white light in the LED treatments played an insignificant role in growth and development when applied as SL with the background ambient light. The LED fixtures in this study consumed substantially less electricity than the HPS lamps while providing the same PPFD, and seedlings produced were of similar quality, making LEDs a suitable technology option for greenhouse SL delivery.

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Heidi Marie Wollaeger and Erik S. Runkle

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 ×hybrida), and tomato (Solanum lycopersicum) under six sole-source light-emitting diode (LED) treatments or one cool-white fluorescent treatment that each delivered a photosynthetic photon flux (PPF) of 160 µmol·m−2·s–1 for 18 h·d−1. 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.

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Roberto G. Lopez and Erik S. Runkle

The vegetatively propagated `Fire Kiss' clone of the hybrid Zygopetalum Redvale orchid has appealing potted-plant characteristics, including fragrant flowers that are waxy lime-green and dark maroon with a broad, three-lobed, magenta and white labellum. We performed experiments to quantify how temperature influenced leaf unfolding and expansion, time from visible inflorescence to flower, and longevity of individual flowers and inflorescences. Plants were grown in controlled-environment chambers with constant temperature set points of 14, 17, 20, 23, 26, and 29 °C and an irradiance of 150 μmol·m-2·s-1 for 9 h·d-1. As actual temperature increased from 14 to 25 °C, the time to produce one leaf decreased from 46 to 19 days. Individual plants were also transferred from a greenhouse to the chambers on the date that an inflorescence was first visible or the first flower of an inflorescence opened. Time from visible inflorescence to open flower decreased from 73 days at 14 °C to 30 days at 26 °C. As temperature increased from 14 to 29 °C, flower and inflorescence longevity decreased from 37 and 38 days to 13 and 15 days, respectively. Data were converted to rates, and thermal time models were developed to predict time to flower and senescence at different temperatures. The base temperature was estimated at 6.2 °C for leaf unfolding, 3.5 °C for time to flower, and 3.7 °C for flower longevity. These models could be used by greenhouse growers to more accurately schedule Zygopetalum flowering crops for particular market dates.

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Catherine M. Whitman and Erik S. Runkle

Flowering of the herbaceous perennial Aquilegia is generally considered to require vernalization after seedlings are mature, whereas photoperiod has little or no effect. We performed experiments to determine the flowering responses for two Aquilegia ×hybrida varieties, one of which reportedly has reduced cold requirements. Seedlings of Aquilegia ‘Origami Blue and White’ and ‘Winky Double Red and White’ with three to five leaves were either placed directly into a 5 °C cooler with low-intensity lighting for 9 hours/day or transplanted to 13-cm containers and grown (bulked) for 0, 3, or 6 weeks at 20 °C under 9-hour short days (SDs) or 16-h long days (LDs). Plants were then cooled at 5 °C for 0, 5, or 10 weeks and placed in a common forcing environment at 20 °C under LDs. Flowering response of the two cultivars differed markedly. All Aquilegia ‘Origami Blue and White’ plants placed directly into the forcing environment flowered and in a mean of 93 days. Flowering percentage of plants cooled in the plug tray decreased with increasing duration of cold treatment, and only 15% flowered after a 10-week cold treatment. All plants bulked for 3 or 6 weeks before cold treatment flowered after 25 to 36 days in the forcing environment. Adding bulking and forcing time together, time to flowering of ‘Origami Blue and White’ was complete and most rapid (62 days) when plants were bulked for 3 weeks under SDs and then forced under LDs. In contrast, no ‘Winky Double Red and White’ plants flowered without cold treatment, and 6 weeks of bulking followed by 10 weeks of cold was required for 100% flowering. These results indicate that ‘Origami Blue and White’ has a relatively short juvenile phase and flowering was promoted by SD bulking or cold treatment, whereas ‘Winky Double Red and White’ has a longer juvenile phase and requires cold for flowering.

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Fumiko Kohyama, Catherine Whitman and Erik S. Runkle

When the natural daylength is short, commercial growers of ornamental long-day plants (LDP) often provide low-intensity lighting to more rapidly and uniformly induce flowering. Incandescent (INC) lamps have been traditionally used for photoperiodic lighting because their spectrum, rich in red [R (600 to 700 nm)] and far-red [FR (700 to 800 nm)] light, is effective and they are inexpensive to purchase and install. Light-emitting diodes (LEDs) are much more energy efficient, can emit wavelengths of light that specifically regulate flowering, and last at least 20 times longer. We investigated the efficacy of two new commercial LED products developed for flowering applications on the LDP ageratum (Ageratum houstonianum), calibrachoa (Calibrachoa ×hybrida), two cultivars of dianthus (Dianthus chinensis), and two cultivars of petunia (Petunia ×hybrida). Plants were grown under a 9-hour short day without or with a 4-hour night interruption (NI) delivered by one of three lamp types: INC lamps (R:FR = 0.59), LED lamps with R and white (W) diodes [R + W (R:FR = 53.35)], and LED lamps with R, W, and FR diodes [R + W + FR (R:FR = 0.67)]. The experiment was performed twice, both at a constant 20 °C, but the photosynthetic daily light integral (DLI) during the second replicate (Rep. II) was twice that in the first (Rep. I). In all crops and in both experimental replicates, time to flower, flower or inflorescence and node number, and plant height were similar under the R + W + FR LEDs and the INC lamps. However, in Rep. I, both petunia cultivars developed more nodes and flowering was delayed under the R + W LEDs compared with the INC or R + W + FR LEDs. In Rep. II, petunia flowering time and node number were similar under the three NI treatments. Plant height of both dianthus cultivars was generally shorter under the NI treatment without FR light (R + W LEDs). These results indicate that when the DLI is low (e.g., ≤6 mol·m−2·d−1), FR light is required in NI lighting for the most rapid flowering of some but not all LDP; under a higher DLI, the flowering response to FR light in NI lighting is apparently diminished.

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Heidi Marie Wollaeger and Erik S. Runkle

Light-emitting diodes (LEDs) are of increasing interest in controlled environment plant production because of their increasing energy efficiency, long lifetime, and colors can be combined to elicit desirable plant responses. Red light (600–700 nm) is considered the most efficient wavelength for photosynthesis, but little research has compared growth responses under different wavelengths of red. We grew seedlings of impatiens (Impatiens walleriana), petunia (Petunia ×hybrida), tomato (Solanum lycopersicum), and marigold (Tagetes patula) or salvia (Salvia splendens) at 20 °C under six sole-source LED lighting treatments. In the first experiment, a photosynthetic photon flux (PPF) of 160 μmol·m−2·s–1 was provided for 18 h·d−1 by 10% blue (B; peak = 446 nm) and 10% green (G; peak = 516 nm) lights, with the remaining percentages consisting of orange (O; peak = 596 nm)–red (R; peak = 634 nm)–hyper red (HR; peak = 664 nm) of 20–30–30, 0–80–0, 0–60–20, 0–40–40, 0–20–60, and 0–0–80, respectively. There were no consistent effects of lighting treatment across species on any of the growth characteristics measured including leaf area, plant height, or shoot fresh weight. In a second experiment, seedlings were grown under two light intensities (low, 125 μmol·m−2·s–1 and high, 250 μmol·m−2·s–1) consisting of 10% B and 10% G light and the following percentages of R–HR: 0–80, 40–40, 80–0. Shoot fresh weight was similar in all light treatments, whereas shoot dry weight was often greater under the higher light intensity, especially under the 40–40 treatments. Leaf chlorophyll concentration under 40–40low, 80–0low, or both was often greater than that in plants under the high light treatments, indicating that plants acclimated to the lower light intensity to better use photons available for photosynthesis. We conclude that O, R, and HR light have generally similar effects on plant growth at the intensities tested when background G and B lights are provided and thus, selection of red LEDs for horticultural applications could be based on other factors such as economics and durability.

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Heidi M. Wollaeger and Erik S. Runkle

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 and morphology 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 µmol·m−2·s–1 for 18 h·d−1. The following treatments were provided with B (peak = 446 nm) and R (peaks = 634 and 664 nm) LEDs: B160 (160 µmol·m−2·s−1 of B light only), B80+R80, B40+R120, B20+R140, B10+R150, and R160. Seedlings of impatiens (Impatiens walleriana), salvia (Salvia splendens), petunia (Petunia ×hybrida), and tomato (Solanum lycopersicum) were grown for 31 to 37 days at a constant 20 °C. Plants with as little as 10 µmol·m−2·s−1 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 µmol·m−2·s−1. 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 µmol·m−2·s−1 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.