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Kim D. Bowman and Ute Albrecht

rootstock types under modern greenhouse conditions and in response to the use of modern supplemental HPS or LED lights. Supplemental light has been studied in many plant systems and has been shown to typically have large effects to improve or change plant

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Ricardo Hernández and Chieri Kubota

is HPS. HPS are well accepted as a result of their relatively high fixture PPF efficiency. For example, single-ended and double-ended magnetic and electronic HPS PPF efficiencies range between 0.93 to 1.85 μmol·J −1 ( Nelson and Bugbee, 2013

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Wesley C. Randall and Roberto G. Lopez

(HID) lights. High-pressure sodium (HPS) lamps are the most commonly used HID light sources, and several characteristics contribute to their use. However, HPS lamps primarily emit light in the spectral range of 565 to 700 nm, which is primarily yellow

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Joshua K. Craver, Jennifer K. Boldt, and Roberto G. Lopez

-quality young plants can be grown during times of the year when a lack of solar radiation may limit uniform and consistent production ( Hernández and Kubota, 2012 ). Numerous studies have reported that increasing the DLI with SL from HPS lamps improves young

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Will Healy and David Graper

Petunia `Red Flash' seedling were grown under HPS (175 μmolm-2 s-1) photoperiod treatments of 10, 12, 14 or 16 hr at 20C soil temperature in a shaded glasshouse where the maximum peak PPF was reduced to 150 μmolm-2s-1. Seedling dry weight and individual leaf area were determined daily. The photosynthetic rate was determined when seedlings reached the second true leaf stage.

The dry weight response to increasing photoperiod durations was cubic with a peak at 14 hr. Seedling dry weight increased slowly during days 5 through 10 then increased rapidly during the next 7 to 10 days. This increase coincided with the unfolding of leaves one through four. The total leaf area showed a cubic response to the photoperiod treatments. The leaf area increased slowly then began an exponential increase after day 10. The photosynthetic rate per gram dry weight was increased by the 10 hr photoperiod treatment when compared to the 16 hr treatment. The increased photosynthetic rate was not observed when the data was calculated on a fresh weight or leaf area basis.

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Michael P. Dzakovich, Celina Gómez, and Cary A. Mitchell

active radiation ( PAR ) received by a plant, during winter months ( Dorais et al., 1991 ; Korczynski et al., 2002 ). Greenhouse supplemental lighting is typically provided from overhead HPS (OH-HPS) lamps, emitting an orange-biased spectrum and

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W. Garrett Owen and Roberto G. Lopez

during propagation. Previous research has investigated the effects of DLI and SL from high-pressure sodium (HPS) lamps and LEDs during AR development and subsequent rhizogenesis of numerous genera of vegetatively propagated annual bedding plants ( Currey

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Vincent Martineau, Mark Lefsrud, Most Tahera Naznin, and Dean A. Kopsell

to install HPS lamps and use them to extend the photoperiod of the crops to increase yields ( McAvoy, 1984 ). However, this practice can be onerous for large installations, both in equipment and energy costs. Some other disadvantages of HPS lamps

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Cary A. Mitchell, Tina Leakakos, and Tameria L. Ford

This study evaluated the potential of high photosynthetic photon flux (PPF) from high-pressure sodium (HPS) lamps, alone or in combination with metal halide (MH) plus quartz iodide (QI) incandescent lamps, to support lettuce growth, with or without N supplementation. Varying exposures to radiation from combined HPS, MH, and QI lamps influenced dry weight gain and photosynthetic pigment content of hydroponically grown `Black-Seeded Simpson' lettuce (Lactuca sativa L.) seedlings. Cumulative leaf dry weight declined with increasing exposure, up to 20 hours per day, to 660 μmol·m-2·s-1 of photosynthetically active radiation (PAR) from HPS lamps concomitant with constant 20 hours per day of 400 m mol·m-2·s-1 from MH + QI lamps. Leaves progressively yellowed with increasing exposure to radiation from the three-lamp combination, corresponding to lower specific chlorophyll content but not to specific carotenoid content. Lettuce grown under 20-hour photoperiods of 400, 473, or 668 μmolm·m-2·s-1 from HPS radiation alone had the highest leaf dry weight at a PPF of 473 μmol·m-2·s-1. Chlorophyll, but not carotenoid specific content, decreased with each incremental increase in PPF from HPS lamps. Doubling the level of N in nutrient solution and supplying it as a combination of NH4 + and NO3 - partially ameliorated adverse effects of high PPF on growth and pigment content relative to treatments using single-strength N as NO3 .

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D. Sean Moodv and Douglas A. Hopper

Four cut rose cultivars (`Royalty', `Samantha', `Sonia', and `Gabriella') were exposed to supplemental radiation for 2 years of production at the W.D. Holley Plant Environmental Research Center (PERC) at Colorado State Univ. and 1 year at Jordan's Greenhouse (cultivars Royalty and Kardinal). At PERC the house was divided into two treatments: l) natural light, and 2) supplemental radiation at ≈100 μmol·m–2·s–1 (750 fc) from 1000-W high-pressure sodium (HPS) lamps for 10 h each night. Jordan's had a third treatment of supplemental radiation at ≈50 μmol·m–2·s–1 (400 fc). Nutrient solution recirculation was tested with one bench in each of the treatments. Each rose was counted and measured for stem length and fresh weight. At PERC, all the cultivars showed no significant differences in the weekly number of flowers produced or the total flower fresh weight when grown under nonrecirculation vs. recirculation of nutrient solution. From 1993 results, grade A production for 60 `Royalty' plants increased from 455 flowers under natural conditions to 522 flowers under lighted conditions over 7 months, a 97-flower increase (21%) due to lighting.