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are as effective as lamps traditionally used in greenhouses at controlling flowering of photoperiodic plants. Introduction Most plants can be classified into one of three categories according to their photoperiodic responses: long-day plants (LDPs), short-day (SD) plants (SDPs), and day-neutral plants (Thomas and Vince-Prue, 1997). Flowering of LDPs is promoted when the night length is shorter than a species- or cultivar-specific critical skotoperiod, whereas flowering of SDPs is inhibited or delayed when the uninterrupted dark period is shorter than a critical skotoperiod (Thomas and Vince-Prue, 1997). When the ambient photoperiod is short, low-intensity photoperiodic lighting can be used to control flowering of LDPs and SDPs. This can be achieved by delivering light beginning at the end of the day until the desired photoperiod is met (day extension) or during the middle of the night (night interruption, NI). Although the minimum duration of effective NI lighting can vary among species, 4 h of NI lighting is typically sufficient to regulate flowering of photoperiodic crops (Runkle and Heins, 2003; Runkle et al., 1998). Generally, NI lighting is most effective when delivered during the middle of the long night. For example, to promote flowering of dianthus (Dianthus chinensis) or inhibit flowering of ‘Dream Land’ zinnia (Zinnia elegans), 4-h NI lighting during a night lasting from 1700 HR to 0800 HR was most effective starting at 2200 HR, rather than 1800 HR or 0200 HR (Park et al., 2013). Conventional light sources, such as incandescent (INC), fluorescent (FL), and high- pressure sodium (HPS) lamps typically are used by commercial growers to deliver photoperiodic lighting. Incandescent lamps emit both red (R) and far-red (FR) light and are effective for a wide range of crops (Thomas and Vince-Prue, 1997). Because FL lamps emit little FR light, direct 31PDF Image | LED REGULATE FLOWERING OF PHOTOPERIODIC ORNAMENTAL CROPS
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