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form a p-n semiconductor junction, where extra electrons from the atoms in the n-type material fall into the electron holes of the atoms in the p-type material to create photons. Various colors can be obtained in LEDs using different elements (e.g., AlInGaP and InGaN) to produce photons at different specific wavelengths. In contrast, conventional broad-spectrum light sources, such as INC, FL, and HPS lamps, are restricted in the controllability of spectral composition. Using LEDs allows the selection of the most efficacious spectral composition for desirable growth and development responses (Heo et al., 2002; Schubert and Kim, 2005). Moreover, some (but not all) LEDs as low-voltage devices are considerably more energy efficient than traditional light sources (Pimputkar et al., 2009). The photosynthetic photon efficiency of commercially available LEDs in 2014 range from 0.9 to 1.7 μmol∙J–1 (Nelson and Bugbee, 2014), which indicates that the best LEDs are as efficient as the best HPS lamps, while the worst LEDs are as efficient than the worst HPS lamps. The power of LEDs is mainly used to generate light and conductive heat, but not radiated heat. On the contrary, conventional light sources generate a significant amount of infrared radiation that is not effective for photosynthesis. An LED lamp has a significantly long useful life time, ranging between 20,000 and 55,000 h (Morrow, 2008; Tähkämö et al., 2012). In comparison, the longevity of a traditional INC bulb is 1,000 h in most cases, while that of a compact FL lamp is 8,000 to 10,000 h (Tähkämö et al., 2012). The effectiveness of LEDs in photoperiodic lighting can depend on their spectral composition. For example, FR LEDs were less effective at promoting flowering of the LDP cyclamen (Cyclamen persicum) than FL lamps, but LEDs emitting both B and R light were more effective than FL lamps (Shin et al., 2010). Appropriate combinations of LEDs can be equally or more effective, yet substantially more energy efficient, than traditional light sources. For example, LEDs with an R to FR light ratio (R:FR) of 0.66 (PFR/PR+FR = 0.63) or 1.07 (PFR/PR+FR 7PDF Image | LED REGULATE FLOWERING OF PHOTOPERIODIC ORNAMENTAL CROPS
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