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PFR/PR+FR) decreased (Runkle and Heins, 2001). Therefore, the use of LEDs in photoperiodic lighting at low intensities is a feasible way to inhibit or promote extension growth, irrespective of flowering. For example, a 30-min DE provided by R LEDs at 5 μmol·m−2·s−1 suppressed stem elongation of poinsettia (Euphorbia pulcherrima) grown under a 10-h SD (Islam et al., 2012). In Arabidopsis, B light can induce the suppression of gibberellin biosynthesis genes and promote the expression of gibberellin inactivation genes, and the transcriptional regulation of these genes was mediated by both cryptochromes (cry1 and cry2) and phytochrome (phyA) (Zhao et al., 2007). In rice, B light caused a reduction in active gibberellin concentration, and the suppression of gibberellin biosynthesis and promotion of gibberellin inactivation was mediated by phytochromes (phyA, phyB, and phyC) and cryptochrome (cry1), respectively (Hirose et al., 2012). Therefore, although low-intensity B light may not regulate flowering of a wide range of plants in greenhouse production, it could potentially have an inhibitory effect on stem elongation. For example, internode elongation of chrysanthemum was suppressed by 60% under a 4-h NI provided by B LEDs at a PPF of 1.7 μmol·m−2·s−1 compared with FL lamps at a PPF of 150 μmol·m−2·s−1 (Shimizu et al., 2005); however, different light intensities under B LEDs and FL lamps in this study might have confounded the results. In high-intensity sole-source lighting, B light generally inhibits extension growth when added to R light. For example, B light and a combination of B and R light resulted in a shorter peduncle length of cyclamen than R light or FL light (Heo et al., 2003). In contrast, other studies suggest B light can promote stem elongation of some plants. For example, chrysanthemum grown under an 11-h SD provided by R and B LEDs for 42 d flowered similarly but were 18 cm taller under an 11-SD with a 4-h DE provided by B LEDs at a PPF of 100 μmol·m−2·s−1 (Jeong et al., 2014). Furthermore, as the duration of B light was prolonged, 15PDF Image | LED REGULATE FLOWERING OF PHOTOPERIODIC ORNAMENTAL CROPS
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