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Light is perceived through different classes of photoreceptors in higher plants, including five red (R; 600 to 700 nm) and far-red (FR; 700 to 800 nm) light-absorbing phytochromes, ultraviolet-A/blue (B; 400 to 500 nm) light-absorbing cryptochromes, and B light-absorbing phototropins, as identified in the LDP Arabidopsis thaliana (Casal, 2000). These photoreceptors interact to mediate flowering and photomorphogenesis (Cashmore et al., 1999). There are similarities as well as differences in how SDPs and LDPs respond to the spectral distribution of photoperiodic lighting. R light is the most effective waveband at inhibiting flowering of SDPs; a moderate to high R to FR light ratio (R:FR; ≥0.66) was generally effective during an NI (Craig and Runkle, 2013). In contrast, a combination of R and FR light is more effective at promoting flowering of LDPs than either R or FR light alone (Thomas and Vince-Prue, 1997); the optimum R:FR was between 0.66 and 1.07 (Craig and Runkle, 2012). Phytochromes are primarily R and FR light-absorbing photoreceptors that regulate flowering by establishing their active and inactive forms primarily in response to R and FR light (Sager et al., 1988). The most effective 10-nm wavebands of R and FR light are 655 to 665 nm and 725 to 735 nm, respectively, which correspond to the absorption peak wavelengths of extracted oat phytochromes (Butler et al., 1965; Sager et al., 1988). In addition, both active and inactive forms of phytochromes also absorb B light approximately equally, but to a lesser extent than R and FR light, although absorption peak wavelengths shift slightly from each other (Butler et al., 1965; Sager et al., 1988). Specific functions of B light in flowering-time regulation are not as well understood. Previous studies showed that B light (peak wavelength = 455 nm) at 3 μmol∙m−2∙s−1 did not “create” (i.e., mimic) LDs when delivered as 4-h NI lighting for a range of photoperiodic ornamental plants (Craig, 2012; Ho et al., 2012). Similarly, chrysanthemum (Dendranthema × grandiflorum) ‘Zembla’ flowered similarly under an 11-h photoperiod with or without 4-h DE 96PDF Image | LED REGULATE FLOWERING OF PHOTOPERIODIC ORNAMENTAL CROPS
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