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expression of the CO gene oscillates in a circadian rhythm and must coincide with irradiance. Under inductive photoperiodic conditions, the CO protein induces the expression of a signal, FLOWERING LOCUS T (FT) protein, which is produced in leaves and serves as a long-distance stimulus to induce flowering at the apical meristem (Corbesier et al., 2007; Imaizumi and Kay, 2006). Depending on species, multiple phytochromes (phyA, phyB, phyC, phyD, and phyE) and cryptochromes (cry1 and cry2) can exist (Clack et al., 1994; Sharrock and Quail, 1989). In at least some species of the Brassicaceae, phytochromes and cryptochromes can interact and overlap in function (Cashmore et al., 1999). The R/FR reversibility refers to the paradigm that phytochrome-mediated responses, such as flowering and seed germination, can be at least partially reversed by converting phytochromes between their inactive R light-absorbing form, PR, and active FR light-absorbing form, PFR. Irradiance and the R to FR light ratio (R:FR) elicit formation of PR and PFR, the proportions of which determine an estimated phytochrome photoequilibrium (PFR/PR+FR) (Sager et al., 1988). The effectiveness of spectral wavebands in NI lighting to control flowering is somewhat different for SDPs and LDPs. In SDPs, R light is the most effective waveband to inhibit flowering (Thomas and Vince-Prue, 1997). The effect can be at least somewhat reversed by subsequent exposure to FR light, indicating involvement of phytochromes. A particular intensity threshold is required for specific wavelengths to interrupt the night effectively. For example, monochromatic light of 450, 550, 650, or 750 nm all inhibited flowering of the SDP duckweed (Lemna paucicostata), but the light intensity required for 50% flowering inhibition was 10, 0.5, 0.1, and 3 μmol∙m−2∙s−1, respectively (Saji et al., 1982). In some LDPs such as Arabidopsis, R light was effective at promoting flowering, but B and FR light were both more effective than R light at a similar intensity of 0.8 to 1.0 μmol∙m−2∙s−1 (Goto et al., 1991). However, B light at 3.3 63PDF Image | LED REGULATE FLOWERING OF PHOTOPERIODIC ORNAMENTAL CROPS
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