Mixing Blue, Red, and Far–Red Light for Night-interruption Lighting of Short-day Plants

The spectral quality of photoperiodic lighting affects flowering of short-day plants (SDPs) and long-day plants differently. A moderate to high red (R, 600 to 700 nm) to far-red (FR, 700 to 800 nm) light ratio inhibits flowering of SDPs when delivered during the middle of the night (night interruption, NI) while low-intensity (≈ 2 μmol∙m⁻²∙s⁻¹) blue (B, 400 to 500 nm) or FR light alone does not. However, the effects of low-intensity B light with R and/or FR light have been inconclusive. We investigated the effects of mixed B, R, and FR light on flowering of four SDPs: chrysanthemum (Dendranthema × grandiflorum) ‘Golden Cheryl’, dahlia (Dahlia pinnata) ‘Leanne’ and ‘Gallery Pablo’, and marigold (Tagetes erecta) ‘American Antigua Yellow’. Plants were grown in a greenhouse at a constant 20 °C, receiving a truncated 9-hour short day with or without 4 hours of NI lighting from incandescent (INC) lamps or white (peak = 606 nm), B (peak = 465 nm), B+R (peak = 659 nm), B+FR (peak = 737 nm), B+R+FR, or R+FR light-emitting diodes (LEDs). Each lighting treatment delivered a photon flux density of 2 μmol∙m⁻²∙s⁻¹ between 400 and 800 nm; and LED treatments with multiple colors delivered equal intensities. For all SDPs, white LEDs were most effective at inhibiting flowering while B light alone was not perceived as a long day. The B+R NI was as effective as the white LED NI at creating a long day for all cultivars except chrysanthemum. The B+FR NI did not interrupt the night for chrysanthemum or dahlia ‘Gallery Pablo’ whereas it did inhibit flowering of marigold and dahlia ‘Leanne’ compared with the SD. For marigold, the B+FR NI was less effective than other lighting treatments with R light. Flowering of all cultivars except dahlia ‘Gallery Pablo’ was delayed similarly under the B+R+FR and R+FR NI. All cultivars were tallest under the B+R+FR, R+FR, and INC NI, where lamps emitted both R and FR light. We conclude that for at least the SDPs studied, when added to R and FR light, low-intensity B light does not influence flowering. In addition, white LEDs emitting little FR light are effective at creating long days for these crops.