When the natural photoperiod is short, lighting during the middle of the night (night interruption, NI) can promote flowering of long-day plants (LDPs) and inhibit flowering of short-day plants (SDPs). Unlike some conventional lamps, light-emitting diodes (LEDs) are energy efficient, durable, and controllable. We coordinated a trial with five commercial greenhouses to compare the efficacy of 4-hour NI lighting from red (R; 600 to 700 nm)+white (W)+far-red (FR; 700 to 800 nm) LEDs and conventional lamps to regulate flowering of eight photoperiodic ornamental crops. In most instances, the R+W+FR LEDs were as effective at controlling flowering as conventional lamps. Therefore, these LEDs specifically developed for flowering applications emit an effective spectrum and can replace less energy-efficient conventional lamps. In another experiment, we investigated the role of low-intensity (1 to 2 μmol·m−2·s−1) blue (B; 400 to 500 nm) light in regulating flowering of four LDPs and five SDPs. Low-intensity B light, alone and when added to R and FR light, did not influence flowering or plant morphology. In a third experiment, we determined whether B light at higher intensities (15 and 30 μmol·m−2·s−1) controlled flowering of five LDPs and one SDP. B light at 30 μmol·m−2·s−1 created long days in all crops as effectively as low-intensity R+W+FR light. However, the addition of B light to R+W+FR light did not further accelerate flowering. Therefore, the effectiveness of B light in NI lighting apparently depends on some threshold intensity and does not modify the response to R+W+FR light. The promotion of flowering from a higher irradiance of B light could be mediated by cryptochromes, phytochromes, or both.
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