Low-intensity blue light in night-interruption lighting does not influence flowering of herbaceous ornamentals

Abstract

The spectral quality of photoperiodic lighting can affect flowering of short-day plants (SDPs) and long-day plants (LDPs) differently. When delivered during the middle of the night (night interruption, NI), red (R; 600–700nm) light alone can inhibit flowering of SDPs, whereas a combination of R and far-red (FR; 700–800nm) light promotes flowering of some LDPs. However, whether or not low-intensity (≈1–2μmolm−2s−1) blue (B; 400–500nm) light, when added to R and/or FR light, influences flowering has not been established. We investigated the effects of mixed B, R, and FR light on flowering of five SDPs [chrysanthemum (Chrysanthemum×morifolium), cosmos (Cosmos sulfureus), two cultivars of dahlia (Dahlia pinnata), and marigold (Tagetes erecta)] and two LDPs [dianthus (Dianthus chinensis) and rudbeckia (Rudbeckia hirta)]. Plants were grown in a greenhouse at a constant set point of 20°C and received a truncated 9-h short day (SD) with or without 4-h NI lighting from incandescent (INC) lamps or white (W), B, B+R, B+FR, B+R+FR, or R+FR light-emitting diodes (LEDs). Each NI lighting treatment delivered a mean photon flux of 1.5μmolm−2s−1 between 400 and 800nm at plant height. Blue light alone was not perceived as a long day by all SDPs and LDPs tested. For all SDPs, W LEDs inhibited flowering most effectively. B+R NI was as effective as W NI at creating a long day for all SDPs except chrysanthemum. B+FR NI inhibited flowering of marigold and dahlia ‘Leanne’, but not chrysanthemum or dahlia ‘Gallery Pablo’. For marigold, B+FR NI was less effective than other NI lighting treatments with R light. B+R+FR and R+FR NI similarly delayed flowering of all SDPs except dahlia ‘Gallery Pablo’. NI lighting treatments containing R light similarly promoted flowering of rudbeckia. We conclude that in at least the crops studied, low-intensity B light during the night does not influence flowering. In addition, W LEDs that emit little FR light are effective at creating long days for SDPs but only some LDPs.