An intermediate phytochrome photoequilibria from night-interruption lighting optimally promotes flowering of several long-day plants

• Six long-day plants were grown under seven red to far-red ratios during the night.
• Far-red light during the night was usually not perceived as a long day.
• An intermediate phytochrome photoequilibrium induced the earliest flowering.
• In Petunia, early flowering was associated with the greatest stem extension.

Flowering of long-day (LD) plants is promoted by low-intensity (photoperiodic) lighting during an otherwise long night. Conventional lamps that emit a relatively low red (R; 600–700 nm) to far-red (FR; 700–800 nm) light ratio (e.g., incandescent lamps) create an intermediate phytochrome photoequilibria (PPE) and are sometimes more effective at promoting flowering of LD plants (LDP) than lamps that emit a higher R:FR (e.g., fluorescent lamps) and establish a higher PPE. Thus, we postulated that flowering of several LDPs would be increasingly promoted as the fraction of FR radiation increased relative to R light and the established PPE decreased. Rudbeckia (Rudbeckia hirta), snapdragon (Antirrhinum majus), Fuchsia (Fuchsia × hybrida), and three cultivars of Petunia (Petunia × hybrida) were grown at 20 °C under a truncated 9-h ambient photoperiod with or without 4-h NI lighting by incandescent lamps or light-emitting diodes that emitted seven different R:FR and created estimated PPE from 0.16 to 0.89. For all three Petunia cultivars and snapdragon, flowering was earliest under an NI with an intermediate PPE and delayed under short days (SDs) or an NI that elicited the highest or lowest PPE. For Rudbeckia and Fuchsia, all NI treatments promoted flowering except for the highest PPE NI, which was perceived as an SD. There were relatively subtle effects of the NI treatments on extension growth except in Petunia, in which all three cultivars showed a quadratic response to the PPE under the NI treatments, where plants were tallest at flowering under intermediate PPE. We conclude that an NI that establishes an intermediate PPE optimally promotes flowering of a variety of LDPs. These results are not consistent with the established paradigm for how light quality regulates flowering of LDPs, particularly in Arabidopsis, suggesting that the paradigm is not necessarily applicable to plants outside of the Brassicaceae.