Blue radiation signals and saturates photoperiodic flowering of several long-day plants at crop-specific photon flux densities

Highlights

• Flowering of long-day plants is promoted by ≥15 μmol m⁻² s⁻¹ blue radiation.

• Day extension or night-interruption lighting providing blue radiation photon flux densities are effective.

• blue radiation photon flux density between 0 and 30 μmol m  Negative dose-response relationships between flowering time and blue radiation was between 0 and 30 μmol m⁻² s⁻¹.

• The minimum blue radiation photon flux density to saturate the flowering response of some long day plants was as low as 5 μmol m⁻² s⁻¹.

• The threshold blue radiation photon flux density to saturate flowering was 15 μmol m⁻² s⁻¹.

Abstract

When the natural photoperiod is short, electric lighting can be used to promote flowering of a wide range of ornamental long-day plants (LDP) grown inside greenhouses. A combination of red (R; 600–700 nm) and far-red (FR; 700–800 nm) radiation is effective when delivered at a low intensity (1–2 μmol m⁻² s⁻¹), but recent research shows blue (B; 400–500 nm) radiation can also be effective. We performed an experiment, replicated in time, that identified the B photon flux density that controlled flowering of four LDPs when delivered as a 4-h night interruption (NI) or 7-hour day extension (DE) during an otherwise 15-h night. Seedlings of four annual bedding plants were initially grown under a 9-h day, and then were transferred to one of seven lighting treatments, where subscripts indicate their photon flux densities: R+FR_2–3 NI; B₅, B₁₅, or B₃₀ NI; and B₅, B₁₅, or B₃₀ DE. At a sufficiently high photon flux density, B radiation delivered as a 4-h NI or 7-h DE promoted flowering of all four LDPs. All species exhibited a dose-response relationship between B photon flux density and flowering time. The threshold B photon flux density above which flowering was promoted varied among the four LDPs, and was 5 μmol m⁻² s⁻¹ for coreopsis (Coreopsis grandiflora) and snapdragon (Antirrhinum majus) and 15 μmol m⁻² s⁻¹ for petunia (Petunia × hybrida) and rudbeckia (Rudbeckia hirta). A B photon flux density of 15 or 30 μmol m⁻² s⁻¹ delivered as an NI or DE was usually as effective as 2–3 μmol m⁻² s⁻¹ of R + FR radiation for all LDPs tested. We conclude that while flowering of LDPs is more sensitive to R + FR than B radiation, relatively low B photon flux densities are perceived as a long day when delivered as a DE or NI.

Introduction

Photoperiodic flowering is dependent on the light environment, which includes the photoperiod (duration), spectral distribution (quality), and photon flux density (intensity). Long-day plants (LDPs) flower earlier when the night period is shorter than some species-specific critical duration. Under natural short days (SD), prolonged electric lighting can promote flowering of LDPs in greenhouse crop production. One method is night-interruption (NI) lighting, which breaks a long night into two short dark periods. Another strategy is day-extension (DE) lighting, which extends a natural SD to a long day (LD). When provided for sufficient durations, an NI and a DE are both effective in flowering applications (Thomas and Vince-Prue, 1997). For example, under a 9-h SD, a 4-h NI was as effective as a 6-h DE at promoting flowering of LDPs campanula (Campanula carpatica) ‘Pearl Deep Blue’, coreopsis (Coreopsis grandiflora) ‘Early Sunrise’, cyclamen (Cyclamen persicum) ‘Metis Red’, and rudbeckia (Rudbeckia hirta) ‘Becky Cinnamon Bicolor’ (Oh et al., 2013; Runkle et al., 2012). However, a 6-h DE was more effective than a 4-h NI for petunia (Petunia × hybrida) ‘Classic Wave Purple’ under incandescent lamps, but not fluorescent lamps (Runkle et al., 2012). This suggests that comparative effectiveness of NI and DE lighting depends on crop species and light quality.

Plants use photoreceptors, such as phytochromes (A–E) and cryptochromes (1 and 2), to sense the light environment and regulate a series of signaling cascade events related to flowering. Phytochrome A and phytochrome B have antagonistic functions in flowering. Phytochrome B is activated by red (R; 600–700 nm) radiation to inhibit floral initiation, whereas phytochrome A is activated by far-red (FR; 700–800 nm) radiation to promote flowering (Lin, 2000). Depending on incident light quality, phytochromes partially convert between two forms: an R-absorbing inactive form (P_R) and an FR-absorbing active form (P_FR). The ratio of P_FR in the total phytochrome pool is the phytochrome photoequilibrium (PPE), which ranges from 0 to 0.9 (Sager et al., 1986). The PPE in plants can be estimated based on the spectral distribution of incident radiation and phytochrome absorption spectra. NI or DE lighting that contains a combination of R and FR radiation (creating an intermediate PPE of 0.6–0.7) is generally most effective at accelerating flowering of LDPs, whereas R radiation (creating a high PPE of 0.8–0.9) inhibits flowering of short-day plants (Craig and Runkle, 2013, 2016). Photoperiodic R + FR radiation reaches maximal effectiveness at very low photon flux densities such as 1–2 μmol m⁻² s⁻¹ (Whitman et al., 1998). Therefore, incandescent lamps and R + FR light-emitting diodes that create an estimated PPE of 0.6–0.7 are effective at controlling flowering of LDPs (Kohyama et al., 2014).

Cryptochromes are the major photoreceptors mediating photoperiodic flowering under blue (B; 400–500 nm) radiation (Lin, 2002). They can act antagonistically with phytochromes to regulate flowering of the LDP arabidopsis (Arabidopsis thaliana). The cryptochrome 2 mutant of arabidopsis flowers later than the wild type under white or B + R radiation, but flowers at the same time as the wild type under continuous B or R radiation (Guo et al., 1998; Mockler et al., 1999; Lin, 2002). The dependence of cryptochrome 2 on B + R radiation in photoperiodic flowering underscores an interplay between cryptochromes and phytochromes (Guo et al., 1998; Lin, 2002; Mockler et al., 1999). Under R radiation, phytochrome B mediates inhibition of flowering with partially overlapping functions of phytochromes D and E. With additional B radiation, cryptochrome 2 suppresses this phytochrome-mediated regulatory pathway (Mockler et al., 2003). On the other hand, cryptochrome 1, cryptochrome 2, and phytochrome A promote flowering of arabidopsis redundantly under continuous B radiation (Mockler et al., 2003). Moreover, cryptochrome 2 (activated by B radiation) and phytochrome A (activated by FR radiation) can act in parallel to promote flowering (Lin, 2002; Mas et al., 2000). Therefore, under natural photoperiods, regulation of flowering by B radiation depends not only on functions of cryptochromes, but on their antagonistic and redundant interactions with phytochromes (Lin, 2000).

At least for NI lighting, the effectiveness of B radiation at regulating flowering depends on its photon flux density (Meng and Runkle, 2017). When delivered as a 4-h NI during the middle of a 15-h skotoperiod, B radiation regulated flowering of various photoperiodic ornamental crops at 30 μmol m⁻² s⁻¹, but not at 2 μmol m⁻² s⁻¹ (Meng and Runkle, 2015, 2017). However, intermediate B photon flux densities between 2 and 30 μmol m⁻² s⁻¹ may elicit or saturate flowering responses. The objectives of the present study were to establish threshold B photon flux densities for photoperiodic control of LDPs and to compare the effectiveness of B radiation when delivered as DE or NI lighting.