Photoperiod is an environmental signal that controls bud dormancy and break, tuber formation, and flowering. A photoperiodic response, such as flowering, is determined primarily by the duration of the dark period (skotoperiod). Based on photoperiodic flowering responses, many ornamental plants are classified as long- or short-day plants (Thomas and Vince-Prue, 1997). Long- or short-day plants flower most rapidly when the skotoperiod is within or exceeds a critical duration, respectively. Extending or truncating the photoperiod can control flowering time of these photoperiodic plants. Plants use photoreceptors such as phytochromes and cryptochromes to perceive radiation and mediate developmental processes and morphological traits such as flowering and stem elongation. The radiation signal generally saturates flowering responses at a low intensity of 1–2 μmol·m−2·s−1 (Thomas and Vince-Prue, 1997; Whitman et al., 1998).
When the natural skotoperiod is long, such as during winter in northern latitudes, greenhouse growers can use low-intensity electric lighting to shorten long nights and promote or inhibit flowering of long- or short-day plants, respectively. Accelerating flowering of long-day plants shortens production time, whereas delaying flowering of short-day plants is often accompanied by desirable vegetative growth. Electric lighting is typically delivered at the beginning of the night (i.e., as DE lighting) or in the middle of the night [i.e., night-interruption (NI) lighting]. The minimum duration to operate electric lighting varies among plant species and cultivars because of their different critical skotoperiods. Generally, DE lighting creating a 16-h day or 4-h NI lighting is effective for most ornamental crops (Runkle et al., 1998; Whitman et al., 1998).
The spectral distribution of photoperiodic lighting influences regulation of flowering in photoperiodic crops. Red [R (600–700 nm)] and far-red [FR (700–800 nm)] radiation can mediate activities of phytochrome B and phytochrome A. Red radiation converts phytochrome from an inactive form, PR, to an active form, PFR, whereas FR radiation can at least partially reverse R radiation effects by converting PFR back to PR (Thomas and Vince-Prue, 1997). A combination of R and FR radiation promotes flowering of a variety of long-day plants (Craig and Runkle, 2016; Thomas and Vince-Prue, 1997), whereas R radiation is the major waveband for inhibition of flowering in short-day plants (Craig and Runkle, 2013). Blue [B (400–500 nm)] radiation mediates both phytochrome and cryptochrome activities and can regulate flowering, but only when its intensity is sufficiently high (i.e., ≥30 μmol·m−2·s−1) (Meng and Runkle, 2017).
Incandescent, compact fluorescent, and high-intensity discharge (HID) lamps are light sources traditionally used for photoperiodic control of flowering in greenhouses. However, most incandescent lamps have been phased out of production because of their energy inefficiency and short life span. Compact fluorescent and HID lamps have spectral distributions deficient in FR radiation, which is crucial for rapid flowering of some long-day plants (Blanchard and Runkle, 2010; Runkle et al., 2012). The emergence of long-lasting, energy-efficient LEDs allows for a wide array of adjustable spectral distributions relevant to regulation of flowering. Lighting manufacturers have developed LED lamps emitting varying intensities of B, R, or FR radiation for flowering applications of photoperiodic ornamental crops.
The photosynthetic DLI is the accumulated photosynthetic photon flux density [PPFD (400–700 nm)] in 1 d. A DLI increase from 5 to 20 mol·m−2·d−1 can hasten flowering and increase flower number and dry mass of plants such as petunia (P. ×hybrida Vilm.-Andr.) and french marigold (Tagetes patula L.) (Faust et al., 2005; Moccaldi and Runkle, 2007). In addition, the DLI may influence the role of FR radiation in promotion of flowering in some long-day plants such as petunia. For example, the addition of FR radiation to R and W radiation in NI lighting promoted flowering of some ornamental species grown under a low DLI (≤6 mol·m−2·d−1), but not under a higher DLI (<12 mol·m−2·d−1) (Kohyama et al., 2014). However, this discovery was based on two experimental replications with different DLIs and therefore, warrants additional research on the interaction between the DLI and FR radiation. Our objective was to quantify how DE photoperiodic lighting from two commercially available low-intensity LED lamps emitting R + W or R + W + FR radiation would interact with the DLI to influence stem elongation and flowering of several long-day, short-day, and day-neutral ornamental species.
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