Height control is crucial for easter lily (Lilium longiflorum) production to attain acceptable aesthetics and to enable shipping to retailers in standard-sized boxes. To manage plant height, easter lily growers have commonly relied on plant growth regulators (PGRs) (Francescangeli et al., 2007; Jiao et al., 1986; Wulster et al., 1987) and temperature control (Blom and Kerec, 2003; Erwin and Heins, 1995; Erwin et al., 1989; Wilfret, 1987). Nonchemical methods of controlling height are increasingly becoming desirable alternatives for height control as PGRs face increasing environmental concerns and tighter regulations (Bergstrand, 2017).
Various environmental manipulation strategies to control easter lily have been demonstrated, although they each have challenges that may be barriers to commercial adoption. For example, doubling the intensity (without changing photoperiod) resulted in height reductions of 17% (13.4 cm) in one study (Kohl and Nelson, 1963). Similarly, halving the intensity from emergence to flowering increased total height by ≈40% (Heins et al., 1982a). Higher light intensities can be achieved by maintaining high light transmission through greenhouse coverings, reducing light-blocking overhead structures, or through supplemental lighting, but the energy and infrastructure costs of the latter are trade-offs. Another method to reduce height of many plant species, including easter lily, is to provide a negative day vs. night temperature differential (DIF) (i.e., by providing warmer temperatures during the night compared with the day). For example, raising night temperature from 14 to 30 °C after flower initiation (determined through meristem examination) reduced the height of easter lily from 43.8 to 31.3 cm (29%) when day temperature was held at 14 °C (Erwin et al., 1989), although the additional energy required for nighttime heating must be considered for this height control strategy. Reducing the temperature of irrigation solution, when applied to the shoot apex, was found to reduce easter lily height linearly by 1.75 cm·°C−1 between 18 and 2 °C (Blom et al., 2004). However, focused applications of cooled water to the shoot apex may be challenging to scale-up to commercial production, and irrigation temperature had no effect when applied directly to the substrate.
Light-emitting diodes (LEDs) are a promising tool for manipulating plant morphology due to the customizable nature of LED spectra, as targeted light spectral modifications can have substantial photomorphological effects. For example, the red [R (600–700 nm)] to far red [FR (700–800 nm)] photon flux ratio (R:FR) can dramatically affect plant morphology in sole-source lighting scenarios (Carvalho and Folta, 2014; Demotes-Mainard et al., 2016; Hernández et al., 2016; Holmes and Smith, 1977a; Kong et al., 2018; Mah et al., 2018). The challenge remains to design suitable and cost-effective growth control treatments using LEDs in greenhouse production environments, where targeted supplemental light treatments must overcome the background intensity and quality characteristics of natural light to elicit photomorphogenic effects.
One notable feature of the solar spectrum on Earth is that the R:FR tends to decrease at the ends of the natural photoperiod (Hughes et al., 1984). This leads to the question of whether this short-duration decrease in R:FR at the end of day (EOD) reduces plant compactness, as many species exhibit “shade avoidance” responses when exposed to low R:FR (Demotes-Mainard et al., 2016; Smith and Whitelam, 1997). In easter lily, a typical response to additional FR includes increased stem elongation (Blom and Kerec, 2003; Blom et al., 1995).
During the daytime (i.e., solar elevation > 10°–15°), the R:FR of natural sunlight is ≈1.15, but during twilight (solar elevation < 10°–15° until darkness), the R:FR gradually drops ≈35% (Holmes and Smith, 1977b). The dynamics of light intensity and quality at twilight are impacted by weather, time of year, latitude, and light pollution (Kishida, 1986; Spitschan et al., 2016). Although few studies directly investigate whether the changes in light quality during twilight actually influence stem elongation, Lund et al. (2007) found that height of chrysanthemum (Chrysanthemum ×morifolium) increased as R:FR was reduced from 2.4 to 0.7 or 0.4 to mimic twilight (using LEDs), under very low photosynthetic photon flux density [PPFD (400–700 nm)] by maintaining either R at 1 µmol·m−2·s−1 or FR at 1 µmol·m−2·s−1 for 30 min at the end of photoperiod in growth chambers. A previous growth chamber study showed that four bedding plant species grown under a light spectrum with R:FR of 0.7 vs. 1.1 had longer stems (Mah et al., 2018), although it is uncertain whether this result would still be observed if R:FR was altered during twilight only.
Instead of competing with the native intensity and spectrum characteristics of sunlight in a greenhouse environment, targeted spectral treatments from artificial sources (such as LEDs) can be applied near the EOD; i.e., at the tail end of the photoperiod just before darkness, to manipulate crop morphology in greenhouse environments. During the twilight periods, artificial lighting can easily modify the spectrum received by plants, as PPFD of natural light is low during that time. Based on solar measurements in Guelph, ON across several months, PPFD of natural light dropped below 10 µmol·m−2·s−1 after sunset, and diminished to less than 2 µmol·m−2·s−1 when apparent solar elevation was just 2° below the horizon (Mah, 2019). Altering the light spectrum at EOD using FR-absorbing filters has been shown to reduce height of vegetable seedlings, although increasing the R:FR at the EOD was less effective at reducing height than the same R:FR applied over the entire photoperiod (Cerny et al., 2004). Nonetheless, EOD treatments using LEDs still warrant investigation, as they may require less infrastructure or energy than all-day treatments, and do not have the drawback of simultaneously reducing PPFD associated with FR-absorbing filters.
Adding narrow-band FR at EOD has been shown to have a strong promoting effect on internode elongation in numerous species (Casal and Smith, 1989; Chia and Kubota, 2010; Ilias and Rajapakse, 2005; Vince-Prue, 1977; Zahedi and Sarikhani, 2016), including easter lily (Blom et al., 1995). However, these experiments tended to compare EOD FR to EOD R treatments, while far fewer experiments have compared EOD R lighting to control treatments that are more relevant to greenhouse environments. In one experiment, adding 15 min of R (2.1 W·m−2) at EOD in a greenhouse using filtered fluorescent light reduced the height of petunia (Petunia ×hybrida) from 12.8 to 10.2 cm (20%) compared with the ambient control (Ilias and Rajapakse, 2005).
Blackout strategies (BO) have been in use for over a decade in easter lily production to suppress stem elongation (Blom et al., 2004; Miller, 1992). By shortening the natural daylength to 8 h, BO reduced height of ‘Nellie White’ easter lily by 15% (44.8 vs. 38.0 cm) (Blom et al., 1995), up to 26% (52.4 vs. 39.0 cm) (Blom and Kerec, 2003), or even 45% (36.8 vs. 20.4 cm) (Heins et al., 1982b). It has been suggested that BO curtains control stem elongation by eliminating the dynamic changes in R:FR during twilight, therefore preventing any stem-lengthening associated with the natural decrease in R:FR at EOD (Blom et al., 1995), and also by shortening the photoperiod (Kohl and Nelson, 1963). However, most studies that investigated photoperiod for easter lily height control used incandescent lights, which have a R:FR of ≈0.6 (Craig and Runkle, 2016), to extend the photoperiod of the treatments (Roh and Wilkins, 1977a, 1977b; Smith and Langhans, 1962), or compared BO with natural days without controlling for light quality changes at twilight (Blom and Kerec, 2003; Blom et al., 1995; Heins et al., 1982b). In these cases, photoperiod and spectral factors were confounded. The spectrum of photoperiod extension lighting is a known factor for manipulating easter lily growth, with 28% greater height when photoperiod extension lighting was provided by incandescent lamps vs. high-pressure sodium lamps (Heins et al., 1982a). Nonetheless, at least one study (in growth chambers) demonstrated that a short photoperiod (8 h) dramatically reduced easter lily height by 29% (24.8 cm) compared with a 16-h photoperiod with the same PPFD and light quality (Kohl and Nelson, 1963). Shorter photoperiod with the same PPFD also resulted in lower dry weight, attributed to the reduced daily light integral (DLI), while leaf number was unaffected. Combining a short photoperiod (8 h) with high PPFD resulted in shorter plants than either high PPFD or short photoperiod alone (Kohl and Nelson, 1963).
The objective of this study was to evaluate the efficacy of EOD BO and low fluence rates of EOD R on height and flowering of greenhouse-grown easter lily. The hypothesis was that both EOD R and BO would reduce height compared with a control, given that both strategies should prevent the plants from experiencing the natural drop in R:FR at the EOD. Unlike most BO studies that shorten the photoperiod to 8 h, this study proposed to close the BO curtains shortly before sunset to minimize reductions in natural DLI while still cutting off the natural reductions in R:FR during the twilight period.
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