Foliage annuals are an increasingly important part of the U.S. floriculture industries’ $1.86 billion bedding and garden plant sector (USDA, 2016). Unlike most bedding and garden plants, they are primarily grown for the aesthetic appeal of their brightly colored, variegated, or patterned leaves rather than for their flowers (Walters et al., 2017). Production of foliage annuals should theoretically be easier, as growers do not have to schedule flowering plants for specific market dates. However, when foliage annuals do initiate flowers, many species become unsightly and growth often stalls (Healy et al., 1980). As such, inhibiting flowering during stock plant production, finishing, and in the retail setting is an important aspect of producing foliage annuals (Lopez, 2007).
Flower induction is influenced by a variety of factors, including maturity, vernalization, daily light integral, light quality, and photoperiod (Owen et al., 2018; Yuan et al., 1998). Photoperiod is the hours of light during a 24-h period; however, it is the skotoperiod, or uninterrupted dark period, that regulates flowering (Thomas and Vince-Prue, 1997). Plants can be classified as either SD, day neutral, or long day (LD) by their photoperiodic responses. SD plants flower when the photoperiod is less than a certain duration, whereas LD plants flower when the photoperiod is longer than a certain duration. This duration is called the critical photoperiod (Thomas and Vince-Prue, 1997) and often varies among species and cultivars. Photoperiodic responses can be further categorized as obligate or facultative. Obligate plants require a certain photoperiod or they will not flower, whereas flowering of facultative plants is only hastened if plants are grown under a certain photoperiod.
Photoperiodic responses are regulated by phytochrome, a photoreceptor. When plants perceive R (600–700 nm) radiation, phytochrome converts to the biologically active form (PFR). In contrast, when FR (700–800 nm) radiation is perceived, phytochrome is converted to the inactive form [(PR) Sager et al., 1988]. In general, during the night, PFR gradually converts to PR. When the skotoperiod is long, the greater conversion to PR generally promotes flowering of SD plants, whereas shorter skotoperiods result in greater PFR, promoting flowering of LD plants (Craig and Runkle, 2016).
In LD plants, FR-deficient environments often delay flowering (Kim et al., 2000; van Haeringen et al., 1998), but providing photoperiodic lighting with only FR is not consistently effective at promoting flowering (Craig and Runkle, 2016; Nishidate et al., 2012). Therefore, a combination of R and FR radiation can be used for photoperiodic control of LD plants. Similarly, providing only FR radiation for NI lighting also is perceived as an SD in SD plants and is not effective at inhibiting flowering, whereas a combination of R and FR radiation is effective (Craig and Runkle, 2013). However, flowering of SD plants also can be inhibited with NI lighting providing only R radiation (Borthwick et al., 1952; Cathey and Borthwick, 1957; Downs, 1956).
Traditionally, incandescent lamps have been used for photoperiodic lighting to control crop development because they emitted both R and FR radiation, were inexpensive, and were commonly available (Craig and Runkle, 2013). However, incandescent lamps have been phased out of production because of their short life span and energy inefficiency (Waide, 2010). Currently, fluorescent and LED lamps are widely available. Fluorescent lamps are not as effective at promoting flowering of LD plants because of their low FR emittance (Whitman et al., 1998). The emergence of LEDs has provided a more energy-efficient, long-lasting lamp option that can vary in spectra, providing the opportunity to optimize the photoperiodic spectrum for various floriculture crops (Owen et al., 2018).
Although photoperiodic flowering and cutting yield and quality of some foliage annuals, including aluminum plant (Pilea cadierei and Pilea ‘Moon Valley’), baby rubber plant (Peperomia obtusifolia), coleus (Coleus ×hybridus), Joseph’s coat (Alternanthera amoena), English ivy (Hedera helix), and Persian shield (Strobilanthes dyerianus), have been investigated, researchers did not determine the critical photoperiod for these crops nor provide recommendations for non-stock plant producers (Gamrod, 2003; Healy et al., 1980). Therefore, the objectives of this study were 1) to quantify how photoperiod influences growth and development during production and stock plant cutting yield of five foliage annuals and 2) to determine how NI lighting with or without FR radiation influences their growth and development.
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