The production of young plants from seed (plugs) for the annual bedding plant market commonly occurs during winter and early spring (Styer, 2003). However, in northern latitudes, the natural photosynthetic daily light integral (DLI) is not sufficient to produce high-quality young plants in the greenhouse (Lopez and Runkle, 2008; Pramuk and Runkle, 2005). Previous research has shown that a minimum DLI of 10 to 12 mol·m−2·d−1 is recommended to produce high-quality ornamental young plants (Pramuk and Runkle, 2005; Randall and Lopez, 2014). Thus, to efficiently produce seedlings in northern latitudes, where the DLI in the greenhouse can be as low as 1 to 5 mol·m−2·d−1 during winter and early spring, supplemental lighting is recommended (Pramuk and Runkle, 2005). Currently, high-pressure sodium (HPS) lamps are the industry standard for providing supplemental lighting, with a photosynthetic photon flux density (PPFD) of 70 to 90 µmol·m−2·s–1 commonly targeted (Lopez et al., 2017). One alternative to traditional greenhouse production is multilayer or vertical production indoors in repurposed shipping containers, warehouses, or chambers under sole-source lighting (SSL) provided by light-emitting diodes (LEDs). While SSL applications may not be suitable for all crops, young plant production is one area that may benefit substantially from this technology as growers strive to produce a uniform, high-quality crop during months of the year where greenhouse environmental conditions are both unpredictable and unfavorable. Additionally, young plant production may provide one of the most cost-effective applications for SSL due to the small size and high value of plugs and relatively short production cycle (Park and Runkle, 2017).
Previous research has shown that LED SSL in controlled environments is a promising lighting method for the production of annual bedding plant seedlings (Randall and Lopez, 2015; Wollaeger and Runkle, 2014). Specifically, Randall and Lopez (2015) evaluated seedlings of vinca (Catharanthus roseus ‘Titan Red Dark’), impatiens (Impatiens walleriana ‘Super Elfin XP Blue Pearl’), geranium (Pelargonium ×hortorum ‘Bullseye Red’), petunia (Petunia ×hybrida ‘Dreams Midnight’), and French marigold (Tagetes patula ‘Durango Yellow’) under SSL using LEDs providing a red:blue radiation ratio (%) of either 87:13 or 70:30. Generally, they found that seedlings produced under controlled-environment SSL were more compact, darker in foliage color (higher relative chlorophyll content), and had a higher root mass than those produced under greenhouse supplemental lighting or ambient lighting conditions.
Increases in radiation intensity and DLI have been reported to increase seedling quality and decrease subsequent time to flower (TTF) for many bedding plant species (Oh et al., 2010; Pramuk and Runkle, 2005). Seedlings produced under lower DLIs generally show decreased growth rates and possess more water in the plant tissues, ultimately leading to seedlings that growers would refer to as being less “toned” (Faust et al., 2005). For example, Pramuk and Runkle (2005) found that as the DLI increased from 4.1 to 14.2 mol·m−2·d−1 during seedling production, the average shoot dry mass (SDM) per internode increased linearly for celosia (Celosia argentea var. plumosa ‘Gloria Mix’), impatiens ‘Accent Red’, French marigold ‘Bonanza Yellow’, and pansy (Viola ×wittrockiana ‘Crystal Bowl Yellow’).
One of the benefits LEDs provide is the ability to select specific wavelengths of radiation to elicit desired morphological or physiological plant responses. Red wavelengths are most commonly associated with their role in photosynthesis and dry mass accumulation based on their action at the absorption peaks of chlorophylls (Massa et al., 2008). Compared with red radiation, blue wavelengths are believed to be less photosynthetically efficient due to their absorption by pigments other than chlorophyll and resulting low quantum efficiency (Barnes et al., 1993; Cope et al., 2014; Franklin, 2008; Massa et al., 2008). Another reason for the loss in photosynthetic activity may be due to smaller leaf area, which has been observed under high percentages of blue radiation and ultimately leads to decreased radiation interception (Cope et al., 2014). However, this inhibition response to blue wavelengths may be desirable for many crops as a means of controlling excessive growth (Cope et al., 2014; Cosgrove, 1981; Kigel and Cosgrove, 1991; Runkle and Heins, 2001). Thus, LEDs can be manufactured with a variety of plant responses in mind, such as the control of stem elongation or matching the absorbance peaks of photoreceptors involved in photosynthesis (Mitchell et al., 2012).
Additionally, Stutte (2009) found that the phytochrome photostationary state, the relative proportion of the active form to total phytochrome, could be manipulated using LEDs to either initiate earlier flowering or promote continued growth in the vegetative state for photoperiod-sensitive species. Far-red radiation has a significant effect in the processes of stem elongation and flowering (Downs and Thomas, 1982). For example, a deficiency in far-red radiation has often been found to delay flower initiation or development in species with a long-day photoperiodic response such as campanula (Campanula carpatica ‘Blue Clips’), coreopsis (Coreopsis ×grandiflora ‘Early Sunrise’), and pansy ‘Crystal Bowl Yellow’ (Runkle and Heins, 2001). Thus, reductions in radiation intensity as well as the lack of critical wavelengths in environments using SSL may lead to delays in flowering and a reduction in seedling quality for some species.
While limited research has been conducted on the effects of radiation quality for young-plant production in controlled environments under SSL, to our knowledge, no research to date has evaluated how the manipulation of radiation quality across various radiation intensities might further impact bedding plant seedling quality and TTF under SSL conditions. Additionally, by furthering our understanding regarding the impacts of LED SSL on nutrient uptake, a more thorough outlook on how to optimize production within these environments may be provided. We also postulated that the inclusion of far-red radiation during seedling propagation would initiate earlier flowering at finishing for species with a long-day photoperiodic response. Thus, the objective of the study was to evaluate the effects of various radiation qualities and intensities in a SSL environment on the morphology, nutrient uptake, and subsequent flowering of annual bedding plant seedlings.
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