Herbaceous perennials are propagated from seed (plugs), cuttings (liners), divisions, and tissue-cultured plantlets, and in 2015 had a reported wholesale value of $120 million for the 15 top-producing states (U.S. Department of Agriculture, 2016). Although herbaceous perennials can be successfully and economically propagated by seed, many are vegetatively propagated by shoot-tip, stem, basal, rhizome, or root cuttings, thus maintaining genetic uniformity, producing sterile cultivars, and hastening production (Pilon, 2006). Rhizome and root cuttings rarely yield uniform results and are labor intensive (Scoggins, 2006); therefore, shoot-tip, stem, and basal cuttings are recommended for vegetative perennial propagation.
According to Owen and Lopez (2016), 29%, 39%, and 18% of U.S. propagators receive and root herbaceous perennial cuttings during spring (March–May), summer (June–August), and fall months (September–November), respectively. During these months, seasonal outdoor daily temperatures and photosynthetic DLIs differ greatly, hindering the ability to maintain consistent environmental parameters during propagation. For instance, in 2015, the national average daily temperatures during spring, summer, and fall months were 12.0 ± 3.5, 22.0 ± 0.6, and 14.0 ± 2.9 °C, respectively (NOAA, 2016). Seasonal temperatures influence the need to heat or cool the propagation environment to achieve recommended air and root-zone temperatures of 20 to 23 °C and 18 to 25 °C, respectively (Pilon, 2006). The rate at which callus and adventitious root (AR) initials develop is temperature-dependent, thereby effecting AR formation (ARF) in cuttings. In addition, outdoor DLIs during late winter to early spring months are relatively low (5 to 20 mol·m−2·d−1) compared with summer and fall months (30 to 50 mol·m−2·d−1; Korczynski et al., 2002) and can be reduced by 50% or more from the greenhouse glazing material (Hanan, 1998) with further reductions from greenhouse infrastructure shading, white wash, and shade curtains (Lopez and Runkle, 2008). Although low DLIs (≤5 mol·m−2·d−1) during the early stages of propagation may be beneficial for minimizing stress and developing callus, excessively low DLIs (≤2 mol·m−2·d−1) can result in little to no ARF in cuttings. Overall, these seasonal variations pose a challenge to maintain consistent callusing and rooting of herbaceous perennials (Owen and Lopez, 2016). Therefore, additional temperature management and SL may be necessary during propagation.
Previous research has investigated the effects of DLI and SL from high-pressure sodium (HPS) lamps and LEDs during AR development and subsequent rhizogenesis of numerous genera of vegetatively propagated annual bedding plants (Currey et al., 2012; Hutchinson et al., 2012; Lopez and Runkle, 2008). In controlled environments, the effects of SSL provided by LEDs during seedling (plug) propagation (Randall and Lopez, 2015; Wollaeger and Runkle, 2015) and in vitro propagation (Budiarto, 2010; Gu et al., 2012; Jao et al., 2005) have been documented.
Ex vitro vegetative cutting propagation under SSL LEDs has been investigated for calibrachoa (Calibrachoa Llave and Lex. ‘MiniFamous Neo Royal Blue’; Olschowski et al., 2016), English lavender (Lavandula angustifolia Mill. ‘Hidcote’; Christiaens et al., 2015), garden mum (Chrysanthemum ×morifolium ‘Orlando’; Christiaens et al., 2015), sweet basil (Ocimum basilicum L.; Lim and Eom, 2013), and four genera of woody ornamentals (Christiaens et al., 2015; Van Dalfsen and Slingerland, 2012). These studies examined the photomorphogenic responses to monochromatic and dichromatic light spectra. For instance, Christiaens et al. (2015) propagated garden mum cuttings under SSL LEDs delivering 60 µmol·m−2·s−1 (DLI of 4.1 mol·m−2·d−1) of R (660 nm):B (460 nm) light ratios (%) of R100:B0, R90:B10, R50:B50, R10:B90, or R0:B100. They determined root dry mass (RDM) of garden mum cuttings propagated under R100:B0 or R0:B100 to be 171% to 200% greater, respectively, than for cuttings propagated under all dichromatic ratios tested. In another study, Olschowski et al. (2016) propagated calibrachoa cuttings under SSL LEDs delivering 80 µmol·m−2·s−1 of R100:B0 or R0:B100 or HPS lamps delivering 80 µmol·m−2·s−1 to provide a DLI of 4.6 mol·m−2·d−1 and maintained 95% relative humidity (RH) and 24 °C air temperature. After 21 d, they observed calibrachoa cuttings propagated under SSL LEDs exhibited significantly shorter roots and had smaller shoot dry mass (SDM) and RDM compared with cuttings propagated under HPS lamps. Although the foci of these experiments were only to determine ARF and subsequent root and shoot growth and development, literature determining the effects of SSL LEDs on callus formation and growth in vegetative cuttings has not been documented. Methods to assess in vitro callus formation and growth include fresh and dry callus mass, surface area and volume measurements, visual comparisons, cellular quantification, mitotic indices, and callus respiration (Mottley and Keen, 1987; Sathyanarayana and Varghese, 2007). To date, scientific methods have not been established to measure and quantify ex vitro callus formation and growth.
There is little known about the effects of light quality from SSL on callus formation and growth and on initial ARF during vegetative propagation of herbaceous perennials. The recent interest in SSL LEDs for ornamental seedling production, combined with the potential use for vegetative cutting propagation, provides a unique opportunity to investigate the impact of spectra-specific SSL applications. In addition, a multilayer vertical system for ex vitro cutting propagation provides propagators a controlled environment (light and temperature) to uniformly callus and root vegetative cuttings, maintain consistent liner quality, and maximize space efficiency. Therefore, our objectives were to quantify and compare the effects of SSL from LEDs providing four different light qualities to SL from HPS lamps on callus formation and growth and on early subsequent rhizogenesis of herbaceous perennial cuttings. In addition, we aim to establish new methodologies to quantify callus formation and growth in vegetatively propagated herbaceous perennial cuttings.
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