The U.S. commercial wholesale value of ornamental grasses increased by 22% from 2009 to 2014 (U.S. Department of Agriculture, 2010, 2015). Most of the ornamental grasses commercially produced are perennials. Purple fountain grass [P. ×advena Wipff and Veldkamp (formerly known as P. setaceum Forsk. Chiov. ‘Rubrum’)] is a popular ornamental grass grown commercially as a horticultural annual in the United States (Bailey, 1949; Drake, 1999), although it is a perennial in frost-free areas. Purple fountain grass is a warm-season perennial bunch grass with broad purple leaves and dark purple inflorescences (Simpson and Bashaw, 1969).
Availability of young purple fountain grass liners is limited (Cunliffe et al., 2001) because of seed sterility (Simpson and Bashaw, 1969) and loss of outdoor stock plants from killing frosts (Cunliffe et al., 2001). Propagation methods of ornamental grasses include micropropagation (Gawel et al., 1990; Robacker and Corley, 1992), seed (Miao et al., 1998), production of tillers, rhizomes, stolons, vegetative apomixis (Miao et al., 1998), crown divisions (Corley, 1989; Simon, 1982), and culm or flowering stem cuttings (Barnes, 1994; Corley, 1989; Cunliffe et al., 2001). The most common propagation technique methods for purple fountain grass include division and tissue-cultured plantlets that result in crop uniformity and preserves cultivar identity (Wang et al., 1999).
Growers who propagate purple fountain grass from divisions, tissue-cultured plantlets, or culm cuttings during winter months and early spring have reported slow growth and delayed flowering, especially across the northern United States (lat. ≥40°N), when the average greenhouse PDLI ranges from 5 to 10 mol·m−2·d−1. These low-light conditions increase rooting time, do not maximize root production, and result in low-quality liners. For example, crop time from division to a marketable 50-cell liner tray (106.5-mL individual cell volume) from fall to early winter, late winter to early spring, or spring to summer is 12, 10, and 8 weeks, respectively (M. Goyette, personal communication). Therefore, reduced crop time from spring to summer is consistent with previous research indicating that increased DLI during root development increases rooting, biomass accumulation, and quality of annual bedding plant cuttings (Lopez and Runkle, 2008).
Numerous studies have evaluated effects of PDLI (Currey et al., 2012; Enfield, 2002; Hutchinson et al., 2012; Lopez and Runkle, 2008), and, to a lesser extent, RZT (Iapichino and Bertolino, 2009; Wilkerson et al., 2005) on growth, morphology, and quality of vegetatively propagated annual bedding plants and herbaceous perennials during rhizogenesis of cuttings. Daily light integral during propagation influences photosynthesis, respiration, stomatal conductance, leaf, stem, and root mass ratios, leaf area ratio, specific leaf area, chlorophyll content, and carbohydrate status of cuttings (Currey and Lopez, 2015). During propagation, cuttings require a minimum DLI to provide a supply of carbohydrates for callus and adventitious root initiation and development (Geiss et al., 2009; Lopez and Runkle, 2008).
In addition to PDLI, temperature during propagation generally influences many physiological processes, including photosynthesis, respiration, transpiration, and root and shoot development (Blanchard et al., 2006). Temperature can potentially influence adventitious root capacity in many aspects, such as by influencing water and nutrient uptake and metabolism, thereby promoting or inhibiting enzymatic action (Geiss et al., 2009). Specifically, RZT is a major factor controlling plant growth and development, thus influencing cell division and expansion, and the capacity of plants to construct root tissue as well as root-tissue morphology and function (Pregitzer et al., 2000). Previous investigations have shown RZT to be critical for root initiation in garden mum (Chrysanthemum morifolium L. ‘Bright Golden Anne’ and ‘Ki-Amagahara’) (Dykeman, 1976; Takahashi et al., 1981) and poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch ‘Freedom Dark Red’) (Wilkerson et al., 2005). Studies indicate that increasing RZT hastens the time to visible root formation and increases root density per cutting up to a species-dependent optimum temperature (To), above which increasing temperature has a deleterious impact on rooting (Tmax) (Ochoa et al., 2004; Wilkerson et al., 2005). Suboptimal temperatures may inhibit or limit adventitious root formation because cuttings do not metabolize at a rate sufficient for optimum rooting (Preece, 1993).
Previous cutting propagation studies with ornamental grasses (Cunliffe et al., 2001; Meyer and Hong, 2011) focused on rooting substrate composition, IBA application, and rooting success at different node positions. To our knowledge, no studies have been published on the effects of PDLI and RZT on rhizogenesis of vegetative single-internode purple fountain grass culm cuttings. Therefore, our objectives were to determine the effects of PDLI and RZT on rhizogenesis, biomass accumulation, and quality of single-internode culm cuttings.
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