Potted spring-flowering bulbs and bedding and garden plants collectively account for $1.97 billion (49%) of the total U.S. wholesale value of floriculture crops for the 15 top-producing states (U.S. Department of Agriculture, 2011). These crops often have excessive stem elongation as a result of dense spacing or suboptimal environmental conditions, which results in overgrown, unattractive, and unmarketable plants (Krug et al., 2006a,b; Starman et al., 2004). For many potted flowering plants such as Euphorbia pulcherrima Willd. ex Klotzsch, Lilium longiflorum Thunb. and Lilium L. hybrids, and Narcissus pseudonarcissus L., there are industry target height recommendations for aesthetic value and ease of postharvest packing and shipping (De Hertogh, 1996; Fisher and Heins, 2002; Miller, 1992, 1993). For example, the recommended target height of spring flowering bulbs such as Narcissus grown in 15-cm containers is 25 to 30 cm (De Hertogh, 1996). Conversely, there are currently no target height recommendations for annuals as a result of the large number of seed- and cutting-propagated bedding plants in the market.
Plant growth regulators (PGRs) are commonly used in greenhouse production to produce uniform, compact plants that can be easily shipped and marketed to consumers (Currey et al., 2010). Foliar sprays, substrate drenches, liner dips, or bulb, tuber, and rhizome soaks or dips are common application methods for PGRs and one or more of these techniques are appropriate for nearly every active ingredient (Barrett, 2004; Blanchard and Runkle, 2007; Currey et al., 2010; Whipker and McCall, 2000). The majority of synthetic PGRs suppress stem elongation by inhibiting gibberellin biosynthesis (Davis et al., 1988; Rademacher, 2000). In contrast, ethephon [(2-chloroethyl) phosphonic acid] is a PGR that releases ethylene (C2H4), chlorine (Cl–), and hydrogen phosphate (H2PO4−) on application and is known to inhibit internode elongation, induce branching, and cause abscission of flower buds and leaves (Glady et al., 2007; Leatherwood et al., 2009; Maynard and Swan, 1963; Starman et al., 2004).
Ethephon sprays are applied to many floriculture crops 1 to 2 weeks after transplant and at 1- to 2-week intervals thereafter (Styer, 2002). Application concentrations for annual and perennials generally range from 250 to 1000 mg·L−1 (Glady et al., 2007; Starman et al., 2004; Styer, 2002). Currently, ethephon sprays are the main technique for height control in Hyacinthus orientalis L. and Narcissus with suggested concentrations of 500 to 2000 mg·L−1 depending on the cultivar, length of cooling, and forcing time (De Hertogh, 1996).
Substrate drenches of many PGRs provide more uniform results and increase the duration of effectiveness compared with foliar sprays (Boldt, 2008; Gent and McAvoy, 2000). Applying antigibberellin substrate drenches containing flurprimidol, paclobutrazol, or uniconazole early in the production of Euphorbia, Lilium longiflorum, and bedding plants is a PGR application strategy for height control (Barrett, 2004; Currey et al., 2010; Currey and Lopez, 2011; Lopez and Runkle, 2007; Runkle et al., 2006).
Although ethephon sprays are effective at inhibiting stem elongation and promoting branching, the potential of ethephon substrate drenches for bedding plant and spring bulb stem elongation control is largely unknown. Johnson et al. (1982) demonstrated growth effects from soil-applied ethephon on Ficus benjamina and preliminary results on Narcissus height control from substrate applications of ethephon have been reported (Anonymous, 1973, 1974; Briggs, 1975). However, to our knowledge, no literature on the use of ethephon as a substrate drench on bedding plants exists. Therefore, the objectives of this research were to evaluate ethephon drenches on a range of bedding plant and Narcissus cultivars and to assess the effect of substrate lime concentrations on ethylene release from a peat substrate.
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