Within efficient production systems, ornamental crops need to be uniform in shape and size (Meijón et al., 2009). By regulating plant size and creating a uniform crop, growers can save space during production and transportation and reduce breakage and loss (Hayashi et al., 2001). Controlling plant architecture can also meet the demands of retailers and consumers for more compact, dense (Cameron et al., 2006; Lütken et al., 2012), and symmetrical plants (Glasgow et al., 1998). In addition, growers can manipulate flowering dates to extend shipping periods (Hayashi et al., 2001).
To achieve target uniformity, nursery producers manually prune and, less commonly, use chemical PGRs to enhance plant growth and plant architecture. Manual pruning can be labor intensive (Banko and Stefani, 1996) and expensive (Holland et al., 2007), yet, may be more environmentally sustainable than chemical control. Although PGRs are generally less labor intensive to apply than manual pruning, they can cause phytotoxicity (Meijón et al., 2009) and be perceived as digressing from sustainable production (Lütken et al., 2012).
Plant growth regulators have several modes of action, including branch inducing, chemical pinching (chemicals that suppress apical dominance), and ethylene generation. Plant responses to PGRs can vary across cultural and environmental conditions. In addition, the rates, timing, and number of applications required to elicit the desired response can vary among plant genera and species (Bruner et al., 2002; Kessler and Keever, 2008). For example, benzyladenine at 300 ppm increased BN in ‘Purple Haze’ anise hyssop (Agastache), but not in ‘Provence’ lavender (Lavandula ×intermedia) or ‘May Night’ sage (Salvia ×sylvestris) (Grossman et al., 2012), and 300 ppm benzyladenine and 1000 ppm ethephon increased BN of Marnier’s kalanchoe (Kalanchoe marnieriana), but not flower dust plant (K. pumila) (Currey and Erwin, 2012). These differing plant responses validate the importance of optimizing PGR chemistry, as well as rate and application timing based on the target crop, conditions, and desired effect on plant form.
Little Lime™ hydrangea is a new hydrangea cultivar that is increasingly popular, but is not considered attractive in its juvenile plant form because of sparse foliage and limited branching. Since hydrangea species account for an estimated $73 million in U.S. sales annually [U.S. Department of Agriculture (USDA), 2009], the use of PGRs to control growth could greatly benefit nurseries producing Little Lime™. Recently, dikegulac sodium, benzyladenine, and ethephon, all with different modes of action (DNA synthesis inhibitor, branch promoter, and ethylene generator, respectively), have shown success as PGRs on other hydrangea species (Hester et al., 2013), but efficacy on Little Lime™ is not known. With the variation in plant response, it is recommended to test PGRs on each plant cultivar individually (Currey and Erwin, 2012; Hilgers et al., 2005; Starman et al., 2004). Therefore, the objective of these experiments was to evaluate vegetative and floral development of Little Lime™ following application of dikegulac sodium, benzyladenine, or ethephon.
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