It is desirable for commercial potted, bedding, and foliage plants to be well-branched and compact in habit (Roh and Lawson, 1998). Ideally, these characteristics are achieved genetically through breeding and selection during crop development. However, when compact height and high branch number does not occur naturally, environmental or mechanical manipulation by reducing the difference in day and night air temperatures (DIF) or pinching, respectively, can be used to suppress height and increase branch number (Andersen and Andersen, 2000). Chemical suppression of stem elongation and increased branching is possible with the use of different types of PGRs. For example, applications of inhibitors of gibberellin synthesis or ethylene generators will suppress cell elongation and, thus, stem length (Rademacher, 2000). Additionally, exogenous applications of a cytokinin or ethylene generators can suppress apical dominance and release axillary buds by lowering the ratio of auxin to cytokinin or ethylene, thereby enhancing branching (Srivastava, 2002).
PGRs are regularly applied in commercial ornamental plant production to improve the ornamental and aesthetic characteristics of plants, including stem length and branching (Gent and McAvoy, 2000). There are several methods used to apply PGR solutions, including: bulb, tuber, or rhizome soaks or dips (Currey and Lopez, 2010; Krug et al., 2005); liner dips (Blanchard and Runkle, 2007; Schnelle and Barrett, 2010); substrate drenches (Currey and Lopez, 2011; Sellmer et al., 2001); and foliar sprays (Gibson and Whipker, 2003; Whipker and Dasoju, 1998). Foliar sprays have the advantage of being a relatively easy application method, inexpensive to apply with respect to labor, often use less total chemical, and are appropriate for nearly every PGR. As such, foliar sprays are the most common method employed for PGR application (Gent and McAvoy, 2000).
Though spray application of most PGRs elicit an effective response, the efficacy of different chemicals and concentrations can vary among and within genera, species, and cultivars (Seely, 1979). For example, Warner and Erwin (2003) and Nordwig (1999) reported variation among several hibiscus (Hibiscus sp.) and milkweed species (Asclepias sp.), respectively, to different PGRs at varying concentrations when applied as sprays. Similarly, Boldt (2008), Gibson and Whipker (2001), and Krug et al. (2007) reported variation among cultivars of angelonia (Angelonia angustifolia), ornamental cabbage and kale (Brassica oleracea), and tulip (Tulipa gesneriana) in response to different PGRs.
There are 139 species of kalanchoe (Descoings, 2005). However, only florists’ kalanchoe (Kalanchoe blossfeldiana) is widely grown as an ornamental plant (Dole and Wilkins, 2005). We identified numerous kalanchoe species with commercial ornamental crop potential based on unique flowering and/or vegetative characteristics (Currey, 2009; Currey and Erwin, 2010, 2011a, 2011b). While these species possess unique ornamental floral, foliar, and/or vegetative qualities, they are neither compact nor free-branching (Currey and Erwin, 2011a). The use of chemical PGRs to inhibit stem elongation, enhance branching, or both may aide in adapting these species to ornamental flowering plant culture (Davis and Andersen, 1989).
Various PGRs effectively inhibit stem elongation of K. blossfeldiana and K. porphyrocalyx (Adriansen, 1989; Carlson et al., 1977; Cathey and Stuart, 1961; Hwang et al., 2008; Lee et al., 2003; Nightingale, 1970; Pertuit, 1973; Ueber, 1998; Zimmer, 1985) and increase or enhance branching for K. blossfeldiana and K. tomentosa (Lyons and Hale, 1987; Pertuit, 1973). There is little, or no, information on what PGRs are effective on the species previously identified with ornamental potential. Therefore, our objectives were to assess the efficacy of various PGRs on stem elongation and branching of several kalanchoe species with ornamental characteristics.
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