Plant growth regulators are commonly used during greenhouse ornamental plant production (Latimer and Whipker, 2012; Whipker, 2015) and plant growth retardants (PGRs) are the most common type used (Dole and Wilkins, 2005). Producers use PGRs to inhibit internode extension that results in shorter plants with thicker stems and leaves, as well as leaves that may exhibit a darker green color (Latimer and Whipker, 2012). These smaller, sturdier plants allow growers to use bench space more efficiently and help plants to endure shipping and retail handling. In addition, there are other potential benefits of PGRs including improved plant water-use efficiency (Ahmad et al., 2014; Jungklang and Saengnil, 2012) and disease suppression (Hill and Latimer, 2004).
In the U.S. floriculture industry, the PGR most extensively used is PBZ (Whipker, 2015). This may be because PBZ has a low likelihood for phytotoxicity, can be applied in many different ways (sprayed, drenched, or dipped), and is generally effective on a wide range of crops (Ahmad et al., 2014; Currey and Erwin, 2012; Miller and Armitage, 2002). Typical foliar spray concentrations for PBZ range from 15 to 30 ppm for bedding plants, with lower and higher doses used for northern climates and southern climates, respectively (Whipker, 2015). Most PGR research has investigated controlling plant size and very few studies have reported other factors that may impact the effectiveness of a PGR treatment.
Plant height and growth rate are affected by many environmental factors. Stem elongation is affected by the difference between day and night temperatures (DIF) for many floriculture species (Erwin and Heins, 1995). Light quality and quantity, as well as photoperiod, can interact with DIF to affect plant internode length (Myster and Moe, 1995). The ADT impacts the rate of plant development, with faster rates at higher compared with lower temperatures, assuming the temperature is within the tolerable range for the species and other factors (light, nutrition, etc.) are not limiting (Dole and Wilkins, 2005; Ministry of Agriculture, Food and Rural Affairs, 2014). Research on poinsettia (Euphorbia pulcherrima) sprayed with uniconazole (a PGR in the same group of chemicals as PBZ) demonstrated that bract diameter and shoot length were greater under warmer greenhouse regimes compared with cooler ones (Barrett, 2001). The objective of this study was to build on previous research to evaluate the relative efficacy of three PBZ concentrations on four ornamental annual species grown at three greenhouse ADTs.
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