Paclobutrazol is a triazole plant growth retardant widely used in the horticulture industry. Growth retardation results from inhibition of gibberellin biosynthesis, resulting in shorter internodes and more compact growth habit in many plant species (Barrett and Nell, 1989; Blanchard and Runkle, 2007). Other effects from paclobutrazol include increased stem diameter, decreased inflorescence diameter (Dasoju et al., 1998), delayed flowering, reduced flower number (Blanchard and Runkle, 2007), smaller leaf area, and higher leaf chlorophyll content (Moraes et al., 2005). Applications of paclobutrazol can provide growers an efficient method for regulating plant growth and size when proper concentrations and application timing are used.
Paclobutrazol is stable in water. In sterile aqueous solutions, 94% to 98% of triazole ring–labeled paclobutrazol was present after 30 d at pH ranging from 4 to 7 (U.S. Environmental Protection Agency, 2007). The compound did not undergo appreciable photolysis in water when exposed to 1.94 to 2.50 W·m−2 of electromagnetic radiation at 420 nm in pH 7 buffer (U.S. Environmental Protection Agency, 2007). Barrett (2006) described a commonly suspected and potentially serious situation where paclobutrazol sprays not intercepted by target plant canopies can contact exposed bench or floor surfaces and subsequently leave dried residues. During later irrigation events, dried spray residues may dissolve in excess irrigation water and be transported to either a holding tank or retention pond. Reapplication of irrigation water contaminated with paclobutrazol can cause stunting and deformed growth in nontarget floriculture crops. Million et al. (1999) showed that continuous irrigation with concentrations as low as 5, 17, and 24 µg·L−1 were enough to cause stunting of ‘Gin’ begonia (Begonia ×semperflorens-cultorum), ‘Super Elfin Coral’ impatiens (Impatiens walleriana), and ‘Nob Hill’ chrysanthemum (Dendranthema ×grandiflora), respectively. In comparison, paclobutrazol is commonly applied at a concentration several orders of magnitude higher than this biologically active threshold, at 0.01 to 8 mg·L−1 as a soil application, or 1 to 200 mg·L−1 as a foliar spray (Latimer, 2015; Whipker, 2015).
Water catchment and recirculation is an increasing practice, especially in greenhouse production. Despite the many ecological and economic benefits of water recirculation, accumulation of undesired contaminants is a risk to plant production. The risk of paclobutrazol accumulation in recirculated irrigation indicates a need for periodic monitoring of paclobutrazol in ponds and storage tanks. Although standards exist for collecting water samples for analysis of organic compounds (Rice et al., 2012; U.S. Geological Survey, 2006), no specific directions could be found for collecting water samples for paclobutrazol analysis. An important consideration in collecting water samples is the type of bottle used for sample collection. Virtually all analytical laboratories instruct growers to send water samples in clean plastic bottles for analysis [typically for pH, electrical conductivity (EC), total dissolved solids, alkalinity, etc.]. However, the U.S. Geological Survey instructs that water samples to be analyzed for organic compounds, in general, should be sent in bottles made from fluorocarbon polymer, glass, or metal components. More specifically, it instructs against the use of plastics other than fluorocarbon polymers. The Standard Methods for Examination of Water and Wastewater recommends hard glass containers because of their lower release of analytes (i.e., silica, sodium, and boron) and lower absorption of pesticides and metals compared with soft glass and plastic (Rice et al., 2012). In addition, amber-colored containers are recommended to avoid photodegradation. Most practitioners, by default, will likely use the same plastic bottles for water samples intended for paclobutrazol analysis as they would for traditional pH and nutrient analysis.
Another factor to consider when submitting water samples is how the sample should be treated at the time of collection and stored until the time of analysis. For example, the U.S. Geological Survey recommends methods such as filtration, acidification, or chilling for sample preservation depending on the compound of interest (Shelton, 1994). While parameters such as filtration and acidification might affect organic compounds in general, they are probably beyond the scope of what most greenhouse growers can perform on site. However, storage temperature and storage duration are factors that can be controlled by most greenhouse growers, and the influence of these factors should therefore be quantified. The objective of this research was to determine if the material of the sample container or storage temperature affects paclobutrazol stability after 3, 14, or 30 d of storage.
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Shelton, L.R. 1994 Field guide for collecting and processing stream-water samples for the national water-quality assessment program U.S. Geological Survey Open-file Rpt. 94 455
U.S. Environmental Protection Agency 2007 Paclobutrazol summary document for registration review: Initial Docket. Case Number 7002. 20 May 2015. <http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2006-0109-0003>
U.S. Geological Survey 2006 National field manual for the collection of water-quality data. Chapter A4. Collection of water samples. 26 May 2015. <http://pubs.water.usgs.gov/twri9A/>
Whipker, B.E. 2015 Plant growth regulators for annuals 2015. 10 Aug. 2015. <http://www.ballpublishing.com/pdf/PGR0115.pdf>