Chemical PGRs are commonplace in the horticulture industry, having been used commercially since the 1940s (Nickell, 1994). In high-value floriculture crops, PGRs are used in combination with cultural and environmental control methods to produce crops meeting increasingly specific market window and size specifications. The short production time of annual bedding plants in 10- to 15-cm diameter containers and the marketing advantage of compact plants (improved branching and structure) in flower (earlier and more uniform) and with improved shelf life have contributed to the necessity of using chemical PGRs in the bedding plant industry (Barrett, 2006; Bell, 2001). There is also the possibility of additional benefits derived from PGR application: disease reduction, enhanced foliage color, and increased water use efficiency that contributes to their continued use (Whipker, 2013).
Much research has been conducted on efficient chemical PGR application: timing of application, application method and rates, target tissues, environmental conditions at application, and dosage (Whipker et al., 2003). Spray solution water quality, particularly pH and alkalinity [presence of bicarbonates (HCO3−) and carbonates (CO3−2)], may also play a role in PGR efficiency. Growth regulator solutions made with high pH (greater than 7.0) or highly buffered (greater than 100 mg·L−1 CaCO3) carrier water may reduce effectiveness, as suggested by Hammer (2001). A grower survey (Burns, 2004) indicated that 60% of respondents were aware of potential water quality effects on chemical PGRs and that 45% had a water treatment system in place, typically acid injection to lower pH and neutralize bicarbonates. Water used in U.S. greenhouse production facilities has traditionally come from groundwater wells (Biernbaum, 1999). Because many growers do not use a water treatment system, carrier water for PGR spray solutions can be variable in pH and buffering capacity. Research documents water quality effects on herbicides and insecticides, particularly in agronomic crops, but there is little research detailing how the pH or alkalinity aspects of water quality influence PGRs in horticultural crops. For example, phytotoxicity from glyphosate was reduced when mixed in carrier water with high concentrations of calcium and bicarbonate (Buhler and Burnside, 1983). Carrier water pH greater than 7.0 can cause weak acid herbicides such as glyphosate, 2,4-D, and dicamba to become negatively charged (OH− from water accepts H+ from chemical) decreasing absorption by the leaf cuticle and cell membrane (Chahal et al., 2012). Insecticides and miticides are also sensitive to carrier water alkalinity, and alkaline hydrolysis can occur when spray solution pH is greater than 7.0 (Cloyd, 2007). Mudge and Swanson (1978) found that ethylene release from ethephon was dependent on solution pH and that the addition of buffers increased ethylene concentration to cuttings of Phaseolus aureus Roxb. (mung bean).
There is little information available on the reaction of the PGRs widely used in bedding plant production to carrier water pH and bicarbonate concentration. The objective of this study was to quantify the effects of carrier water pH, bicarbonate concentration, and PGR concentration on the final solution pH of 11 PGRs labeled for horticultural use. Final solution pH was compared with the recommended pH range for optimum PGR performance as indicated by manufacturer (Table 1) or Yates et al. (2011).
Properties of the plant growth regulators used in the study as indicated on manufacturer label or material safety data sheet unless noted.
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Bell, M. 2001 Bedding plants and seed geraniums. Tips on regulating growth of floriculture crops. O.F.A. Services, Inc., Columbus, OH
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Burns, C. 2004 Water quality: Is it an issue? 3 Oct. 2013. <http://www.gpnmag.com/water-quality-it-issue>
Chahal, G., Roskamp, J., Legleiter, T. & Johnson, B. 2012 The influence of spray water quality on herbicide efficacy. 3 Oct. 2013. <https://ag.purdue.edu/btny/weedscience/documents/Water_Quality.pdf>
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Nickell, L.G. 1994 Plant growth regulators in agriculture and horticulture, p. 9−18. In: Heden, P. (ed.) Bioregulators for crop production and pest control. American Chemical Society, Washington, DC
Smith, T. 2010 Effects of pH on pesticides and growth regulators. UMass Ext. 3 Oct. 2013. <http://extension.umass.edu/floriculture/fact-sheets/effects-ph-pesticides-and-growth-regulators>
UNH 2009 ALKCALC Alkalinity calculator. 23 Apr. 2014. <http://extension.unh.edu/Agric/AGGHFL/alk_calc.cfm>
Whipker, B.E. 2013 Additional benefits of PGRs. 3 Oct. 2013. <http://www.ballpublishing.com/pdf/PGR_GUIDE_2013-LowRez.pdf>
Whipker, B.E., Cavins, T.J., Gibson, J.L., Dole, J.M., Nelson, P.V. & Fonteno, W. 2003 Growth regulators, p. 85−112. In: Hamrick, D. (ed.). Ball Redbook crop production Vol. 2. Ball Publishing, Batavia, IL
Yates, R., Lutz, J. & Brubaker, V. 2011 Optimum pesticide spray water pH using Indicate 5. Griffin Greenhouse and Nursery Supplies, Inc. 3 Oct. 2013. <http://www.ggspro.com/new/pdfs/Opt-Pest-Spray.pdf>