Determining the Effect of Carrier Water pH and Bicarbonate Concentration on Final pH of Plant Growth Regulator Solutions

Abstract

<jats:p>Chemical plant growth regulators (PGRs) are important tools in greenhouse ornamental crop production because growers must increasingly meet specifications for plant shipping and marketability. However, the role of water quality parameters such as pH or alkalinity (bicarbonate in this study) on final PGR solution pH is not well documented and could impact efficacy. We assessed the interaction of PGR type and concentration on the final spray solution pH when combined with carrier water of varying pH and bicarbonate concentration. Eleven PGRs commonly used in floriculture (ancymidol, benzyladenine, chlormequat chloride, daminozide, dikegulac-sodium, ethephon, flurprimidol, gibberellic acid, gibberellic acid/benzyladenine, paclobutrazol, and uniconazole) at three concentrations (low, medium, and high recommended rates for each product) were added to reverse osmosis (RO) carrier water adjusted to four pH (5.3, 6.2, 7.2, 8.2) levels or added to tap carrier water adjusted to four bicarbonate concentrations (40, 86, 142, 293 mg·L<jats:sup>−1</jats:sup> of CaCO<jats:sub>3</jats:sub>). Resultant solution pH levels were measured. Plant growth regulators were categorized as acidic, neutral, or basic in reaction based on the change of the carrier water pH on their addition. Benzyladenine, chlormequat chloride, gibberellic acid, and gibberellic acid/benzyladenine acted as weak acids when added to RO water, whereas daminozide, ethephon, and uniconazole reduced final solution pH from 1.25 to 5.75 pH units. Flurprimidol and paclobutrazol were neutral in reaction with final solution pH being similar to that of the RO carrier water before their addition. Ancymidol and dikegulac-sodium were basic in reaction, increasing final solution pH in RO carrier water up to 2.3 units. There was an interaction between chlormequat chloride concentration and RO carrier water pH on change in pH. When added to tap carrier water, final solution pH increased for all except the stronger acids, daminozide, ethephon, and uniconazole, where it decreased up to 3.5 units, and benzyladenine, where it decreased 0.35 units at 40 mg·L<jats:sup>−1</jats:sup> bicarbonate. There was an interaction between PGR concentration and bicarbonate concentration in tap carrier water for daminozide and ethephon. The magnitude of change in pH (final solution pH minus initial carrier water pH) with the addition of each PGR was greater for RO than for tap water containing 40 to 293 mg·L<jats:sup>−1</jats:sup> bicarbonate for all 11 PGRs tested.</jats:p>

Substrate pH Impacts Efficacy of Ethephon Drenches on Growth of Herbaceous Perennials

“…This demonstrates that the additional buffering capacity of the dolomitic lime directly influenced the efficacy of the ethephon drench. As Camberato et al (2014) demonstrated when looking at carrier water pH of PGR solutions, with low concentrations of bicarbonates, there is little buffering capacity and therefore ethephon drives the solution pH down. However, under high bicarbonate concentrations, the buffering capacity is high and ethephon has less effect on final solution pH.…”

“…Consequently, as the speed at which ethephon evolution increases while in solution, ethephon availability for plant uptake decreases, therein reducing the chemical efficacy (Smith, 2010). Ethephon is a relatively strong acid, which will reduce the solution pH; however, in regions with water sources with high alkalinity, the buffering capacity of the bicarbonates in the water may prevent the solution pH from lowering to the recommended range (Camberato et al, 2014). In these circumstances, it is necessary to reduce the pH of the solution by adding an acidic buffer solution to the tank before adding ethephon to prevent ethephon degradation (Yates et al, 2011).…”

Ethephon Foliar Sprays Are Influenced by Carrier Water Alkalinity and Ambient Air Temperature at Application

“…First, there is an increase in the active ingredient, ethephon, and greater ethylene release resulting in more robust plant responses (Currey et al, 2016b). Second, ethephon is a strong acid; therefore, increasing ETH also results in a lower spray solution pH (Camberato et al, 2014). Moreover, because the rate of ethephon conversion into ethylene increases as pH increases (Klein et al, 1979;Warner and Leopold, 1969) and…”

“…In addition, water pH >7.0 can cause weak acid herbicides, such as glyphosate, 2,4-D, and dicamba, to become negatively charged, thus inhibiting absorption by the leaf cuticle and cell membrane (Chahal et al, 2012). Carrier water, particularly its alkalinity and pH, also can play a role in the chemical composition and evolution of PGRs, thus influencing efficacy (Camberato et al, 2014;Hammer, 2001).…”

“…The rapid release of ethylene from solution results in a phase change from liquid to gas, reducing the ethylene influx through stomata, and thus reducing chemical efficacy. Camberato et al (2014) classified ethephon as a strong acid and reported that the addition of 500 mg · L -1 ethephon [Collate Ò (21.7% a.i. ); Fine Americas, Inc., Walnut Creek, CA] to reverse osmosis carrier water with a pH of 5.3 or 8.2, resulted in a final spray solution pH of 2.4 and 2.5, respectively.…”