Lower formulation pH does not enhance bentazone uptake into plant foliage

Abstract

Lower pH generally favours the diffusion of weak acid compounds in vitro into plant cells. Such a rule may not be applicable to the uptake of formulated weak acid herbicides applied to plant foliage in vivo. In this study, the effect of spray formulation pH (5, 7 and 9) on the foliar uptake of a weak acid herbicide, bentazone which is used as a formulated salt, was investigated using three plant species, mustard (Sinapis alba), wheat (Triticum aestivum) and bean (Vicia faba). Greater uptake of the herbicide occurred at pH 9 and pH 7 than at pH 5 on mustard and wheat leaves. Uptake of bentazone into bean was slow (< 20% after 24 h) regardless of carrier pH. However, in the presence of a surfactant, faster uptake was achieved with higher pH. The results are discussed in relation to the lipophilicity and the solubility of weak acid chemicals as influenced by pH.

Increasing and Decreasing pH to Enhance the Biological Activity of Nicosulfuron

“…Herbicide solubilization is required but is not sufficient for high foliar activity (Hess and Foy 2000). Liu (2002) recently coupled the ionic nature of weak acid herbicides and their solubility. When the herbicide is at low concentration and solubility is not a limiting factor, uptake is greater at low pH.…”

Spray carrier pH effect on absorption and translocation of trifloxysulfuron in Palmer amaranth (Amaranthus palmeri) and Texasweed (Caperonia palustris)

“…Greater absorption of trifloxysulfuron at high pH is in contrast to most weak acid herbicides. Liu (2002) noted that the absorption of most weak acid herbicides into plant tissues is greater at a low carrier pH because of a greater proportion of the molecules being present in an undissociated form. However, most of the reviewed studies were conducted with plant tissues immersed or cultured cells incubated in a solution of nonformulated weak acid herbicides (Liu 2002).…”

“…Liu (2002) noted that the absorption of most weak acid herbicides into plant tissues is greater at a low carrier pH because of a greater proportion of the molecules being present in an undissociated form. However, most of the reviewed studies were conducted with plant tissues immersed or cultured cells incubated in a solution of nonformulated weak acid herbicides (Liu 2002). Liu (2002) did observe greater absorption of bentazon by white mustard and wheat with spray carrier pH of 7 and 9 as compared with pH 5.…”

“…However, most of the reviewed studies were conducted with plant tissues immersed or cultured cells incubated in a solution of nonformulated weak acid herbicides (Liu 2002). Liu (2002) did observe greater absorption of bentazon by white mustard and wheat with spray carrier pH of 7 and 9 as compared with pH 5. Another study found that the addition of adjuvants that increased pH of the spray mixture increased the biological activity of high nicosulfuron rates at low spray volumes on difficult-to-control weeds (Green and Cahill 2003).…”

“…Generally, the uptake of weak acids by plant tissues is greater at lower carrier pH because of a higher proportion of the molecules being present in an undissociated form. Yet, Liu et al (2002) reported greater uptake of bentazon by white mustard (Sinapis alba L.) and wheat (Triticum aestivum L.) plant leaves at pH 7 and 9 compared with pH 5. Absorption of bentazon by field bean (Vicia faba L.) was minimal regardless of carrier pH.…”