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.
The effect of sugars on the uptake of bentazone and glyphosate into bean (Vicia faba) and wheat (Triticum aestivum) foliage was studied. The addition of 0.5% glucose or sucrose significantly increased the uptake of bentazone into bean and wheat leaves. This effect was also observed in the presence of a surfactant, especially in wheat. Glucose and sucrose also improved the uptake of glyphosate into wheat leaves, especially in combination with surfactants of low ethylene oxide content. When used at 2% concentration, both sugars completely overcame the antagonistic effect of an organosilicone surfactant (Silwet L-77) on glyphosate uptake into wheat leaves. The results are discussed in relation to the solute dose on the leaf surface as influenced by surfactants and sugars.
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