Interactions of Calcium Ions with Weakly Acidic Active Ingredients Slow Cuticular Penetration:  A Case Study with Glyphosate

Abstract

Potassium and calcium salts of glyphosate were obtained by titrating glyphosate acid with the respective bases to pH 4.0, and rates of penetration of these salts across isolated astomatous cuticular membranes (CMs) were measured at 20 degrees C and 70, 80, 90, and 100% humidity. K-glyphosate exhibited first-order penetration kinetics, and rate constants (k) increased with increasing humidity. Ca-glyphosate penetrated only when the humidity above the salt residue was 100%. At 90% humidity and below, Ca-glyphosate formed a solid residue on the CMs and penetration was not measurable. With Ca-glyphosate, the k value at 100% humidity decreased with time and the initial rates were lower than for K-glyphosate by a factor of 3.68. After equimolar concentrations of ammonium oxalate were added to Ca-glyphosate, high penetration rates close to those measured with K-glyphosate were measured at all humidities. Adding ammonium sulfate or potassium carbonate also increased rates between 70 and 100% humidity, but they were not as high as with ammonium oxalate. The data indicate that at pH 4.0 one Ca2+ ion is bound to two glyphosate anions. This salt has its deliquescence point near 100% humidity. Therefore, it is a solid at lower humidity and does not penetrate. Its molecular weight is 1.82 times larger than that of K-glyphosate, and this greatly slows down rates of penetration, even at 100% humidity. The additives tested have low solubility products and form insoluble precipitates with Ca2+ ions, but only ammonium oxalate binds Ca2+ quantitatively. The resulting ammonium salt of glyphosate penetrates at 70-100% humidity and at rates comparable to K-glyphosate. The results contribute to a better understanding of the hard water antagonism observed with glyphosate. It is argued that other pesticides and hormones with carboxyl functions are likely to respond to Ca2+ ions in a similar fashion. In all of these cases, ammonium oxalate is expected to overcome hard water antagonism.

Size selectivity of aqueous pores in stomatous cuticles of Vicia faba leaves

“…f, h Trichomes of AgNO 3 -treated leaf surfaces with characteristic silver deposits in the base and head of the trichome Kru¨ger (1999) observed that each Ca 2+ ion was accompanied by two nitrate ions. Scho¨nherr and Schreiber (2004b) compared fluxes of 45 Ca-glyphosate and CaÀ 14 C-glyphosate across poplar CMs and found them to be identical. Theoretically, counter-ion diffusion might have occurred, i.e.…”

Glyphosate-induced impairment of plant growth and micronutrient status in glyphosate-resistant soybean (Glycine max L.)

“…Obviously, on the highly weathered Arenosol with low buffering capacity, glyphosate was sufficiently available in the soil solution for interactions with the roots of sunflower as a non-target plant. High Ca 2+ concentrations in the calcareous sub-soil (30% CaCO 3 ) may lead to rapid complexation and immobilization of glyphosate (Gauvrit et al 2001;Schönherr and Schreiber 2004) to make it unavailable for plant roots and to protect it from conversion to AMPA, which can exert phyto- 132.5a (±33.5)…”

Residue determination of glyphosate in environmental water samples with high-performance liquid chromatography and UV detection after derivatization with 4-chloro-3,5-dinitrobenzotrifluoride

“…Due to its strong retention on soil components, high solubility in water and long half-life in the environment (about 47 days), glyphosate may still be detected long after application or even far from the site of application [5]. For decades, the long-term environmental and ecological effects of glyphosate have been the target of research and discussion [6][7][8][9][10]. A reliable method for the determination of glyphosate in environmental samples is therefore a must for this research and environmental monitoring.…”