Biological evidence for a 1:1 Ca²⁺:glyphosate association in deposit residuals on the leaf surface of barley

Abstract

It has long been known that calcium ion antagonizes glyphosate, but it was not clear whether the stoichiometry of their interaction is 1:1 or 1:2. Two independent methods were used to determine which stoichiometry was the most probable. First, dose±response curves of barley (Hordeum vulgare L.) plants treated with glyphosate were determined in the presence of 0, 1.25, 2.5, 5 and 10 mM CaCl 2 . The doses of`free' glyphosate (not inactivated by calcium ion) were computed using the assumptions of 1:1 and 1:2 stoechiometries. The response curves were redrawn as a function of`free' glyphosate. Analysis showed that the 1:2 hypothesis could be rejected, whereas the 1:1 hypothesis was highly probable. Second, kinetics of glyphosate penetration into barley leaves were determined in the presence of 0, 2.5 and 10 mM CaCl 2 . The concentrations of`free' glyphosate were computed as above. The kinetics of glyphosate penetration at these concentrations were established and were compared to the kinetics of glyphosate penetration in the presence of CaCl 2 . Again, the 1:2 hypothesis was rejected, whereas the 1:1 hypothesis was more probable. These results strongly suggest that the stoichiometry of the Ca 2+ :glyphosate association is 1:1 in deposit residuals on the leaf surface.

Spray retention, foliar uptake and translocation of glufosinate and glyphosate in Ambrosia artemisiifolia

“…3). At doses ranging from 67.5 to 1080 g ha −1 , maximum uptake reached nearly 100% of the applied label, and t 1/2 was in the range 2.3–3 h. Contrary to previous experiments (Gauvrit et al. , 2001) with barley ( Hordeum vulgare L.), no lag time was observed before the onset of glyphosate uptake.…”

“…For example, uptake into Apocynum cannabinum L. was 19%, 36% and 53% of the applied label after 12, 72 and 288 h respectively (Schultz & Burnside, 1980). In a previous study on barley, we found t 1/2 values to be around 30 h (Gauvrit et al. , 2001), when compared with 2–3 h in this study.…”

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)…”

Glyphosate Response to Calcium, Ethoxylated Amine Surfactant, and Ammonium Sulfate1

“…This complex or salt is poorly absorbed by plants (Nilsson 1985); in addition, suggested that glyphosate is entrapped in a solid residue on the leaf surface. The stoichiometry of the glyphosatecalcium association was shown to be 1:1 (de Ruiter et al 2002;Gauvrit et al 2000Gauvrit et al , 2001, as suggested by Nalewaja et al (1996). Under conditions in France calcium concentration is rarely higher than 5 mM (Blondlot et al 1999).…”

“…It was confirmed that the chloroform wash recovered more than 96% of nonpenetrated 14 C-glyphosate, although glyphosate and its calcium salts or chelate are not soluble in chloroform (Franz et al 1997). Presumably, the chloroform dissolved the wax on which 14 C-glyphosate and its salts or chelate were present (Gauvrit et al 2001). To 75a D and C denote the upper and lower limits of dry matter at zero and at very large doses of glyphosate; ED 50 and ED 90 denote the doses of glyphosate required to reduce dry matter by 50 and 90%, respectively, between the upper and lower limits; and b is proportional to the slope of the curve around ED 50 .…”