Glyphosate interaction with manganese in tank mixtures and its effect on glyphosate absorption and translocation

Abstract

Recent reports indicate that manganese (Mn), applied as a foliar fertilizer in tank mixtures with glyphosate, has the potential to antagonize glyphosate efficacy and reduce weed control. It was hypothesized that Mn2+ complexed with glyphosate in a similar manner to Ca2+, forming salts that were not readily absorbed and, thereby, reducing glyphosate efficacy. This study was conducted to confirm the interaction of Mn2+ and glyphosate and to measure the effect of Mn on glyphosate absorption and translocation in velvetleaf. In aqueous solutions, Mn2+ binds with solvent molecules and with chelating agents to form hexacoordinate complexes. The distribution of paramagnetic species, both the free manganous ion ([Mn{H2O}6]2+) and the Mn2+–glyphosate complex, in Mn–glyphosate solutions at various pH values were analyzed using electron paramagnetic resonance (EPR) spectroscopy. Glyphosate interaction with Mn appeared to increase as the pH was increased from spray solution levels (2.8 to 4.5) to levels common in the plant symplast (7.5). Growth chamber bioassays were conducted to measure absorption and translocation of 14C-labeled glyphosate in solution with four Mn fertilizers: Mn-ethylaminoacetate (Mn-EAA), Mn-ethylenediaminetetraacetate (Mn-EDTA), Mn-lignin sulfonate (Mn-LS), and Mn-sulfate (MnSO4). Mn-EDTA did not interfere with glyphosate efficacy, absorption, or translocation. However, both MnSO4 and Mn-LS reduced glyphosate efficacy, absorption, and translocation. Mn-EAA severely antagonized glyphosate efficacy, and although glyphosate in tank mixtures with Mn-EAA was absorbed rapidly, little was translocated from the treated leaf. The Mn-EAA fertilizer contained approximately 0.5% iron (Fe) not reported on the fertilizer label. Iron is presumed to be partially responsible for the very limited translocation of glyphosate from the treated leaf in Mn-EAA tank mixtures. Adding ammonium sulfate increased the efficacy, absorption, and translocation of glyphosate for each Mn fertilizer tank mixture.

Foliar-Applied Glyphosate Substantially Reduced Uptake and Transport of Iron and Manganese in Sunflower (Helianthus annuus L.) Plants

“…Accumulation of glyphosate in roots may result in the formation of immobile Fe and Mn complexes and consequently limit root-to-shoot transport of essential metal nutrients. Glyphosate-treated plants can accumulate up to 0.3 mM glyphosate in root tissues (34, cited by 7) that can immobilize micronutrients in roots as found in spray solutions or leaf tissues (20,23,24). The complexing action of glyphosate in roots is probably the main reason for the marked depression in root-to-shoot transport of Mn and Fe by glyphosate ( Figure 2).…”

“…The antagonism between cationic mineral nutrients and glyphosate has been studied primarily in terms of impaired leaf absorption (penetration) and limited translocation of glyphosate to explain reduced effectiveness of glyphosate to kill target plants (19)(20)(21)(22)(23)(24). In sunflower and velvetleaf plants, cationic nutrients such as Mn, Fe, and Ca bind to the glyphosate molecule via its carboxyl and phosphonate groups to form stable complexes with glyphosate (20,22,23). Such complexes severely reduced the absorption and translocation of glyphosate within the treated tissue and thus limited its efficacy in weed control.…”

Residual effect of mixture of glyphosate and 2,4-D in winter maize in different soil textures

“…The herbicide glyphosate, for being a phosphonic acid, can act in the direct degradation of chlorophyll a, through demetallation, forming complex with the magnesium ion, and in the indirect degradation, through immobilization of magnesium and manganese, necessary for the formation of chlorophyll and photosynthesis, respectively (Bernards et al, 2005). Kitchen et al (1981) observed harmful effect caused by glyphosate on the synthesis of aminolevulinic acid (ALA), a precursor in the biosynthesis of chlorophylls a and b, since both enzymes responsible for their biosynthesis, catalase and peroxidase, are very sensitive to the deficiency of iron, which is one of the elements that can be chelated by glyphosate.…”

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

“…Iron and Mn in spray solutions are known to inhibit glyphosate herbicidal activity by limiting absorption and translocation of glyphosate in treated leaves (Bernards et al 2005b).…”