Plants as sources of cations antagonistic to glyphosate activity

It is well known that ammonium sulphate (AMS) overcomes the antagonistic effect of cations found in hard water on glyphosate activity. In this paper we examine the effect of AMS on glyphosate‐trimesium activity when sprayed in deionised water and following hydroponic application to roots of two tolerant broadleaved weeds (Abutilon theophrasti Medik and Convolvulus arvensis L) and one susceptible grass (Sorghum halepense L). The increase in activity from foliar sprays with AMS was greatest on A theophrasti, followed by C arvensis, with no significant improvement on S halepense. Following root treatment of the same weed species, glyphosate‐trimesium showed a marked increase in activity against A theophrasti with AMS, with lower levels of enhancement on the other two species. This matched the higher calcium content of A theophrasti (3.1% dry weight) compared to C arvensis (1.5%) and S halepense (1.0%). Scanning electron microscopy and X‐ray microanalysis were used to identify the presence of antagonistic elements (Mg and Ca) on leaf surfaces of A theophrasti. Misting of leaf surfaces promoted the leaching of substances rich in calcium from within the leaf on to the surface. In addition, species such as A theophrasti have specialised trichomes known as chalk glands which are a source of cations. Data are presented which show that, in uptake studies with [14C]glyphosate‐trimesium, anomalous results may be found unless an acidified wash solution is used to remove residual surface material, which remains as an insoluble deposit with aqueous methanol washing. It is proposed that the reduced potency shown by glyphosate on certain species such as A theophrasti can be partially explained by the interaction of the herbicide anion with antagonistic cations both on and within the plant. © 2000 Society of Chemical Industry