It is well known that ammonium sulphate (AIMS) overcomes the antagonistic e
ffect 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 tol
erant broadleaved weeds (Abutilon theophrasti Medik and Convolvulus arvensi
s L) and one susceptible grass (Sorghum halepense L.). The increase in acti
vity 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 i
ncrease in activity against A theophrasti with AIMS, with lower levels of e
nhancement on the other two species. This matched the higher calcium conten
t of A theophrasti (3.1% dry weight) compared to C arvensis (1.5%) and S ha
lepense (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 the
ophrasti. Misting of leaf surfaces promoted the leaching of substances rich
in calcium from within the leaf on to the surface. In addition, species su
ch as A theophrasti have specialised trichomes known as chalk glands which
are a source of cations. Data are presented which show that, in uptake stud
ies with [C-14]glyphosate-trimesium, anomalous results may be found unless
an acidified wash solution is used to remove residual surface material, whi
ch remains as an insoluble deposit with aqueous methanol washing. It is pro
posed that the reduced potency shown by glyphosate on certain species such
as A theophrasti can be partially explained by the interaction of the herbi
cide anion with antagonistic cations both on and within the plant. (C) 2000
Society of Chemical Industry.