Abstract:
Bean shoot parts that respond to glyphosate (N-
(phosphonomethyl)glycine) in ways useful for bioassay were determined
by applying glyphosate doses of 3.8 to 60.3 ug ae per plant to the
simple leaves when the first trifoliolate leaflets were about 1 cm
long. Dry weights of parts that were almost fully enlarged at
treatment were greater in treated than untreated plants after two
weeks. Maximum weight increase was found with the 15.1 ug dose.
Growth of younger shoot parts was reduced by glyphosate, showing linear
reductions with log dose from 3.8 to 30.2 ug. Growth reduction of
young shoot parts was therefore concluded to be a better measure of
sublethal glyphosate activity than reduction of total shoot growth.
Using the above assay, growth of bean plants from a controlled
environment was evaluated after treatment with a 3.0 mM glyphosate
solution applied in uniform drops of 138, 430, and 1230 um diameter.
The largest drops were less effective than both smaller sizes, and no
difference in activity between the two smaller sizes was found. Bean
plants grown outdoors in pots responded similarly to a 12.2 mM
glyphosate solution applied in drops of 138, 240, 430, 740, and 1230 um
diameter. There were no activity differences between the four smallest
sizes, and the 1230 um size was less effective than all others.
To ascertain effects of leaf coverage on glyphosate activity, 1.0
ul drops of glyphosate solution were applied to the simple leaves of
bean plants and physically spread with the tip of a small, glass rod to
cover areas of different size. Herbicidal activity was reduced with
increased drop spread on plants grown in a controlled environment, but
not with plants grown outdoors. Cuticular adsorption of glyphosate was
assumed not to be a factor in reducing activity because isolated leaf
cuticles from beans grown both outdoors and in a growth chamber were
shaken in an aqueous solution of ¹⁴C-glyphosate at 25 C and showed no
adsorptive tendency throughout an 83-hour period.
The hypothesis that glyphosate applied in low volumes is more
effective because of reduced leaf surface contact is not supported by
drop size data, and is supported only by results from controlled
environment treatments showing less activity from increased drop
spread.
