TITLE:
Comparison of WeatherMax® and EngameTM Formulations of Glyphosate on Cotyledon Surface Structure, Chlorophyll A Fluorescence and Shikimate Levels in Isogenic Cotton Cultivars Differing in Roundup Resistance
AUTHORS:
William T. Molin, Andrew J. Bowling, Kevin C. Vaughn
KEYWORDS:
Glyphosate, EngameTM, WeatherMax®, Cuticle Damage, Shikimate, Uptake
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.11 No.8,
August
11,
2020
ABSTRACT: The effects of Roundup WeatherMax® and EngameTM formulations of glyphosate were investigated on the cotyledons of
glyphosate resistant (GR) and glyphosate sensitive (GS) isogenic cotton
cultivars. EngameTM is a mixture of glyphosate and 1-aminomethanamide dihydrogen tetraoxosulfate
(AMADS). Fully expanded cotton cotyledons treated with EngameTM or AMADS developed surface lesions within 2 hours after treatment
whereas surfactant-treated control or WeatherMax®-treated tissues did not develop lesions. The EngameTM and AMADS damage appeared as depressions which were confirmed by scanning electron microscopy. Light micrographs
of cross sections through the depressions revealed collapsed and compressed
epidermal and mesophyll cells with congealed cytoplasmic contents in the
palisade and spongy mesophyll cells. Changes to photosynthetic electron
transport were evident at 4 hours after treatment (HAT) in all treatments as revealed
by chlorophyll A fluorescence. In GR cotton, the fluorescence perturbations
decreased with time such that at 72 HAT EngameTM-treated cotyledons could not be distinguished from the surfactant-or WeatherMax®-treated plants. The GS cotton continued to show progressive decreases in
the fluorescence parameters Fv/Fm and performance index (PI) to 72 HAT.
Shikimate levels increased following glyphosate treatment in glyphosate
sensitive cotton and EngameTM caused a two-to three-fold greater increase in shikimate compared to WeatherMax®. These results indicate that the EngameTM-based glyphosate formulation involved structural tissue damage which
likely increased glyphosate uptake and subsequently increased inhibition of
photosynthesis and the shikimate pathway.