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

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.