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.
Highlights
Glyphosate-based herbicides are broad spectrum, nonselective, post-emergent products having high unit activity on a wide variety of annual and perennial weeds [1]
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 shiki
The objectives of this research were four-fold: 1) to compare the Roundup Weathermax® and EngameTM formulations of glyphosate on isogenic cotton cultivars differing in resistance to glyphosate (Delta and Pine Land cultivar 491 glyphosate-sensitive (GS) and 494 glyphosate-resistant (GR), 2) to document changes in leaf surface anatomy caused by EngameTM, 3) to determine changes in response time in shikimate accumulation between these formulations as an indicator of glyphosate action, and 4) to determine and compare the chlorophyll A fluorescence (Chl A) parameters, the ratio of variable fluorescence to maximum fluorescence (Fv/Fm), and the performance index (PI), as means to confirm physiological injury
Summary
Glyphosate-based herbicides are broad spectrum, nonselective, post-emergent products having high unit activity on a wide variety of annual and perennial weeds [1]. Glyphosate-based herbicides are generally formulated as a mixture of a soluble salt and proprietary surfactants and adjuvants that increase dispersion and retention on the leaf surface, and penetration through the hydrophobic, transport limiting cuticle [4] [5]. Active research into glyphosate formulations may have occurred in anticipation of Monsanto patents on glyphosate expiring outside the USA in 1991 and in the US in 2000. Additives, such as ammonium sulfate, were already known to increase the phytotoxicity of many water-soluble post-emergence herbicides in glyphosate [2] [11] [18]. Ammonium sulfate reduced the concentration of glyphosate isopropylamine salt to cause a 50% inhibition of velvetleaf dry weight accumulation by two- to five-fold [10]
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