Abstract
The non-judicious use of herbicides has led to a widespread evolution of herbicide resistance in various weed species including Palmer amaranth, one of the most aggressive and troublesome weeds in the United States. Early detection of herbicide resistance in weed populations may help growers devise alternative management strategies before resistance spreads throughout the field. In this study, Raman spectroscopy was utilized as a rapid, non-destructive diagnostic tool to distinguish between three different glyphosate-resistant and four -susceptible Palmer amaranth populations. The glyphosate-resistant populations used in this study were 11-, 32-, and 36-fold more resistant compared to the susceptible standard. The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number for these resistant populations ranged from 86 to 116. We found that Raman spectroscopy could be used to differentiate herbicide-treated and non-treated susceptible populations based on changes in the intensity of vibrational bands at 1156, 1186, and 1525 cm–1 that originate from carotenoids. The partial least squares discriminant analysis (PLS-DA) model indicated that within 1 day of glyphosate treatment (D1), the average accuracy of detecting herbicide-treated and non-treated susceptible populations was 90 and 73.3%, respectively. We also found that glyphosate-resistant and -susceptible populations of Palmer amaranth can be easily detected with an accuracy of 84.7 and 71.9%, respectively, as early as D1. There were relative differences in the concentration of carotenoids in plants with different resistance levels, but these changes were not significant. The results of the study illustrate the utility of Raman spectra for evaluation of herbicide resistance and stress response in plants under field conditions.
Highlights
Weeds compete with crop plants for critical resources and cause severe yield losses, if not managed adequately (Oerke, 2006)
The variation in resistance levels of the tested resistant populations in the current study presented an appropriate case for differentiation through Raman spectroscopy (RS)
The partial least square discriminant analysis (PLS-DA) analysis of Raman spectra indicates that glyphosate-treated and non-treated susceptible populations can be differentiated with an average accuracy of 93.45% and 82.1% at D1 and D2, respectively (Table 3)
Summary
Weeds compete with crop plants for critical resources and cause severe yield losses, if not managed adequately (Oerke, 2006). Herbicides are the most commonly used tool for weed control in modern agriculture; repeated use of few herbicide modes of action (MOA) has led to a widespread evolution of herbicide-resistant weeds in cropping systems. In the United States cropping systems, weeds resistant to glyphosate (the active ingredient in the herbicide Roundup R ) have become a major production challenge (Garetson et al, 2019). The first case of glyphosate resistance in Palmer amaranth Wats.), the most troublesome weed in the United States cropping systems (Van Wychen, 2017), was confirmed in 2004 in Georgia (Culpepper et al, 2006). 30 states in the United States have reported the occurrence of glyphosate-resistant Palmer amaranth (Heap, 2020)
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