Abstract
Drought events are predicted to increase in the future. Evaluating the response of herbicide-resistant and -susceptible weed ecotypes to progressive drought can provide insights into whether resistance traits affect the fitness of resistant weed populations. Two experiments were conducted in the greenhouse between January and May 2021 to evaluate drought tolerance differences between Palmer amaranth accessions resistant to S-metolachlor or glyphosate and their susceptible counterparts. The accessions used were S-metolachlor-resistant (17TUN-A), a susceptible standard (09CRW-A), and glyphosate-resistant (22–165 EPSPS copies) and glyphosate-susceptible (3–10 EPSPS copies) plants from accession 16CRW-D. Daily transpiration of each plant was measured. The daily transpiration rate was converted to normalized transpiration ratio (NTR) using a double-normalization procedure. The daily soil water content was expressed as a fraction of transpirable soil water (FTSW). The threshold FTSW (FTSWcr), after which NTR decreases linearly, was estimated using a two-segment linear regression analysis. The data showed differences between S-metolachlor-resistant and -susceptible accessions (p ≤ 0.05). The FTSW remaining in the soil at the breakpoint for the S-metolachlor-susceptible accession (09CRW-A) was 0.17 ± 0.007. The FTSW remaining in the soil at the breakpoint for the S-metolachlor-resistant accession (17TUN-A) was 0.23 ± 0.004. The FTSW remaining in the soil at the breakpoint for the glyphosate-resistant and glyphosate-susceptible plants (16CRW-D) was 0.25 ± 0.007 and 0.25 ± 0.008, respectively. Although the mechanism endowing resistance to S-metolachlor might have contributed to increased drought tolerance, follow-up experiments are needed in order to verify this finding. Increased EPSPS copy numbers did not improve the drought tolerance of Palmer amaranth. As droughts are predicted to increase in frequency and severity, these results suggest that S-metolachlor-resistant and glyphosate-resistant Palmer amaranth populations will not be at a competitive disadvantage compared to susceptible genotypes. Alternative and diverse management strategies will be required for effective Palmer amaranth control, regardless of herbicide resistance status.
Highlights
IntroductionDrought can negatively affect physiological and biochemical processes, and causes yield reduction [1]
The GST-mediated resistance to S-metolachlor may impart some tolerance to drought stress. This is a logical consequence of the diverse roles played by GSTs in general stress mitigation and intrinsic plant protection
This can be verified by developing near-isolines of resistant and susceptible plants and conducting stress experiments on these materials
Summary
Drought can negatively affect physiological and biochemical processes, and causes yield reduction [1]. The ‘flash drought’ event in the US in 2012 disrupted the production of major crops and affected international grain markets [2]. This most detrimental drought of the past century in the US Midwest had a greater impact on agricultural systems (crops and grasslands) than on forests [3]. Drought frequency and severity are likely to increase in the future [3–5]; yields of crops such as corn and soybeans are predicted to decline by 8–21% [6]. Seasonal water supply and soil water-holding capacity are major determinants of crop productivity [7]. Under irrigated conditions, irrigation does not always fully supply crop water demand during droughts [9]
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