Glyphosate Translocation Research Articles

Proline-106 EPSPS Mutation Imparting Glyphosate Resistance in Goosegrass (Eleusine indica) Emerges in South America

AbstractGlyphosate-resistant (GR) goosegrass [Eleusine indica(L.) Gaertn.] was recently identified in Brazil, but its resistance mechanism was unknown. This study elucidated the resistance mechanism in this species and developed a molecular marker for rapid detection of this target-site resistance trait. The resistance factor for the resistant biotype was 4.4-fold compared with the glyphosate-susceptible (GS) in greenhouse dose–response experiments. This was accompanied by a similar (4-fold) difference in the levels of in vitro andin plantashikimate accumulation in these biotypes. However, there was no difference in uptake, translocation, or metabolism of glyphosate between the GS and GR biotypes. Moreover, both biotypes showed similar values for 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number and transcription. Sequencing of a 330-bp fragment of theEPSPSgene identified a single-nucleotide polymorphism that led to a Pro-106-Ser amino acid substitution in the enzyme from the GR biotype. This mutation imparted a 3.8-fold increase in the amount of glyphosate required to inhibit 50% of EPSPS activity, confirming the role of this amino acid substitution in resistance to glyphosate. A quantitative PCR–based genotyping assay was developed for the rapid detection of resistant plants containing this Pro-106-Ser mutation.

Glyphosate resistant soybean growth and yield affected by glyphosate and phosphate fertilization

Glyphosate is the most used herbicide worldwide and glyphosate-tolerant soybean planted in almost all cultivated areas. In Brazil, the area cultivated with transgenic soybean reaches more than 90%. Recently, the application of glyphosate in tolerant soybean has assumed the possibility to injury the crop under certain conditions and herbicides formulations. Phosphate fertilization can directly influence glyphosate uptake in plants because glyphosate translocation inside cells is related with phosphate transporters. Therefore, a study was conducted in two steps intended to determine if different doses of phosphate fertilization and/or glyphosate mode of spraying could modify glyphosate susceptibility of glyphosate-tolerant soybean, affecting crop development and yield. We tested two glyphosate doses and its interaction with mode of spraying and different glyphosate doses interaction with phosphate soil-fertilization. Experiment I was set up in a two by two factorial design, testing two doses of isopropylamine salt glyphosate (960 and 1,440 g ae ha-1) and two modes of glyphosate application (single spraying and sequential spraying). Experiment II was set up in a two by three factorial design, testing two doses of glyphosate (960 and 1,440 g ae ha-1) and three doses of phosphate fertilization (54, 108 and 162 kg ha-1 of P2O5). In both experiments, weed community was evaluated based on number of individuals and their respective dry mass accumulation. Crop was evaluated in dry mass of leaves, dry mass of stem, dry mass of pods, dry mass of shoot, plant height and grain yield. The use of glyphosate in a sequential spraying (960 + 480 g ae ha-1) or in higher dose (1,440 g ae ha-1) provides a highly efficient weed control and a high-performance crop growth and yield. The additional soil-fertilization with phosphate in this case does not affect soybean since no crop injury was observed.

Glyphosate-Resistant Junglerice (Echinochloa colona) from Mississippi and Tennessee: Magnitude and Resistance Mechanisms

AbstractRecently, several incidents of glyphosate failure on junglerice [Echinochloa colona(L.) Link] have been reported in the midsouthern United States, specifically in Mississippi and Tennessee. Research was conducted to measure the magnitude of glyphosate resistance and to determine the mechanism(s) of resistance to glyphosate inE. colonapopulations from Mississippi and Tennessee. ED50(dose required to reduce plant growth by 50%) values for a resistant MSGR4 biotype, a resistant TNGR population, and a known susceptible MSGS population were 0.8, 1.62, and 0.23 kg ae ha−1of glyphosate, respectively. The resistance index calculated from the these ED50values indicated that the MSGR4 biotype and TNGR population were 4- and 7-fold, respectively, resistant to glyphosate relative to the MSGS population. The absorption patterns of [14C]glyphosate in the TNGR and MSGS populations were similar. However, the MSGS population translocated 13% more [14C]glyphosate out of the treated leaf compared with the TNGR population at 48 h after treatment.EPSPSgene sequence analyses of TNGRE. colonaindicated no evidence of any point mutations, but several resistant biotypes, including MSGR4, possessed a single-nucleotide substitution of T for C at codon 106 position, resulting in a proline-to-serine substitution (CCA to TCA). Results from quantitative polymerase chain reaction analyses suggested that there was no amplification of theEPSPSgene in the resistant populations and biotypes. Thus, the mechanism of resistance in the MSGR population (and associated biotypes) is, in part, due to a target-site mutation at the 106 loci of theEPSPSgene, while reduced translocation of glyphosate was found to confer glyphosate resistance in the TNGR population.

Non‐target‐site mechanism of glyphosate resistance in Italian ryegrass (Lolium multiflorum)

In Shizuoka Prefecture, Japan, glyphosate‐resistant Lolium multiflorum is a serious problem on the levees of rice paddies and in wheat fields. The mechanism of resistance of this biotype was analyzed. Based on LD50, the resistant population was 2.8–5.0 times more resistant to glyphosate than the susceptible population. The 5‐enolpyruvyl‐shikimate‐3‐phosphate synthase (EPSPS) gene sequence of the resistant biotype did not show a non‐synonymous substitution at Pro106, and amplification of the gene was not observed in the resistant biotype. The metabolism and translocation of glyphosate were examined 4 days after application through the direct detection of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) using liquid chromatograph‐tandem mass spectrometer (LC‐MS/MS). AMPA was not detected in either biotype in glyphosate‐treated leaves or the other plant parts. The respective absorption rates of the susceptible and resistant biotypes were 37.90 ± 3.63% and 41.09 ± 3.36%, respectively, which were not significantly different. The resistant biotype retained more glyphosate in a glyphosate‐treated leaf (91.36 ± 1.56% of absorbed glyphosate) and less in the untreated parts of shoots (5.90 ± 1.17%) and roots (2.76 ± 0.44%) compared with the susceptible biotype, 79.58 ± 3.73%, 15.77 ± 3.06% and 4.65 ± 0.89%, respectively. The results indicate that the resistance mechanism is neither the acquisition of a metabolic system nor limiting the absorption of glyphosate but limited translocation of the herbicide in the resistant biotype of L. multiflorum in Shizuoka Prefecture.

The role of plant size in the selection of glyphosate resistance in Sorghum halepense.

The effect of plant size (seedlings versus young plants versus adult plants) on the phenotypic level of glyphosate resistance and selection intensity (SI) in Sorghum halepense with and without a reduced glyphosate translocation resistance mechanism was evaluated. Resistance parameters [the 50% lethal dose (LD50 ) and the dose required to cause a 50% reduction in plant growth (GR50 )] in adult plants were notably higher than in seedlings regardless of the resistance status. However, under similar plant size increases, populations comprised of glyphosate-resistant (R) individuals showed higher survival and growth when glyphosate treated compared with glyphosate-susceptible (S) plants. An increase in SI was always evident with increasing glyphosate doses. However, the rate of increase in SI was higher under glyphosate selection of young R and S plants, followed by seedlings and adult R and S plants. However, in conditions of R seedlings coexisting with adult S plants under glyphosate treatment (1000-4000 g ha-1 ), selection against glyphosate resistance was observed. Any increase in size from the seedling stage of R plants translates into an amplification of resistance. Depending on the particular size combinations of spatially coexisting R and S plants, selection for glyphosate resistance may be faster, slower or even not evident. © 2018 Society of Chemical Industry.

Reduced Translocation of Glyphosate and Dicamba in Combination Contributes to Poor Control of Kochia scoparia: Evidence of Herbicide Antagonism

Kochia scoparia is a troublesome weed across the Great Plains of North America. Glyphosate and dicamba have been used for decades to control K. scoparia. Due to extensive selection, glyphosate- and dicamba-resistant (GDR) K. scoparia have evolved in the USA. Herbicide mixtures are routinely used to improve weed control. Herbicide interactions if result in an antagonistic effect can significantly affect the management of weeds, such as K. scoparia. To uncover the interaction of glyphosate and dicamba when applied in combination in K. scoparia management the efficacies of different doses of glyphosate plus dicamba were evaluated under greenhouse and field conditions using GDR and a known glyphosate- and dicamba-susceptible (GDS) K. scoparia. The results of greenhouse and field studies suggest that the combination of glyphosate and dicamba application controlled GDS, but glyphosate alone provided a better control of GDR K. scoparia compared to glyphosate plus dicamba combinations. Furthermore, investigation of the basis of this response suggested glyphosate and dicamba interact antagonistically and consequently, the translocation of both herbicides was significantly reduced resulting in poor control of K. scoparia. Therefore, a combination of glyphosate plus dicamba may not be a viable option to control GDR K. scoparia.

Restricted glyphosate translocation in Lolium multiflorum is controlled by a single incomplete dominant nuclear gene

ABSTRACTThe mode of inheritance for herbicide resistance was investigated in a population of Lolium multiflorum (Italian ryegrass) with restricted glyphosate translocation mechanism of resistance. The degree of dominance for glyphosate resistance in Italian ryegrass was evaluated in the progenies of F1 families which were created by reciprocal-crossing between resistant (R) and susceptible (S) parental individuals. The results showed an intermediate level of glyphosate resistance for F1 families compared to that of the parental types. The phenotypic resistance segregation was also investigated using backcross families which were created by pair-crossing the F1 individuals to the susceptible population, and the observed segregation values fitted a one-gene model very well. Thus it appears that glyphosate resistance in the Italian ryegrass obtained from a New Zealand vineyard with restricted herbicide translocation is controlled by a single incompletely dominant nuclear gene.

Reduced Translocation Is Associated with Antagonism of Glyphosate by Glufosinate in Giant Foxtail (Setaria faberi) and Velvetleaf (Abutilon theophrasti)

Previous reports have underscored antagonism that may result from mixing glyphosate and glufosinate across a wide range of application rates and for various broadleaf and grass weed species, but no investigation has been conducted to characterize glyphosate absorption and translocation when combined with glufosinate. The objectives of this study were to evaluate herbicide efficacy and assess herbicide interaction and physiological response with combinations of glyphosate and glufosinate on common lambsquarters, velvetleaf, and giant foxtail. Greenhouse studies to determine interaction resulted in strong and persistent antagonism between glyphosate at 110 and 220 g ae ha−1and glufosinate at 20 or 40 g ae ha−1in giant foxtail, whereas only transient and reduced antagonism was noted for velvetleaf and common lambsquarters. Combining glyphosate and glufosinate increased the maximum absorption of glyphosate by 9% and 23% in velvetleaf and giant foxtail, respectively, compared with glyphosate alone. In velvetleaf, averaged over time, only 2.6% of the applied radioactivity translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 9.9% when glyphosate was applied alone. In giant foxtail, 21.6% of the [14C]glyphosate translocated out of the treated leaf when glufosinate was mixed with glyphosate compared with 52.4% when glyphosate was applied alone. Conversely, no change in the radioactive pattern of translocation was noted for common lambsquarters. These results suggest that reduced translocation of glyphosate is the physiological mechanism responsible for the antagonism observed between glyphosate and glufosinate in giant foxtail and, to a lesser extent, in velvetleaf.

Environmental Factors Moderate Glyphosate-induced Antagonism of POST Herbicides on the Rapid Response Biotype of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida)

AbstractIn the rapid response (RR) biotype of glyphosate-resistant (GR) giant ragweed (Ambrosia trifidaL.), exposure to glyphosate elicits H2O2production in mature leaves, resulting in foliage loss and reduced glyphosate translocation. When glyphosate is applied with POST herbicides intended to improve control ofA. trifida, the RR to glyphosate has the propensity to antagonize these herbicide combinations. This research documents how transient changes in air temperature, soil moisture, and light intensity during a 6-d period surrounding herbicide application regulate induction of the RR and the effect on POST herbicide interactions with glyphosate. Air temperature had the greatest influence on H2O2accumulation in leaf disks following treatment with glyphosate, as plants at 30 C produced more than twice the amount of H2O2at 2.5 h after treatment compared with 10 C. Plants under field capacity conditions accumulated nearly 50% more H2O2than those at one-third field capacity, while those under no shade had only 18% more H2O2compared with those in a shaded environment. Despite these initial results, dry weight reduction at 21 d after treatment never differed by more than 8% between levels of environmental factors, thus indicating a negligible influence on glyphosate efficacy. The magnitude of glyphosate-induced antagonism was generally greater at 30 C (12% to 21% less than expected control) versus 10 C (11% to 16%) on atrazine, cloransulam, dicamba, and topramezone and was greater at field capacity (20% to 24%) versus one-third field capacity (11% to 15%) on cloransulam and topramezone. These results indicate air temperatures and soil moisture levels conducive to optimal plant growth accelerate the RR to glyphosate, thereby increasing the likelihood of glyphosate-induced antagonism on several translocated herbicides.

Glyphosate-Induced Antagonism in Rapid Response Giant Ragweed (Ambrosia trifida)

AbstractGlyphosate application to the rapid-response (RR) biotype of glyphosate-resistant (GR) giant ragweed ensues in loss of foliage via rapid tissue death, thereby reducing glyphosate translocation. Experiments were performed to determine if this GR response, in contrast to a non-rapid response (NRR) GR biotype, results in antagonism of the selective herbicides atrazine, cloransulam, dicamba, lactofen, and topramezone. Application of glyphosate at 1,680 g ae ha–1in the greenhouse resulted in antagonism between all five selective herbicides for the RR biotype, whereas glyphosate applied at 420 g ha–1was antagonistic only for cloransulam. Application of selective herbicides 2 d prior to glyphosate treatment avoided the antagonism observed in the RR biotype. In the field, glyphosate mixtures with dicamba and topramezone were antagonistic on the RR biotype across both 2015 and 2016 field seasons. Thus, the RR effectively reduces glyphosate efficacy but also has potential to diminish the activity of glyphosate mixtures with selective herbicides, and the degree of antagonism between these mixtures escalates at increasing glyphosate rates.

Different levels of glyphosate-resistant Lolium rigidum L. among major crops in southern Spain and France

Herbicides are the most effective tools for controlling almost 99% of weeds. However, herbicide resistance is a primary concern in modern agriculture. The characterization in new areas and elucidation of the mechanisms of resistance are of vital importance in maintaining the sustainability of herbicides, including glyphosate. Nine populations of Lolium rigidum, showing different response patterns, were characterized as being glyphosate resistant (GR). The wide range of values in fresh weight reduction, survival, shikimic acid and EPSPS enzyme activity indicates a different or a combination resistance mechanism. The Line-3 population resulted in minimum reduction of fresh weight and survival values with respect to the glyphosate-susceptible (GS) population, showing 16.05- and 17.90-fold higher values, respectively. There were significant differences in the 14C-glyphosate translocation between GR and GS populations. Moreover, there were differences among the nine GR populations, but they exhibited a reduction in the remaining glyphosate translocation in the treated leaf. The EPSPS gene sequence revealed a Pro-106-Ser substitution in four populations, which could be characterized as being GR with non-target-site and target-site resistance mechanisms. This complexity of several resistance mechanisms makes it necessary to develop long-term integrated weed management strategies to limit further resistance dispersal.

Uptake, Translocation, Metabolism, and Distribution of Glyphosate in Nontarget Tea Plant (Camellia sinensis L.).

The uptake, translocation, metabolism, and distribution behavior of glyphosate in nontarget tea plant were investigated. The negative effects appeared to grown tea saplings when the nutrient solution contained glyphosate above 200 mg L-1. Glyphosate was highest in the roots of the tea plant, where it was also metabolized to aminomethyl phosphonic acid (AMPA). The glyphosate and AMPA in the roots were transported through the xylem or phloem to the stems and leaves. The amount of AMPA in the entire tea plant was less than 6.0% of the amount of glyphosate. The glyphosate level in fresh tea shoots was less than that in mature leaves at each day. These results indicated that free glyphosate in the soil can be continuously absorbed by, metabolized in, and transported from the roots of the tea tree into edible leaves, and therefore, free glyphosate residues in the soil should be controlled to produce teas free of glyphosate.

Is glyphosate-resistant ryegrass resistant to paraquat?


 
 
 Sequestration of herbicide into vacuoles is considered to be the main mechanism of resistance to both glyphosate and paraquat worldwide. In New Zealand, the first case of glyphosate resistance was found in ryegrass (Lolium) species, and the restricted herbicide translocation was found to be the main mechanism of resistance in the studied populations, presumably through sequestration. Overseas researchers hypothesised that the mechanism responsible for glyphosate resistance could also cause resistance to paraquat. We examined this hypothesis by comparing a known glyphosate-resistant population of perennial ryegrass with a known susceptible population after spraying with different rates (25—800 g ai/ha) of paraquat. The glyphosate-resistant population responded similarly to the susceptible population at the different rates of paraquat application. This result suggests that the restricted glyphosate translocation mechanism does not necessarily lead to paraquat resistance. These results also suggest that paraquat could be useful for controlling ryegrass when glyphosate resistance has evolved and the application of paraquat is permitted.
 
 

Pro-106-Ser mutation and EPSPS overexpression acting together simultaneously in glyphosate-resistant goosegrass (Eleusine indica)

Glyphosate has been used for more than 15 years for weed management in citrus groves in the Gulf of Mexico, at up to 3–4 applications per year. Goosegrass (Eleusine indica (L.) Gaertn.) control has sometimes failed. In this research, the mechanisms governing three goosegrass biotypes (Ein-Or from an orange grove, and Ein-Pl1 and Ein-Pl2 from Persian lime groves) with suspected resistance to glyphosate were characterized and compared to a susceptible biotype (Ein-S). Dose-response and shikimate accumulation assays confirmed resistance of the resistant (R) biotypes. There were no differences in glyphosate absorption, but the R biotypes retained up to 62–78% of the herbicide in the treated leaf at 96 h after treatment (HAT), in comparison to the Ein-S biotype (36%). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity in the Ein-Or and Ein-S biotypes was over 100-fold lower than the Ein-Pl1 and Ein-Pl2 ones. The latter showed a high EPSPS-basal activity, a mutation at Pro-106-Ser position in the EPSPS gene, and EPSPS overexpression. The EPSPS basal and EPSPS overexpression were positively correlated. The R goosegrass biotypes displayed poor glyphosate translocation. Furthermore, this grassweed showed, for the first time, two mechanisms at the target-site level (Pro-106-Ser mutation + EPSPS overexpression) acting together simultaneously against glyphosate.

Glyphosate resistance in Ambrosia trifida: Part 2. Rapid response physiology and non-target-site resistance.

The glyphosate-resistant rapid response (GR RR) resistance mechanism in Ambrosia trifida is not due to target-site resistance (TSR) mechanisms. This study explores the physiology of the rapid response and the possibility of reduced translocation and vacuolar sequestration as non-target-site resistance (NTSR) mechanisms. GR RR leaf discs accumulated hydrogen peroxide within minutes of glyphosate exposure, but only in mature leaf tissue. The rapid response required energy either as light or exogenous sucrose. The combination of phenylalanine and tyrosine inhibited the rapid response in a dose-dependent manner. Reduced glyphosate translocation was observed in GR RR, but only when associated with tissue death caused by the rapid response. Nuclear magnetic resonance studies indicated that glyphosate enters the cytoplasm and reaches chloroplasts, and it is not moved into the vacuole of GR RR, GR non-rapid response or glyphosate-susceptible A. trifida. The GR RR mechanism of resistance is not associated with vacuole sequestration of glyphosate, and the observed reduced translocation is likely a consequence of rapid tissue death. Rapid cell death was inhibited by exogenous application of aromatic amino acids phenylalanine and tyrosine. The mechanism by which these amino acids inhibit rapid cell death in the GR RR phenotype remains unknown, and it could involve glyphosate phytotoxicity or other agents generating reactive oxygen species. Implications of these findings are discussed. The GR RR mechanism is distinct from the currently described glyphosate TSR or NTSR mechanisms in other species. © 2017 Society of Chemical Industry.

Evidence, Mechanism and Alternative Chemical Seedbank-Level Control of Glyphosate Resistance of a Rigid Ryegrass (Lolium rigidum) Biotype from Southern Spain.

Rigid ryegrass (Lolium rigidum) is one of the most troublesome weeds in different crops in the Mediterranean region. A rigid ryegrass biotype from an olive grove in Jaén province (Andalusía, southern Spain), potentially resistant to glyphosate (RG), was tested for its resistance level through dose-response assays using a susceptible biotype (SG). To test the hypothesis of a non-target-site-based resistance, as point mutations are far less common mechanisms of glyphosate resistance, studies were also conducted to elucidate whether resistance was associated with biochemical, metabolism, molecular and/or physiological mechanisms. Alternative herbicide-based control options, including single-herbicide or herbicide mixtures with glyphosate, applied at seedling, tillering or full heading stages, were tested in field experiments for 2 years for their efficacy against rigid ryegrass plants and their effects on the soil seed bank. Resistance levels of the RG biotype were 23- (LD50) and 7-fold (GR50) higher compared to the SG biotype. The SG biotype exhibited a significantly greater shikimic acid accumulation than the RG one. At 96 HAT, 58 and 89% of applied 14C-glyphosate was up taken by leaves of RG and SG biotype plants, respectively, and, at this time, a significantly higher proportion of the glyphosate taken up by the treated leaf remained in its tissue in RG plants compared to the SG ones. The RG biotype did not reveal any point mutation in the glyphosate target site EPSP synthase. Overall, results confirmed reduced glyphosate uptake and translocation as being the mechanism involved in glyphosate resistance in the RG biotype. RG biotype responses to the alternative treatments tested in situ indicated that herbicide applications at the later growth stage tended to be less effective in terms of immediate effects on population size than earlier applications, and that only in some cases, the removal of at least 85% of the RG biotype was achieved. However, with few exceptions, the alternative treatments tested appeared to be highly effective in reducing the seed bank irrespective of the growth stage. The frequency of the resistant phenotype in the progeny of surviving plants of the RG biotype was dependent on treatment. Results suggest that a potential exists for effective management of glyphosate-resistant rigid ryegrass in olive groves in southern Spain.

Evaluation of the Mechanism Responsible for the Low-Level Resistance to Glyphosate in Goosegrass

ABSTRACT Goosegrass (Eleusine indica) infests various crops, being reported as low-level resistant (LLR) to glyphosate. The objectives of this study were to evaluate the mechanisms of resistance to glyphosate in goosegrass by the cyt-P450 complex to identify changes in leaf anatomy of these plants and quantify the accumulation of shikimate in biotypes susceptible and with LLR. The metabolic rate was assessed by applying the cyt-P450 malathion inhibitor 30 minutes before the herbicide application. The analysis of the leaf anatomy consisted of a characterization of anatomic cross-sections and evaluation of the effect of glyphosate on the leaf epidermis and the conducting vessels. Plant tissue was collected 24 hours after treatment (HAT) of the dose-response curve to evaluate the accumulation of shikimate. The LLR biotype has not shown glyphosate metabolism by the early application of malathion due to the control of about 65% with or without applying the inhibitor. There is no anatomical evidence to determine the differential absorption of glyphosate by LLR biotypes. However, there is evidence of lower absorption and translocation of glyphosate in this biotype by lower stomatal density and a smaller diameter of the phloem. However, the LLR biotype presents diameter of phloem of 0.027 mm and stomatal density of 91.23 mm-2, values which are below the observed ones on the susceptible biotype. The susceptible biotype presents shikimate accumulation 17% higher compared to the biotype with LLR at a dose of 1,080 g e.a. ha-1, allowing to infer that the resistance mechanism is related to the enzyme insensitivity to the herbicide, which enables the use of appropriate management practices to control this weed.

Glyphosate resistance in common ragweed (Ambrosia artemisiifoliaL.) from Mississippi, USA

Glyphosate is one of the most commonly used broad‐spectrum herbicides over the last 40 years. Due to the widespread adoption of glyphosate‐resistant (GR) crop technology, especially corn, cotton and soybean, several weed species have evolved resistance to this herbicide. Research was conducted to confirm and characterize the magnitude and mechanism of glyphosate resistance in two GR common ragweed ( A mbrosia artemisiifolia L.) biotypes from Mississippi, USA. A glyphosate‐susceptible (GS) biotype was included for comparison. The effective glyphosate dose to reduce the growth of the treated plants by 50% for the GR1, GR2 and GS biotypes was 0.58, 0.46 and 0.11 kg ae ha−1, respectively, indicating that the level of resistance was five and fourfold that of the GS biotype for GR1 and GR2, respectively. Studies using 14 C‐glyphosate have not indicated any difference in its absorption between the biotypes, but the GR1 and GR2 biotypes translocated more 14 C‐glyphosate, compared to the GS biotype. This difference in translocation within resistant biotypes is unique. There was no amino acid substitution at codon 106 that was detected by the 5‐enolpyruvylshikimate‐3‐phosphate synthase gene sequence analysis of the resistant and susceptible biotypes. Therefore, the mechanism of resistance to glyphosate in common ragweed biotypes from Mississippi is not related to a target site mutation or reduced absorption and/or translocation of glyphosate.

Reduced absorption of glyphosate and decreased translocation of dicamba contribute to poor control of kochia (Kochia scoparia) at high temperature

Plant growth temperature is one of the important factors that can influence postemergent herbicide efficacy and impact weed control. Control of kochia (Kochia scoparia), a major broadleaf weed throughout the North American Great Plains, often is unsatisfactory when either glyphosate or dicamba are applied on hot summer days. We tested effects of plant growth temperature on glyphosate and dicamba phytotoxicity on two Kansas kochia populations (P1 and P2) grown under the following three day/night (d/n) temperature regimes: T1, 17.5/7.5°C; T2, 25/15°C; and T3, 32.5/22.5°C. Visual injury and above-ground dry biomass data from herbicide dose-response experiments indicated greater susceptibility to both glyphosate and dicamba when kochia was grown under the two cooler temperature regimes, i.e. T1 and T2. At T1, the ED50 of P1 and P2 kochia were 39 and 36 g ha-1 of glyphosate and 52 and 105 g ha-1 of dicamba, respectively. In comparison, at T3 the ED50 increased to 173 and 186 g ha-1 for glyphosate and 106 and 410 g ha-1 for dicamba, respectively, for P1 and P2. We also investigated the physiological basis of decreased glyphosate and dicamba efficacy under elevated temperatures. Kochia absorbed more glyphosate at T1 and T2 compared to T3. Conversely, there was more dicamba translocated towards meristems at T1 and T2, compared to T3. Reduced efficacy of dicamba or glyphosate to control kochia under elevated temperatures can be attributed to decreased absorption and translocation of glyphosate and dicamba, respectively. Therefore, it is recommended to apply glyphosate or dicamba when the temperature is low (e.g. d/n temperature at 25/15°C) and seedlings are small (less than 12 cm) to maximize kochia control. © 2016 Society of Chemical Industry.

The Evidence for Reduced Glyphosate Efficacy on Acetolactate Synthase–Inhibiting Herbicide-Resistant Yellow Nutsedge (Cyperus esculentus)

A yellow nutsedge biotype (Res) from an Arkansas rice field has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides. TheResbiotype previously exhibited cross-resistance to ALS inhibitors from four chemical families (imidazolinone, pyrimidinyl benzoate, sulfonylurea, and triazolopyrimidine). Experiments were conducted to evaluate alternative herbicides (i.e., glyphosate, bentazon, propanil, quinclorac, and 2,4-D) currently labeled in Arkansas rice–soybean production systems. Based on the percentage of aboveground dry weight reduction, control of the yellow nutsedge biotypes with the labeled rate of bentazon, propanil, quinclorac, and 2,4-D was < 44%. Glyphosate (867 g ae ha−1) resulted in 68 and > 94% control of theResand susceptible yellow nutsedge biotypes, respectively, at 28 d after treatment. Dose-response studies were conducted to estimate the efficacy of glyphosate on theResbiotype, three susceptible yellow nutsedge biotypes, and purple nutsedge. Based on the dry weights, theResbiotype was ≥ 5- and ≥ 1.3-fold less responsive to glyphosate compared to the susceptible biotypes and purple nutsedge, respectively. Differences in absorption and translocation of radiolabeled glyphosate were observed among the yellow nutsedge biotypes and purple nutsedge. The susceptible biotype had less14C-glyphosate radioactivity in the tissues above the treated leaf and greater radioactivity in tissues below the treated leaf compared to theResbiotype and purple nutsedge. Reduced translocation of glyphosate in tissues below the treated leaf of theResbiotype could be a reason for the lower glyphosate efficacy in theResbiotype. No amino acid substitution that would correspond to glyphosate resistance was found in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene of theResbiotype. However, an amino acid (serine) addition was detected in the EPSPS gene of theResbiotype; albeit, it is not believed that this addition contributes to lower efficacy of glyphosate in this biotype.