Synthetic Auxin Herbicides Research Articles

Sensitivity to sublethal rates of dicamba for selected mid-Atlantic vegetable crops

AbstractDicamba is a synthetic auxin herbicide that may be applied over the top of transgenic dicamba-tolerant crops. The increasing prevalence of herbicide-resistant weeds has resulted in increased reliance on dicamba-based herbicides in soybean production systems. Because of the high volatility of dicamba it is prone to off-target movement, and therefore concern exists regarding its drift onto nearby specialty crops. The present study evaluates 12 mid-Atlantic vegetable crops species for sensitivity to sublethal rates of dicamba. Soybean, snap bean, lima bean, tomato, eggplant, bell pepper, cucumber, summer squash, watermelon, pumpkin, sweet basil, lettuce, and kale were grown in a greenhouse and exposed to dicamba at 0, 0.056, 0.11, 0.28, 0.56, 1.12, 2.24 g ae ha−1, which is, respectively, 0, 1/10,000, 1/5,000, 1/2,000, 1/1,000, 1/500, and 1/250 of the maximum recommended label rate for soybean application (560 g ae ha−1). Vegetable crop injury was evaluated 4 wk after treatment using visual rating methods and leaf deformation index measurements. Overall, snap bean was the most sensitive crop, with dicamba rates as low as 0.11 g ae ha−1 resulting in significantly higher leaf deformation levels compared with the nontreated control. Other Fabaceae and Solanaceae species also demonstrated high sensitivity to sublethal rates of dicamba with rates ranging 0.28 to 0.56 g ae ha−1 causing higher leaf deformation compared with the nontreated control. While cucumber, pumpkin, and summer squash were no or moderately sensitive to dicamba, watermelon showed greater sensitivity with unique symptoms at rates as low as 0.056 g ae ha−1 based on visual evaluation. Within the range of tested dicamba rates, sweet basil, lettuce, and kale demonstrated tolerance to dicamba with no injury observed at the maximum rate of 2.24 g ae ha−1.

An in-frame deletion mutation in the degron tail of auxin coreceptor IAA2 confers resistance to the herbicide 2,4-D in Sisymbrium orientale.

The natural auxin indole-3-acetic acid (IAA) is a key regulator of many aspects of plant growth and development. Synthetic auxin herbicides such as 2,4-D mimic the effects of IAA by inducing strong auxinic-signaling responses in plants. To determine the mechanism of 2,4-D resistance in a Sisymbrium orientale (Indian hedge mustard) weed population, we performed a transcriptome analysis of 2,4-D-resistant (R) and -susceptible (S) genotypes that revealed an in-frame 27-nucleotide deletion removing nine amino acids in the degron tail (DT) of the auxin coreceptor Aux/IAA2 (SoIAA2). The deletion allele cosegregated with 2,4-D resistance in recombinant inbred lines. Further, this deletion was also detected in several 2,4-D-resistant field populations of this species. Arabidopsis transgenic lines expressing the SoIAA2 mutant allele were resistant to 2,4-D and dicamba. The IAA2-DT deletion reduced binding to TIR1 invitro with both natural and synthetic auxins, causing reduced association and increased dissociation rates. This mechanism of synthetic auxin herbicide resistance assigns an in planta function to the DT region of this Aux/IAA coreceptor for its role in synthetic auxin binding kinetics and reveals a potential biotechnological approach to produce synthetic auxin-resistant crops using gene-editing.

Auxin‐based herbicide program for weed control in auxin resistant soybean

AbstractSoybean [Glycine max (L.) Merr.] cultivars resistant to synthetic auxin herbicides have provided another mode of action for the postemergence broadleaf weed control. This field study was conducted at three South Dakota locations [Northeast, NERF; east‐central, ARF; and Southeast, SERF) in 2019 and two locations (ARF and SERF) in 2020. The Enlist E3 and Roundup Ready 2 Xtend cultivars were planted at three dates (early, mid‐, and late season) to examine weed control, agronomic characteristics, nodulation, and yield. Preemergence (PRE) treatment was flumioxazin + metribuzin + S‐metolachlor + glyphosate + pendimethalin. Two postemergence (POST) treatments, based on cultivar, were compared with PRE‐only. The PRE‐only treatment had numerous grasses {including green foxtail [Setaria viridis (L.) P. Beauv.] and yellow foxtail [S. pumila (Poir.) Roem. & Schult.], volunteer corn (Zea mays L.), barnyard grass [Echinochola crus‐galli (L.) Beauv.], large crabgrass [Digitaria sanguinalis (L.) Scop.], woolly cupgrass [Eriochloa villosa (Thunb.) Kunth]} and broadleaf weeds (including redroot pigweed [Amaranthus retroflexus L.], common lambsquarters [Chenopodium album L.], waterhemp [Amaranthus rudis Sauer]) with high density and biomass. POST treatments controlled most of the broadleaf species, although some grasses remained. Yields were similar within a location and year, although differences occurred among planting dates. In 2019, planting date did not influence final yield at ARF (average yield 3,084 kg ha−1). Yield was greatest for the early (NERF) and mid‐planting dates (NERF and SERF) compared with late‐season planting. In 2020, dry conditions occurred, and yields at ARF and SERF were lowest for the late‐season plantings (ranging from 37 to 73% lower depending on cultivar) compared with the early season planting. In 2020, dicamba + glyphosate treatment of the Xtend cultivar had 10% (ARF) and 20% (SERF) greater yield than the acifluorfen + clethodim treatment.

Non-Target-Site Resistance Mechanisms Endow Multiple Herbicide Resistance to Five Mechanisms of Action in Conyza bonariensis.

The repeated use of herbicides can lead to the selection of multiple resistance weeds. Some populations of Conyza bonariensis occurring in olive groves from southern Spain have developed resistance to various herbicides. This study determined the resistance levels to 2,4-D, glyphosate, diflufenican, paraquat, and tribenuron-methyl in a putative resistant (R) C. bonariensis population, and the possible non-target-site resistance (NTSR) mechanisms involved were characterized. Resistance factors varied as follows: glyphosate (8.9), 2,4-D (4.8), diflufenican (5.0), tribenuron-methyl (19.6), and paraquat (85.5). Absorption of 14C-glyphosate was up to 25% higher in the susceptible (S) population compared to the R one, but 14C-paraquat absorption was similar (up to 70%) in both populations. S plants translocated more than 60% of both 14C-glyphosate and 14C-paraquat toward shoots and roots, while R plants translocated less than 10%. The R population was able to metabolize 57% of the 2,4-D into nontoxic metabolites and 68% of the tribenuron-methyl into metsulfuron-methyl (10%), metsulfuron-methyl-hydroxylate (18%), and conjugate-metsulfuron-methyl (40%). Among the NTSR mechanisms investigated, absorption and translocation could be involved in glyphosate resistance, but only translocation for paraquat. Proofs of the presence of enhanced metabolism as a resistance mechanism were found for tribenuron-methyl and 2,4-D, but not for diflufenican. This research informs the first occurrence of multiple resistance to five herbicide classes (acetolactate synthase inhibitors, 5-enolpyruvylshikimate-3-phosphate synthase inhibitors, photosystem I electron diverters, photosystem II inhibitors, and synthetic auxin herbicides) in C. bonariensis.

The phytotoxicity mechanism of florpyrauxifen-benzyl to Echinochloa crus-galli (L.) P. Beauv and weed control effect

Weeds infest rice causing high yield losses, leading to the increasing use of herbicides for weed control. However, many weeds have evolved resistance to common commercial herbicides, including penoxsulam, metamifop and quinclorac. This study investigated the weed control effect and the phytotoxicity mechanism of florpyrauxifen-benzyl, a novel synthetic auxin herbicide registered for weed management in rice fields in China. The greenhouse study showed that florpyrauxifen-benzyl was highly efficient (GR50 < 6 and GR90 < 15 g a.i ha−1) at controlling 10 weed species commonly found in rice fields, including penoxsulam- and quinclorac- resistant(R) biotypes of Echinochloa Beauv. and bensulfuron-methyl-R biotype of Ammannia arenaria. The typical plant hormone content showed that following florpyrauxifen-benzyl treatment, indole-3-acetic acid (IAA) production changed only slightly at 12 h, while abscisic acid (ABA) production increased with time in the treated group, whose content was significantly higher than that of the control. Besides, ethylene biosynthesis was stimulated by florpyrauxifen-benzyl, ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) content, and 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO) activities, which evidently increased in the treated group, and ethylene peaked at 36 h. For the antioxidant enzyme activities and malondialdehyde (MDA) content in the treated group, results showed that MDA content continuously increased with time and was greater than that in the untreated group at 48 h and 72 h, superoxide dismutase (SOD) activity changed with exposure time and was significantly higher in the treatment group than the control at 48 h. A similar phenomenon was observed in peroxidase (POD) activity, which reached a peak at 48 h, and no distinct difference in catalase (CAT) activity was observed among groups except for the higher activity in the treated groups than control at 36 h and 48 h. Our results showed that that the stimulation ethylene biosynthesis and accumulation of ABA and reactive oxygen species (ROS) play important roles in the phytotoxicity mechanism of florpyrauxifen-benzyl in plants. Our findings demonstrate the potential of florpyrauxifen-benzyl to provide an alternative weed management strategy for rice fields.

Resistance in smallflower umbrella sedge (Cyperus difformis) to an acetolactate synthase–inhibiting herbicide in rice: first case in India

AbstractSmallflower umbrella sedge is one of the most problematic weeds in direct-seeded rice in India. Bispyribac-sodium (acetolactate synthase [ALS]-inhibiting herbicide) is commonly used in rice, but growers have recently reported lack of smallflower umbrella sedge control with this herbicide. An extensive survey was carried out in two rice-growing states, Chhattisgarh and Kerala, where 53 putative bispyribac-sodium-resistant (BR) biotypes were collected. Studies were conducted to confirm resistance to bispyribac-sodium and to test the efficacy of the newly developed synthetic auxin herbicide florpyrauxifen-benzyl on putative BR biotypes. A whole-plant bioassay revealed that bispyribac-sodium is no longer effective. Of 53 putative BR biotypes, 17 biotypes survived the recommended label rate of 25 g ai ha−1. The effective bispyribac-sodium rate required to control 50% of the plants in most of the BR biotypes (ED50) ranged from 19 to 96 g ha−1, whereas it was 10 g ha−1 in a susceptible biotype. In two highly resistant biotypes, the ED50 was beyond the maximum tested rate, 200 g ha−1. This suggests 2- to &gt;20-fold resistance in BR biotypes. An ALS enzyme activity assay suggests an altered target site as mechanism of resistance to bispyribac-sodium. This study confirms the first case of evolved resistance to bispyribac-sodium in smallflower umbrella sedge in India. However, the newly developed synthetic auxin florpyrauxifen-benzyl effectively controlled all BR biotypes at the field use rate of 31.25 g ai ha−1.

Damage and recovery from drift of synthetic-auxin herbicide dicamba depends on concentration and varies among floral, vegetative, and lifetime traits in rapid cycling Brassica rapa

Herbicides can drift from intended plants onto non-target species. It remains unclear how drift impacts plant functional traits that are important for fitness. To address this gap, we conducted an experiment where fast cycling Brassica rapa plants were exposed to one of three drift concentrations (0.5%, 1%, 10%) of synthetic-auxin dicamba. We evaluated damage to and capacity of floral and vegetative traits to recover as well as lifetime fitness by comparing treated plants to controls. Response to dicamba exposure was concentration-dependent across all traits but varied with trait type. At 0.5% dicamba, three out of five floral traits were affected, while at 1% dicamba, four floral traits and one out of two vegetative traits were negatively impacted. At 10% dicamba all floral and vegetative traits were stunted. Overall, floral traits were more responsive to all dicamba drift concentrations than vegetative traits and displayed a wide range of variation ranging from no response (e.g., pistil length) to up to 84% reduction (ovule number). However, despite floral traits were more affected across the dicamba drift concentrations they were also more likely to recover than the vegetative traits. There was also variation among lifetime traits; the onset of flowering was delayed, and reproductive fitness was negatively affected in a concentration-dependent manner, but the final biomass and total flower production were not affected. Altogether, we show substantial variation across plant traits in their response to dicamba and conclude that accounting for this variation is essential to understand the full impact of herbicide drift on plants and the ecological interactions these traits mediate.

Antitranspirants Partially Mitigate Auxin Herbicide Injury on Tomato Plants

The use of dicamba and 2,4-D products on herbicide-tolerant crops has resulted in numerous cases of off-target movement and injury to sensitive plants, including tomato (Solanum lycopersicon L.). Two greenhouse studies were conducted to determine whether ‘Big Beef’ (‘BB’) or ‘Florida 91’ (‘FL’) tomato plants pretreated with an antitranspirant, including Moisture-Loc (ML) at 100 mL·L−1, TransFilm (TF) at 50 g·L−1, or Wilt-Pruf (WP) at 100 mL·L−1, mitigated injury from synthetic auxin herbicides. Dicamba or 2,4-D was applied at a rate corresponding to 1/200 of the manufacturer’s labeled rate of 0.56 kg ae/ha or 1.06 kg ae/ha, respectively. At 2 weeks after treatment (WAT), plants treated with ML or WP before either herbicide exhibited injury symptoms, but they were always less severe than those treated with the herbicide alone for both cultivars. However, shoot length measurements indicated that none of the antitranspirants consistently provided protection against herbicide injury at 2 WAT. By 12 WAT, ML or WP used before either herbicide increased the number of live reproductive organs compared with dicamba or 2,4-D alone for both cultivars. Floral abortion on tomato plants was also reduced when ML or WP was applied before an herbicide treatment by 12 WAT. Although WP and ML did not provide complete protection against synthetic auxin herbicide injury, the concept of using film-forming barriers may be useful in mitigating some of the short-term effects of drift on plants.

Response of Brazilian peppertree (Schinus terebinthifolia) and four mangrove species to imazamox and carfentrazone-ethyl herbicides

AbstractMangroves are a critical component of many coastal ecosystems in Florida. Woody species, including Brazilian peppertree (Schinus terebinthifolia Raddi), have invaded thousands of hectares of mangrove habitat. The difficulty associated with ground-based management of invasive plants in mangrove communities has warranted a need to identify selective herbicides that can be applied aerially. Recent work suggests that Florida mangrove species are extremely sensitive to synthetic auxin herbicides; however, other herbicides have yet to be tested for selectivity. Greenhouse studies in 2018 and 2019 evaluated broadcast foliar applications of the acetolactate synthase inhibitor imazamox and protoporphyrinogen oxidase inhibitor carfentrazone-ethyl, both as individual treatments and in combinations, for control of S. terebinthifolia and injury to four non-target mangrove species. Across all posttreatment sample dates and species tested, there were no significant interactions between imazamox applied at 0.28 or 0.56 kg ai ha−1 in combination with carfentrazone-ethyl applied at 0 or 0.1 kg ha−1. Main effects of imazamox applied at 0.56 kg ai ha−1 and carfentrazone-ethyl applied at 0.1 kg ha−1 resulted in 99% and 97% defoliation, respectively, of Schinus terebinthifolia at 180 DAT. However, S. terebinthifolia percent survival was 56% and 44% for the same treatments. Both herbicides severely injured all four mangroves by 90 DAT and resulted in 58% to 100% defoliation across species. At 180 DAT, significant increases in percent cambium kill were also observed for all four species. Across species, mangrove survival varied, but red mangrove (Rhizophora mangle L.) survival was reduced to 6% when imazamox was applied at 0.56 kg ha−1. These results indicate both imazamox and carfentrazone-ethyl exhibit activity on S. terebinthifolia but also injure all four mangroves enough to preclude their use as selective treatments.

Imaging analysis method to quantify leaf deformation in response to sub-lethal rates of dicamba

AbstractDicamba is a synthetic auxin herbicide that is prone to off-target movement, including drift and volatilization. Due to the increased acreage of dicamba-resistant soybean to control glyphosate-resistant weeds, dicamba drift injury to neighboring vegetable crops is of concern. A method to quantify leaf deformation (often referred to as leaf cupping) caused by dicamba injury was developed and compared to visual rating techniques to determine its accuracy and suitability. A second objective was to determine the relative dicamba sensitivity of several economically important vegetable crops. Soybean, snap bean, tomato, and cucumber were grown in a greenhouse and exposed to dicamba at 0, 56, 112, 280, 560, 1,120, and 2,240 mg ae ha−1, which is, respectively, 0, 1/10,000, 1/5,000, 1/2,000, 1/1,000, 1/500, and 1/250 of the maximum recommended label rate for soybean application (560 g ae ha−1). Plants were evaluated visually and using an imaging analysis technique that measures the leaf deformation index (LDI) with a leaf area scanner. LDI is calculated by dividing the two-dimensional projection of the area of the leaf in its natural configuration by the area of the flattened leaf. Across all four crops, log-logistic regression analysis indicated the LDI method had lower I50 values with lower standard error, demonstrating that the LDI method gives more precise estimates of sensitivity. This novel method provides an objective, quantitative method for measuring dicamba drift injury and determining relative sensitivities of valuable specialty crops.

Fluroxypyr-resistant kochia [Bassia scoparia (L.) A.J. Scott] confirmed in Alberta

Recent confirmation of dicamba-resistant kochia [Bassia scoparia (L.) A.J. Scott] in Alberta warrants investigation of resistance to other commonly used synthetic auxin herbicides like fluroxypyr. A randomized-stratified survey of 305 sites in Alberta was conducted in 2017 to determine the status of fluroxypyr-resistant kochia. Overall, 13% of the kochia populations were fluroxypyr-resistant. Only 4% of the populations were both fluroxypyr- and dicamba-resistant, indicating that different mechanisms may confer resistance to these herbicides. When combined with estimates of dicamba resistance, about 28% of the kochia populations sampled in Alberta in 2017 were resistant to at least one synthetic auxin herbicide.

Application time affects synthetic auxins herbicides in tank-mixture with paraquat on hairy fleabane control

Conyza sp. was the first glyphosate-resistant weed in Brazil’s soybean crop. Synthetic auxin herbicides followed by paraquat has improved the hairy fleabane control, and time of day of herbicide application can affect the control efficacy. There are no studies reporting the effects of the application time of synthetic auxins in tank-mixture with paraquat, or in sequential application with paraquat at two growth stages of hairy fleabane, applied at different times of the day. The herbicides were sprayed during the morning and night. 2,4-D applied alone was more effective applied during the day, while dicamba efficiency was higher when applied at night in the rosette stage. The mixture of 2,4-D and paraquat was more efficient when applied during the night. Tall hairy fleabane were more effectively controlled by dicamba + paraquat as well as any synthetic auxin followed by paraquat. When herbicides were applied at night, efficiency was slightly higher.

Phytotoxicity of the chiral herbicide dichlorprop: Cross-talk between nitric oxide, reactive oxygen species and phytohormones

Nitric oxide (NO), reactive oxygen species (ROS), and phytohormones in plants often initiate responses to sources of abiotic stress. However, we have a poor understanding of the cross-talk between NO, ROS, and phytohormones during exogenous chiral auxin-induced phytotoxicity. In this study, the toxicity of the chiral synthetic auxin herbicide dichlorprop (DCPP) to Arabidopsis thaliana, as well as the mutual regulation of NO, hydrogen peroxide (H2O2), superoxide anion (O2.-), and phytohormones at the enantiomeric level was investigated. The ROS production exhibited an enantioselective manner, further, that was positively correlated with the change of the morphological indicators. This confirmed that ROS played an important role in the enantioselective effect of DCPP. The distribution of ROS and NO was partially overlapped, indicating that the production of NO may be affected by ROS, and also related to the degree of plant damage. In terms of phytohormones, the level of salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) in the whole plant increased as the (R)-DCPP concentration applied increased, however, the trend has changed, when the data of leaves and roots was discussed separately. The results revealed that the redistribution of phytohormones may exist between leaves and roots, caused by the joint action of ROS and NO. The differences in the biological activity identified between the two enantiomers in this study enhance our understanding of the toxicity mechanism of exogenous auxin via their effects on phytohormones.

Cytotoxic and genotoxic effects of clopyralid herbicide on Allium cepa roots.

Clopyralid is one of the synthetic pyridine-carboxylate auxin herbicides and used to control perennial and annual broadleaf weeds in wheat, sugar beets, canola, etc. In this study, dose-dependent cytotoxicity and genotoxicity of clopyralid at different concentrations (25, 50, and 100 μg/mL) have been evaluated on the Allium cepa roots. The evaluation has been performed at macroscopic (root growth) and microscopic levels [mitotic index (MI), chromosome aberrations (CAs) in ana-telophase cells, and DNA damage] using root growth inhibition, Allium ana-telophase, and comet tests. The percentage of root growth inhibition and concentration of reducing root growth by 50% (EC50) of clopyralid were determined compared with the negative control by using various concentrations of clopyralid (6.25-1000 μg/L). The 96 h EC50 of clopyralid was recorded as 50 μg/L. The gradual decrease in root growth and the MI reveals the cytotoxic effects of clopyralid. All the tested concentrations of clopyralid induced total CAs (polyploidy, stickiness, anaphase bridges, chromosome laggards, and disturbed ana-telophase) and DNA damage dose and time dependently. These results confirm the cytotoxic and genotoxic effects of clopyralid on non-target organism.

Fluroxypyr-1-methylheptyl ester interferes with the normal embryogenesis of zebrafish by inducing apoptosis, inflammation, and neurovascular toxicity

Fluroxypyr-1-methylheptyl ester (FPMH) is a synthetic auxin herbicide used to regulate the growth of post-emergence broad-leaved weeds. Although acute exposure to FPMH increases the mortality of several fish species in the juvenile stage, the developmental toxicity of FPMH in aquatic vertebrates has not yet been investigated. In the present study, we assessed the developmental toxicity of FPMH using zebrafish models that offer many advantages for studying toxicology. During embryogenesis, survival rates gradually decreased with increasing FPMH concentrations and exposure times. At 120 h post-fertilization, FPMH-exposed zebrafish larvae showed various abnormalities such as small eye size, heart defects, enlarged yolk sac, and shortened body length. The study results confirmed the induction of apoptosis in the anterior body of zebrafish and upregulation of inflammatory gene expression. Further, defects in vascular networks, especially the loss of central arteries and abnormal aortic arch structures, were seen in the fli1:eGFP transgenic zebrafish model. Neurotoxicity of FPMH was examined using mbp:eGFP zebrafish and which displayed compromised myelination following FPMH administration. Our study has demonstrated the mechanisms underlying FPMH toxicity in developing zebrafish that is a representative model of vertebrates.

Asp-376-Glu substitution endows target-site resistance to AHAS inhibitors in Limnocharis flava, an invasive weed in tropical rice fields.

Limnocharis flava (L.) Buchenau is a problematic weed in rice fields and water canals of Southeast Asia, and in Malaysia this invasive aquatic weed species has evolved multiple resistance to synthetic auxin herbicide and acetohydroxyacid synthase (AHAS) inhibitors. In this study, it was revealed that, a single nucleotide polymorphism (SNP) at amino acid position 376, where C was substituted to G at the third base of the same codon (GAC to GAG), resulting in Aspartate (Asp) substitution by Glutamate (Glu) was the contributing resistance mechanism in the L. flava population to AHAS inhibitors. In vitro assay further proved that, all the L. flava individuals carrying AHAS resistance mutation exhibited decreased-sensitivity to AHAS inhibitors at the enzyme level. In the bensulfuron-methyl whole-plant bioassay, high resistance indices (RI) of 328- and 437-fold were recorded in the absence and presence of malathion (the P450 inhibitor), respectively. Similarly, translocation and absorption of bensulfuron-methyl in both resistant and susceptible L. flava populations showed no remarkable differences, hence eliminated the possible co-existence of non-target-site resistance mechanism in the resistant L. flava. This study has confirmed another new case of a target-site resistant weed species to AHAS-inhibitors.

Effect of Synthetic Auxin Herbicides Applied During Sugarcane Grand Growth on Starch Accumulation and Color

Morning glory (Ipomoea spp.) and other vining weeds infest Louisiana sugarcane fields and synthetic auxin herbicides are applied to control such weeds. Sugarcane exposed to herbicides during the critical growth period (grand growth) may influence starch and colorant accumulation in some cultivars and reduce processing efficiency. In this study, field sugarcane composed of three commercial cultivars were treated with 2,4-D, dicamba, or a premix of 2,4-D plus dicamba synthetic auxin herbicides to prevent yield losses and maximize harvesting efficiency. No visible abnormalities in sugarcane growth were observed following application of the three herbicide treatments. Treatments with dicamba or 2,4-D did not increase or decrease total or insoluble starch in extracted sugarcane juice. However, when compared to the nontreated control, 16 to 25% more soluble starch was present in juice from L 01–299, HoCP 96–540, and HoCP 04–838 sugarcane that were exposed, in July, to dicamba and the premix formulation of dicamba plus 2,4-D. This is most likely related to natural diastase activity in the sugarcane and further investigation is now warranted. Late-season applied synthetic auxin herbicides did not significantly influence color measured at pH 4.0, 7.0, 8.5, or 9.0; however, pH by cultivar interactions influenced color at pH 7.0 or greater, i.e., phenolic colorants. Overall, this study documented that the synthetic auxin herbicides dicamba and 2,4-D plus dicamba were beneficial in controlling morning glory species and pose no threat to processing difficulties often caused by excessive starch and colorants in processing streams at both the factory and refinery.

Design, synthesis and mode of action of novel 3-chloro-6-pyrazolyl picolinate derivatives as herbicide candidates.

Picolinate/picolinic acid compounds are an important class of synthetic auxin herbicides. To explore the herbicidal activity of 6-pyrazolyl picolinate compounds, a series of 3-chloro-6-pyrazolyl-picolinate derivatives was designed and synthesized. Twenty-five 3-chloro-6-pyrazolyl-picolinate derivatives synthesized were tested for herbicidal activity and the IC50 value of compound c5 to the growth of Arabidopsis thaliana root was 27 times lower than that of the commercial herbicide clopyralid. Compound c5 displayed better post-emergence herbicidal activity and broader (Picloram, Clopyralid, Aminopyralid) herbicidal spectrum at a dosage of 400 g ha-1 in comparison with clopyralid; it also was safe to wheat and maize at this dosage. Arabidopsis thaliana phenotypes and expression of auxin-response genes demonstrated that compound c5 might be a novel auxin-type herbicide. Molecular docking analyses revealed that compound c5 had stronger binding ability to receptor AFB5 (auxin signaling F-box protein 5) than clopyralid. These 6-pyrazolyl picolinate compounds could be used as potential lead structures for the discovery of a novel synthetic auxin herbicide. © 2021 Society of Chemical Industry.

Cytochrome P450 metabolism-based herbicide resistance to imazamox and 2,4-D in Papaver rhoeas

Papaver rhoeas biotypes displaying multiple herbicide resistance to ALS inhibitors and synthetic auxin herbicides (SAH) are spreading across Europe. In Spain, enhanced metabolism to imazamox was confirmed in one population, while cytochrome-P450 (P450) based metabolism to 2,4-D in another two. The objectives of this research were to further confirm the presence of P450 mediated enhanced metabolism and, if so, to confirm whether a putative common P450 is responsible of metabolizing both 2,4-D and imazamox. Metabolism studies were undertaken in five P. rhoeas populations with contrasted HR profiles (herbicide susceptible, only HR to ALS inhibitors, only HR to SAH, or multiple HR to both), and moreover, three different P450 inhibitors were used. The presence of enhanced metabolism to these SoA was confirmed in three more HR P. rhoeas populations. This study provides the first direct evidence that imazamox metabolism in these biotypes is P450-mediated, also in one population without an altered target site. Additionally, it was further confirmed that enhanced metabolism of 2,4-D in biotypes only HR to SAH or multiple HR to ALS inhibitors and SAH involves P450 as well. No metabolism was detected using the three inhibitors in all the herbicide-metabolizing P. rhoeas biotypes, suggesting that a common metabolic system involving P450s is responsible of degrading herbicides affecting both SoAs. Thus, selection pressure with either SAH or imidazolinone ALS inhibitors can select not only for resistance to each of them, but it can also confer cross-resistance between them in P. rhoeas.

Adjuvant interference on spray solution properties of 2,4-D and dicamba herbicides and their efficacy for Ipomoea SPP. control

Synthetic auxin herbicides constitute major alternatives for managing tough-to-kill weeds such as Ipomoea spp. Adjuvant use is known to positively affect the biological efficacy of pesticides by modifying key spraying solution and droplet properties. Determining to what extent the use of adjuvants could change spray solution parameters and affect synthetic auxin herbicides’ efficiency for Ipomoea spp. control were the research goals. The study was conducted in two phases: laboratory and field, respectively. In the laboratory, the pH, the surface tension, and the resources of the herbicide drops were measured. In the field, weed control was evaluated. All adjuvants modified spray solution properties, lowering surface tension values. Most adjuvants decreased pH values as well as number and density of droplets due to an increase in droplet size. Regardless of adjuvant usage, Ipomoea spp. control levels rose more rapidly following 2,4-D spraying rather than dicamba, resulting in lower biomass accumulation when the former was used. Dicamba-containing treatments displayed slightly but significantly lower Ipomoea spp. control levels at the end of the evaluation period. Herbicide efficacy for Ipomoea spp. control was not improved upon the addition to the spray solution of any of the tested adjuvants. Adjuvant use altered spraying solution and droplet properties. 2,4-D spraying allowed for lower Ipomoea spp. biomass and greater control levels relative to dicamba, suggesting it might constitute a better option for Ipomoea spp. control. Even though herbicide efficacy was not improved with adjuvants, their use should still be considered given favorable spraying solution alterations, mainly with some alteration in droplet sizes despite the use of similar spray nozzles tips - maintaining weed control efficacy.