No evidence for early fitness penalty in glyphosate‐resistant biotypes of Conyza canadensis: Common garden experiments in the absence of glyphosate

Strong selection from herbicides has led to the rapid evolution of herbicide‐resistant weeds, greatly complicating weed management efforts worldwide. In particular, overreliance on glyphosate, the active ingredient in RoundUp^®, has spurred the evolution of resistance to this herbicide in ≥40 species. Previously, we reported that Conyza canadensis (horseweed) has evolved extreme resistance to glyphosate, surviving at 40× the original 1× effective dosage. Here, we tested for underlying fitness effects of glyphosate resistance to better understand whether resistance could persist indefinitely in this self‐pollinating, annual weed. We sampled seeds from a single maternal plant (“biotype”) at each of 26 horseweed populations in Iowa, representing nine susceptible biotypes (S), eight with low‐level resistance (LR), and nine with extreme resistance (ER). In 2016 and 2017, we compared early growth rates and bolting dates of these biotypes in common garden experiments at two sites near Ames, Iowa. Nested ANOVAs showed that, as a group, ER biotypes attained similar or larger rosette size after 6 weeks compared to S or LR biotypes, which were similar to each other in size. Also, ER biotypes bolted 1–2 weeks earlier than S or LR biotypes. These fitness‐related traits also varied among biotypes within the same resistance category, and time to bolting was inversely correlated with rosette size across all biotypes. Disease symptoms affected 40% of all plants in 2016 and 78% in 2017, so we did not attempt to measure lifetime fecundity. In both years, the frequency of disease symptoms was greatest in S biotypes and similar in LR versus ER biotypes. Overall, our findings indicate there are no early growth penalty and possibly no lifetime fitness penalty associated with glyphosate resistance, including extremely strong resistance. We conclude that glyphosate resistance is likely to persist in horseweed populations, with or without continued selection pressure from exposure to glyphosate.     

Double herbicide-resistant biotypes of wild oat (<Emphasis Type="Italic">Avena fatua</Emphasis>) display characteristic metabolic fingerprints before and after applying ACCase- and ALS-inhibitors

Plant herbicides inhibit specific enzymes of biosynthetic metabolism, such as acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS). Herbicide resistance can be caused by point mutations at the binding domains, catalytic sites and other regions within multimeric enzymes. Direct-injection electrospray mass spectrometry was used for high-throughput metabolic fingerprinting for finding significant differences among biotypes in response to herbicide application. A Mexican biotype of wild oat (Avena fatua) that displays multiple resistances to ACCase- and ALS-inhibiting herbicides was characterized. The dose–response test showed that the double-resistant biotype had a resistance index of 3.58 for pinoxaden and 3.53 for mesosulfuron-methyl. Resistance was accompanied by characteristic mutations at the site of action: an I-1781-L substitution occurred in the ACCase enzyme and an S-653-N mutation was identified within the ALS enzyme. Other mutations were also detected in the genes of the Mexican biotypes. The ionomic fingerprint showed that the multiple-resistant biotype had a markedly different metabolic pattern under control conditions and that this difference was accentuated after herbicide treatment. This demonstrates that single changes of amino acid sequences can produce several holistic modifications in the metabolism of resistant plants compared to susceptible plants. We conclude that in addition to genetic resistance, additional mechanisms of metabolic adaptation and detoxification can occur in multiple-resistant weed plants.     

iMAR: An Interactive Web-Based Application for Mapping Herbicide Resistant Weeds

Herbicides are the major weed control tool in most cropping systems worldwide. However, the high reliance on herbicides has led to environmental issues as well as to the evolution of herbicide-resistant biotypes. Resistance is a major concern in modern agriculture and early detection of resistant biotypes is therefore crucial for its management and prevention. In this context, a timely update of resistance biotypes distribution is fundamental to devise and implement efficient resistance management strategies. Here we present an innovative web-based application called iMAR (interactive MApping of Resistance) for the mapping of herbicide resistant biotypes. It is based on open source software tools and translates into maps the data reported in the GIRE (Italian herbicide resistance working group) database of herbicide resistance at national level. iMAR allows an automatic, easy and cost-effective updating of the maps a nd provides two different systems, “static” and “dynamic”. In the first one, the user choices are guided by a hierarchical tree menu, whereas the latter is more flexible and includes a multiple choice criteria (type of resistance, weed species, region, cropping systems) that permits customized maps to be created. The generated information can be useful to various stakeholders who are involved in weed resistance management: farmers, advisors, national and local decision makers as well as the agrochemical industry. iMAR is freely available, and the system has the potential to handle large datasets and to be used for other purposes with geographical implications, such as the mapping of invasive plants or pests.     

Germination ecology of Ambrosia artemisiifolia L. and Ambrosia trifida L. biotypes suspected of glyphosate resistance

The germination ecology of Ambrosia artemisiifolia and A. trifida glyphosate susceptible biotypes sampled in marginal areas, was compared with that of the same species but different biotypes suspected of glyphosate resistance, common and giant ragweed, respectively. The suspected resistant biotypes were sampled in Roundup Ready® soybean fields. Within each weed species, the seeds of the biotype sampled in marginal area were significantly bigger and heavier than those of the biotype sampled in the soybean fields. A. artemisiifolia biotypes exhibited a similar dormancy and germination, while differences between A. trifida biotypes were observed. A. artemisiifolia biotypes showed similar threshold temperature for germination, whereas, the threshold temperature of the susceptible A. trifida biotype was half as compared to that of the resistant A. trifida biotype. No significant differences in emergence as a function of sowing depth were observed between susceptible A. artemisiifolia and suspected resistant A. trifida biotype, while at a six-cm seedling depth the emergence of the A. artemisiifolia susceptible biotype was 2.5 times higher than that of the A. trifida suspected resistant biotype. This study identified important differences in seed germination between herbicide resistant and susceptible biotypes and relates this information to the ecology of species adapted to Roundup Ready® fields. Information obtained in this study supports sustainable management strategies, with continued use of glyphosate as a possibility.     

Plant biotechnology: ecological case studies on herbicide resistance

The emerging field of molecular ecology aims to improve the ecological predictability of transgenic crop plants. The most widely cultivated lines are Roundup-Ready plants, which are genetically modified to be resistant to the broad-spectrum herbicide glyphosate. Recent publications demonstrate two ecological effects that were not anticipated: the widespread emergence of glyphosate-resistant weed biotypes and the formation of a metabolic herbicidal residue. Both effects appear to be due to the increased use of glyphosate rather than the genetic modification in the transgenic crop plant. With one prominent exception, opinions collected from the literature point towards a certain degree of resistance mismanagement and an inadequate testing of the ecological effects of extensive glyphosate use.     

农作物抗除草剂遗传工程研究进展

控制杂草提高农作物产量是农业生产中共同面临的问题,发展抗除草剂农作物将是最经济最方便控制杂草的技术。由于对除草剂的作用模式和除草剂代谢途径的了解,弄清了除草剂的关键靶酶及其基因,因此分离除草剂靶酶基因,克隆能解毒除草剂的酶基因,通过转化技术可获得抗除草剂农作物,大量的抗除草剂转基因农作物大田试验表明,将最有希望在２０００年进入市场。     

Mechanism of isoproturon resistance in <Emphasis Type="Italic">Phalaris minor:</Emphasis> in silico design,synthesis and testing of some novel herbicides for regaining sensitivity

Isoproturon, 3-p-cumenyl-1 dimethylurea was the only herbicide controlling Phalaris minor, a major weed growing in wheat fields till the early 1980s. Since it has acquired resistance against isoproturon, like other substituted urea herbicides, where the identified target site for isoproturon is in the photosynthetic apparatus at D1 protein of Photosystem-II (PS-II). Nucleotide sequence of susceptible and resistant psbA gene of P. minor has been reported to have four point mutations. During the present work D1 protein of both susceptible and resistant biotypes of P Minor has been modeled. Transmembrane segments of amino acids were predicted by comparing with the nearest homolog of bacterial D1 protein. Volume and area of active site of both susceptible and resistant biotypes has been simulated. Isoproturon was docked at the active site of both, susceptible and resistant D1 proteins. Modeling and simulation of resistance D1 protein indicates that the resistance is due to alteration in secondary structure near the binding site, resulting in loss in cavity area, volume and change in binding position, loss of hydrogen bonds, hydrophobic interaction and complete loss of hydrophobic sites. To regain sensitivity in resistant biotype new derivatives of isoproturon molecules have been proposed, synthesized and tested. Among the 17 derivatives we found that the N-methyl triazole substituted isoproturon is a potential substitute for isoproturon.     

An individual‐based model of seed‐ and rhizome‐propagated perennial plant species and sustainable management of Sorghum halepense in soybean production systems in Argentina

Perennial plants which propagate through both seeds and rhizomes are common in agricultural and nonagricultural systems. Due to their multifaceted life cycle, few population models are available for studying such species. We constructed a novel individual‐based model to examine the effects of ecological, evolutionary, and anthropogenic factors on the population dynamics of perennial species. To exemplify the application of the model, we presented a case study of an important weed, Sorghum halepense (L.) Pers. (Johnsongrass), in soybean productions in Argentina. The model encompasses a full perennial weed life cycle with both sexual (seeds) and asexual (rhizomes) propagations. The evolution of herbicide resistance was modeled based on either single genes or quantitative effects. Field experiments were conducted in the species' native environment in Argentina to parameterize the model. Simulation results showed that resistance conferred by single‐gene mutations was predominantly affected by the initial frequency of resistance alleles and the associated fitness cost. Population dynamics were influenced by evolved resistance, soil tillage, and rhizome fecundity. Despite the pivotal role of rhizomes in driving the population dynamics of Johnsongrass, most herbicides target the aboveground biomass, and chemical solutions to control rhizomes are still very limited. To maintain effective (short‐term) and sustainable (long‐term) weed management, it is recommended to combine soil tillage with herbicide applications for suppressing the rhizomes and delaying the evolution of resistance. This novel model of seed‐ and rhizome‐propagated plants will also be a useful tool for studying the evolutionary processes of other perennial weeds, cash crops, and invasive species.     

油菜田看麦娘对高效氟吡甲禾灵产生抗药性的生理响应

采用种子检测法,鉴定了对除草剂高效氟吡甲禾灵产生抗药性的看麦娘(Alopecu-rus aequalis Sobol.)生物型JXRII。为探讨看麦娘对高效氟吡甲禾灵产生抗性的生理反应,以敏感生物型JJSII为对比,喷施高效氟吡甲禾灵后,测定抗药生物型和敏感生物型几个重要生理指标变化情况的差异。结果表明:高效氟吡甲禾灵处理下,JJSII的叶片电解质泄漏率、MDA含量极显著上升,叶绿素、类胡萝卜素含量下降;JXRII叶片的电解质渗漏率、MDA含量、叶绿素、类胡萝卜素均与对照无显著差异;高效氟吡甲禾灵处理下,JJSII的GSH含量在处理后就开始逐渐下降,JXRII的GSH含量始终高于对照;二者可溶性糖含量均增加,敏感生物型JJSII可溶性糖含量增加的幅度远远大于抗性生物型JXRII;这2种群体经除草剂胁迫下,除草剂对敏感性生物型的生理变化影响大,抗性生物型表现为一定的生理适应性。     

Cytochrome P450 CYP81A10v7 in Lolium rigidum confers metabolic resistance to herbicides across at least five modes of action

Rapid and widespread evolution of multiple herbicide resistance in global weed species endowed by increased capacity to metabolize (degrade) herbicides (metabolic resistance) is a great threat to herbicide sustainability and global food production. Metabolic resistance in the economically damaging crop weed species Lolium rigidum is well known but a molecular understanding has been lacking. We purified a metabolic resistant (R) subset from a field evolved R L. rigidum population. The R, the herbicide susceptible (S) and derived F₂ populations were used for candidate herbicide resistance gene discovery by RNA sequencing. A P450 gene CYP81A10v7 was identified with higher expression in R vs. S plants. Transgenic rice overexpressing this Lolium CYP81A10v7 gene became highly resistant to acetyl-coenzyme A carboxylase- and acetolactate synthase-inhibiting herbicides (diclofop-methyl, tralkoxydim, chlorsulfuron) and moderately resistant to hydroxyphenylpyruvate dioxygenase-inhibiting herbicide (mesotrione), photosystem II-inhibiting herbicides (atrazine and chlorotoluron) and the tubulin-inhibiting herbicide trifluralin. This wide cross-resistance profile to many dissimilar herbicides in CYP81A10v7 transgenic rice generally reflects what is evident in the R L. rigidum. This report clearly showed that a single P450 gene in a cross-pollinated weed species L. rigidum confers resistance to herbicides of at least five modes of action across seven herbicide chemistries.     

Herbicide resistant rice (Oryza sativa L.): Global implications for weedy rice and weed management*

Rice cultivars resistant to broad‐spectrum herbicides have been developed and their commercial release is imminent, especially for imidazolinone and glufosinate resistant varieties in the USA and Latin America. Glyphosate‐resistant rice should follow within a few years. Rice growers throughout the world could benefit from the introduction of herbicide‐resistant rice cultivars that would allow in‐crop, selective control of weedy Oryza species. Other perceived benefits are the possibility to control ‘hard‐to‐kill’ weed species and weed populations that have already evolved resistance to herbicides currently used in rice production, especially those of the Echinochloa species complex. Weed management could also be improved by more efficient post‐emergence control. Introduction of herbicide resistant rice could also bring areas heavily infested with weedy rice that have been abandoned back to rice production, allow longer term crop rotations, reduce consumption of fossil fuels, promote the replacement of traditional chemicals by more environmentally benign products, and provide more rice grain without adding new land to production. There are also concerns, however, about the impact of releasing herbicide‐resistant rice on weed problems. Of most concern is the possibility of rapid transfer of the resistance trait to compatible weedy Oryza species. Development of such herbicide resistant weedy rice populations would substantially limit the chemical weed management options for farmers. Herbicide‐resistant rice volunteers also could become problematic, and added selection pressure to weed populations could aggravate already serious weed resistance problems. Because of the risk of weedy Oryza species becoming resistant to broad‐spectrum herbicides, mitigating measures to prevent gene flow, eventually attainable by both conventional breeding and molecular genetics, have been proposed. With commercialisation of the first herbicide resistant varieties planned for 2001, these mitigating measures will not be available for use with this first generation of herbicide resistant rice products. Release of herbicide resistant rice should depend on a thorough risk assessment especially in areas infested with con‐specific weedy rice or intercrossing weedy Oryza species. Regulators will have to balance risks and benefits based on local needs and conditions before allowing commercialisation of herbicide‐resistant rice varieties. If accepted, these varieties should be considered as components of integrated weed management, and a rational herbicide use and weedy rice control should be promoted to prevent losing this novel tool.     

Physiological tests for early detection of rigid ryegrass (Lolium rigidum Goud.) resistance to fenoxaprop-P

The study aimed to determine the usefulness of isothermal calorimetry and FT-Raman spectroscopy for the early evaluation of rigid ryegrass resistance to fenoxaprop-P ethyl (active ingredient one of aryloxyphenoxypropionate herbicides). The calorimetric measurements were done on the 4-day-old seedlings of susceptible and resistant biotypes of rigid ryegrass (Lolium rigidum Goud.) for 72 h, at 20 °C. It was observed that the specific thermal power–timecurves of the susceptible and resistant biotypes growing on water (control) were qualitatively similar. Herbicides changed the shape of the specific thermal power–time curves of both biotypes. Furthermore, the total specific thermal energy was significantly higher for the seedlings of resistant biotype, growing both on water or herbicide, as compared to the susceptible ones. The analysis of the seedlings’ endosperm, conducted using FT-Raman spectroscopy, showed a weaker intensity of the bands in the spectra derived from the resistant biotype. Differences in the specific thermal power–time curves and FT-Raman spectra between susceptible and resistant biotypes growing on water indicate that the sensitive and resistant biotypes are metabolically and chemically different already in the early stages of the seedling growth. We conclude that isothermal calorimetry and FT-Raman spectroscopy are efficient tools for the early detection of rigid ryegrass resistance to fenoxaprop-P ethyl.     

Resistant Acetyl-CoA Carboxylase is a Mechanism of Herbicide Resistance in a Biotype of Avena sterilis ssp. ludoviciana

A biotype of Avena sterilis ssp. ludoviciana is highly resistantto a range of herbicides which inhibit a key enzyme in fattyacid synthesis, acetyl-CoA carboxylase (ACCase). Possible mechanismsof herbicide resistance were investigated in this biotype. Acetyl-CoAcarboxylase from the resistant biotype is less sensitive toinhibition by herbicides to which resistance is expressed. I₅₀values for herbicide inhibition of ACCase were 52 to 6 timesgreater in the resistant biotype than in the susceptible biotype.This was the only major difference found between the resistantand susceptible biotypes. The amount of ACCase in the meristemsof the resistant and susceptible is similar during ontogenyand no difference was found in distribution of ACCase betweenthe two biotypes. Uptake, translocation and metabolism of [¹⁴C]diclofop-methylwere not different between the two biotypes. In vivo, ACCaseactivity in the meristems of the susceptible biotype was greatlyinhibited by herbicide application whereas only 25% inhibitionoccurred in the resistant biotype. Depolarisation of plasmamembrane potential by 50 µM diclofop acid was observedin both biotypes and neither biotype showed recovery of themembrane potential following removal of the herbicide. Hence,a modified form of ACCase appears to be the major determinantof resistance in this resistant wild oat biotype. (Received February 10, 1994; Accepted March 11, 1994)     

The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

二氯喹啉酸诱导稗草EcDnaJ基因表达

DnaJ作为分子伴侣在植物抗逆中起重要作用. 但目前在二氯喹啉酸逆境下,其在抗药性稗草中表达特点却鲜有报道. 本研究采用RACE技术从抗二氯喹啉酸稗草中克隆了1个DnaJ基因, 命名为EcDnaJ1 (GenBank登录号：JX518598), 其cDNA全长为2 154 bp, 开放阅读框为1 350 bp,编码449 个氨基酸, 理论分子量为48.4 kD, 等电点为9.5. 该蛋白质氮端含有1个保守的J结构域, 中部含有4个模式为CxxCxGxG的锌指结构. Real-time PCR分别测定EcDnaJ1在二氯喹啉酸抗性和敏感的稗草生物型苗期叶、根及成株期根、茎、叶和种子中的表达, 在感、抗生物型的相对表达量分别是0.8～20.9和7.4～30.2, 其中在抗性稗草苗期叶片中相对表达量最高为30.2, 而在敏感稗草种子中最低为0.8, 抗性稗草是敏感稗草的1.4～9.2 倍. 受二氯喹啉酸诱导后, 其在感、抗稗草的相对表达量分别为35.8～72.5和84.9～261.9, 在抗性稗草苗期叶片中相对表达量最高为261.9, 而在敏感稗草的种子中相对表达量最低为35.8, 抗性稗草是敏感稗草的2.4～3.6 倍. 诱导前后, 无论是在苗期根和叶中还是成株期的根、茎、叶和种子中, EcDnaJ1表达量均是抗性稗草高于敏感稗草. 抗、感稗草生物型的EcDnaJ1在mRNA水平差异表达暗示,它可能参与了稗草对二氯喹啉酸的抗药性.     

Occurrence of an herbicide‐resistant plant trait in agricultural field margins

Agricultural environments allow study of evolutionary change in plants. An example of evolution within agroecological systems is the selection for resistance to the herbicide glyphosate within the weed, Conyza canadensis. Changes in survivorship and reproduction associated with the development of glyphosate resistance (GR) may impact fitness and influence the frequency of occurrence of the GR trait. We hypothesized that site characteristics and history would affect the occurrence of GR C. canadensis in field margins. We surveyed GR occurrence in field margins and asked whether there were correlations between GR occurrence and location, crop rotation, GR crop trait rotation, crop type, use of tillage, and the diversity of herbicides used. In a field experiment, we hypothesized that there would be no difference in fitness between GR and glyphosate‐susceptible (GS) plants. We asked whether there were differences in survivorship, phenology, reproduction, and herbivory between 2 GR and 2 GS populations of C. canadensis in agrestal and ruderal habitats. We found that geographic location was an important factor in the occurrence of GR C. canadensis in field margins. Although not consistently associated with either glyphosate resistance or glyphosate susceptibility, there were differences in phenology, survivorship, and herbivory among biotypes of C. canadensis. We found equal or greater fitness in GR biotypes, compared to GS biotypes, and GR plants were present in field margins. Field margins or ruderal habitats may provide refugia for GR C. canadensis, allowing reproduction and further selection to occur as seeds recolonize the agrestal habitat. Agricultural practices may select for ecological changes that feed back into the evolution of plants in ruderal habitats.     

Evidence for vacuolar sequestration of paraquat in roots of a paraquat-resistant Hordeum glaucum biotype

Paraquat resistance in the grass weed Hordeum glaucum Steud. has been proposed to result from herbicide sequestration away from the growing points. In the present study, we used roots as a model system to investigate cellular transport of paraquat in resistant (R) and susceptible (S) H. glaucum biotypes. Both time- and concentration-dependent kinetics of paraquat influx across the root cell plasma membrane were similar in the S and R biotype. However, compartmentation analysis indicated greater herbicide accumulation in root vacuoles of the R seedlings. In contrast, the amount of paraquat accumulated in the cytoplasm of S was double that found in R biotype. While paraquat efflux from the cytoplasm back into the external solution was similar in the two biotypes, efflux across the tonoplast from the vacuole back into the cytoplasm was 5 times slower in the R than in the S biotype. At the end of a 48-h efflux period, nearly 7-fold more herbicide was retained in the roots of the R compared with those of the S biotype. These results suggest that paraquat resistance in H. glaucum may be due to the herbicide sequestration in the vacuole.     

Effects of 4-Chloro-2-methylphenoxypropionate (an Auxin Analogue) on Plasma Membrane ATPase Activity in Herbicide-Resistant and Herbicide-Susceptible Biotypes of Stellaria media L.

ATPase activity was examined in plasma membrane (PM) fractionsprepared from mecoprop-resistant and -susceptible biotypes ofStellaria media L. (chickweed). Treatment with the herbicidecaused an 18% increase in ATP hydrolysis, but this was not significantlydifferent from control plants and was similar for both biotypes.However, there was an overall significant biotype effect, herbicide-resistantplants having greater enzyme activity than susceptible ones.Proton-pumping was readily demonstrated in PM fractions obtainedfrom both biotypes using the fluorescent probe amino-chloro-methoxyacridine(ACMA), indicating a relatively large proportion of 'inside-out'vesicles. Proton-pumping was significantly greater in PM preparationsobtained from the resistant compared with susceptible plants.The differences in ATPase activity between the two biotypescould not be attributed to differences in the main sterol orphospholipid components of the PM. There were no effects ofthe herbicide on ATP hydrolysis in vitro, but proton-pumpingwas affected in a herbicide concentration-dependent manner.At 1·0 mol m mecoprop caused an increase in the rateof proton-pumping, whereas at 10 and 100 mol m^–6, an inhibitionin this rate was observed. Both biotypes behaved similarly,irrespective of mecoprop concentration. These data indicatethat mecoprop resistance in chickweed is unlikely to be dueto a direct effect of the herbicide on PM H+-ATPase activity. Key words: Stellaria media, mecoprop, ATPase, plasma membrane, herbicide resistance