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1.
The mechanisms of AOPP herbicide resistance in twoAlopecurus myosuroidesbiotypes were investigated. Resistant biotype Peldon A1, which is highly resistant to the phenyl-urea chlorotoluron, is moderately resistant to the AOPP herbicides diclofop-methyl, fenoxaprop-ethyl, fluazifop-P-butyl, and the CHD tralkoxydim. Resistant biotype Lincs. E1, which is only moderately resistant to chlorotoluron, is highly resistant to the AOPP herbicide fenoxaprop-ethyl, and moderately resistant to diclofop-methyl, fluazifop-P-butyl, and the CHD tralkoxydim. There is no clear evidence of resistance to the CHD sethoxydim in either biotype. Both Peldon A1 and Lincs. E1 exhibited moderately enhanced metabolism of diclofop-methyl. The approximate half life of diclofop was 8 and 9 HAT, respectively, compared to 17 HAT for the susceptible Rothamsted biotype. Peldon A1 showed moderately enhanced metabolism of fenoxaprop-P-ethyl. However, in the highly resistant Lincs. E1, fenoxaprop-P-ethyl metabolism rates were intermediate between Peldon A1 and the susceptible biotype. Fenoxaprop-P-ethyl metabolism inA. myosuroideswas not significantly reduced by inhibitors of cytochrome P450: PBO, tetcyclasis, or ABT. While enhanced herbicide metabolism can account for the moderate AOPP/CHD resistance observed in Peldon A1in vivo, it cannot account in total for fenoxaprop-ethyl resistance in Lincs. E1. Lincs. E1 may possess one or more additional resistance mechanism.  相似文献   

2.
Propanil-resistant barnyardgrass populations, previously verified in Arkansas rice fields and in greenhouse tests, were examined in the laboratory to ascertain if the resistance mechanism in this weed biotype was herbicide metabolism. Propanil-resistant barnyardgrass was controlled >95% in the greenhouse when carbaryl (an aryl acylamidase inhibitor) was applied two days prior to propanil. Laboratory studies with 14C-radiolabelled propanil indicated that the herbicide was hydrolysed in propanil-resistant barnyardgrass and rice to form 3,4-dichloroaniline, but no detectable hydrolysis occurred in susceptible barnyardgrass. Two additional polar metabolites were detected in propanil-resistant barnyardgrass and rice and tentatively identified by thin layer chromatography. Overall, metabolites in the resistant barnyardgrass had Rf values similar to those in rice, indicating similar metabolism for both species. These data, coupled with data from a previous report on the resistant biotype showing no differential absorption/translocation or molecular modification of the herbicide binding site in the resistant biotype, indicate that the resistance mechanism is metabolic degradation of propanil. © of SCI.  相似文献   

3.
An Avena sterilis biotype was found to be highly resistant to aryloxyphenoxypropionate (APP) herbicides, especially diclofop-methyl. At the enzyme level, this biotype contained a modified acetyl-coenzyme A carboxylase (ACCase) with six-fold resistance to diclofop acid. Absorption and translocation of [14C]diclofop-methyl applied to the leaf axil of the two-leaf stage plants were similar in both susceptible and resistant biotypes. However, the rate of metabolism of [14C]diclofop was increased 1·5-fold in this resistant biotype compared to the susceptible. Experiments with tetcyclacis, a cytochrome P450 monooxygenase inhibitor, indicated that inhibition of this enhanced diclofop metabolism increased diclofop-methyl phytotoxicity in this biotype. Studies with ten individual families of the resistant biotype indicated that both mechanisms of resistance, an altered target site and enhanced metabolism, are present in each individual of the population. Hence, it is likely that these two mechanisms of resistance both contribute to resistance in this biotype. © 1997 SCI.  相似文献   

4.
Experiments were conducted to investigate the absorption, translocation, and metabolism of propoxycarbazone-sodium in acetolactate synthase-inhibitor resistant (AR and MR) and susceptible (AS and MS) Bromus tectorum biotypes. Absorption and translocation of l4C-propoxycarbazone-sodium were similar in all biotypes. One major and three minor metabolites were identified using reverse-phase high performance liquid chromatography. In all biotypes, 80% of the propoxycarbazone-sodium was metabolized by 72 h after treatment (HAT). However, propoxycarbazone-sodium was metabolized more rapidly in the MR biotype than in the other biotypes. The half-life of propoxycarbazone-sodium in the MR biotype was 8.9 h, which was 30, 36, and 40% shorter than in the AS, AR, and MS biotypes, respectively. When 14C-propoxycarbazone-sodium was applied with 1-aminobenzotriazole, a known cytochrome P450 inhibitor, metabolism decreased 20% 12 HAT in the MR biotype. These results indicate that resistance of the MR biotype to propoxycarbazone-sodium is due to a relatively rapid rate of propoxycarbazone-sodium metabolism compared to other B. tectorum biotypes and that cytochrome P450s may be involved in the metabolism. The fact that these populations evolved so quickly and with different resistance mechanisms is a concern as more ALS inhibitors are introduced into the production systems.  相似文献   

5.
In the UK biotypes of black-grass (Alopecurus myosuroides Huds) showing resistance to both chlorotoluron (CTU) and aryloxyphenoxypropionate graminicides are increasingly being observed. Although the precise mechanisms involved in this resistance have yet to be identified, increased herbicide metabolism has been implicated as being involved in at least some cases of resistance. Glutathione S-transferases (GSTs) are a group of enzymes which have been demonstrated to metabolise herbicides in some plants, and the resistant black-grass biotype Peldon contains approximately double the GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) of susceptible biotypes. To investigate further the possible role of GSTs in herbicide resistance in black-grass, a purification procedure has been developed for these enzymes. A 27.5 kDa polypeptide possessing GST activity was purified from the susceptible biotype Herbiseed. Purification of GSTs from the resistant biotype Peldon also identified this polypeptide along with an additional 30 kDa polypeptide. An in-vitro kinetic study of both crude and purified GST extracts, and western blot analysis using antisera raised against the 27.5 kDa polypeptide, suggest that the 30 kDa polypeptide may possess GST activity, and is not a precursor of the 27.5 kDa polypeptide. These results are discussed and compared to GST profiles for other weeds and crops demonstrating herbicide resistance or tolerance. © 1999 Society of Chemical Industry  相似文献   

6.
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.  相似文献   

7.
In hydroponic experiments, seed-dressing with the herbicide safener 1,8-naphthalic anhydride (NA), significantly enhanced the tolerance of maize, (Zea mays L., cv. Monarque) to the imidazolinone herbicide, AC 263222, (2-[4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl]-5-methylnicotinic acid). Uptake, distribution and metabolism studies where [14C]AC 263222 was applied through the roots of hydroponically grown maize plants showed that NA treatment reduced the translocation of radiolabel from root to shoot tissue and accelerated the degradation of this herbicide to a hydroxylated metabolite. Reductions in the lipophilicity and, therefore, mobility of this compound following hydroxylation may account for NA-induced retention of radiolabel in the root system. Hydroxylation of AC 263222 suggested that NA may stimulate the activity of enzymes involved in oxidative herbicide metabolism, such as the cytochrome P450 mono-oxygenases. In agreement with this theory, the cytochrome P450 inhibitor, 1-aminobenzotriazole (ABT), synergized AC 263222 activity and inhibited its hyroxylation in vivo. NA seed-dressing enhanced the total cytochrome P450 and b5 content of microsomes prepared from etiolated maize shoots. Isolated microsomes catalyzed AC 263222 hydroxylation in vitro. This activity possessed the characteristics of a cytochrome P450 mono-oxygenase, being NADPH-dependent and susceptible to inhibition by ABT. Activity was stimulated four-fold following NA seed treatment. Differential NA enhancement of AC 263222 hydroxylase and the cytochrome P450-dependent cinnamic acid-4-hydroxylase (CA4H) activity, suggested that separate P450 isozymes were responsible for each activity. These results indicate that the protective effects of NA result from enhancement of AC 263222 hydroxylation and concomitant reduction in herbicide translocation. This may be attributed to the stimulation of a microsomal cytochrome P450 system. © 1998 SCI.  相似文献   

8.
To investigate the selectivity and safening action of the sulfonylurea herbicide pyrazosulfuron‐ethyl (PSE), pyrazosulfuron‐ethyl O‐demethylase (PSEOD) activity involving oxidative metabolism by cytochrome P‐450 was studied in rice (Oryza sativa L cv Nipponbare) and Cyperus serotinus Rottb. Cytochrome P‐450‐dependent activity was demonstrated by the use of the inducers 1,8‐naphthalic anhydride and ethanol, the herbicides PSE, bensulfuron‐methyl, dimepiperate and dymron, or the inhibitor piperonyl butoxide (PBO). Growth inhibition in C serotinus seedlings was more severe than that in rice seedlings. O‐Dealkylation activities of PSE were induced differently in rice and in C serotinus, with distinctly higher activity in rice seedlings. The induced PSEOD activities were slightly inhibited by PBO in rice seedlings, whereas they were strongly inhibited in C serotinus seedlings. Dimepiperate and dymron were effective safeners of rice against PSE treatment. Treatments with herbicide alone resulted in less induction of PSEOD activity compared with combined treatments of the herbicide and safener. PSEOD activity in rice seedlings induced with herbicide alone was strongly inhibited by PBO, whereas it was weakly inhibited in rice seedlings induced with combinations of PSE and two safeners. These results suggest that O‐demethylation by cytochrome P‐450 enzymes may be involved in the metabolism of PSE and may contribute to its selectivity and safening action. Furthermore, these results suggest the existence of a multiple form of cytochrome P‐450 in plants. © 2001 Society of Chemical Industry  相似文献   

9.
The degradation of chlorotoluron, 1-(3-chloro-4-methylphenyl)-3,3-dimethylurea, was investigated in laboratory and field-grown wheat and soil. Thin-layer cochromatography and, partially, derivatization and mass spectroscopy were used to elucidate the structures of the metabolites. Wheat treated with 4-methyl[14C]-phenyl-labeled chlorotoluron rapidly metabolized the herbicide using two independent mechanisms: (I) oxidation of the 4-methylphenyl group to yield 4-hydroxy-methylphenyl and 4-carboxyphenyl derivatives; and (II) N-demethylation. Mechanism (I) clearly predominated over mechanism (II). Young wheat degraded the herbicide mainly to 4-hydroxy-methylphenyl derivatives with only a small fraction being additionally N-monodemethylated. Most of both metabolites was conjugated, most probably, with glucose. In straw and grains of mature field-grown summer wheat treated postemergence with labeled chlorotoluron at a rate corresponding to 2 kg active ingredient/hectare 2.8 ppm and 0.12 ppm radioactivity equivalent to chlorotoluron were found, respectively. About 50% of this terminal radioactivity was nonextractable by organic solvents. No chlorotoluron or its N-demethylated derivatives were present in either plant part. About 40% of the radioactivity in straw consisted of 4-carboxyphenyl derivatives half of which were N-mono- or didemethylated. The rest of the terminal radioactivity was mainly in form of the 4-hydroxymethylphenyl derivative of chlorotoluron. Less than 20% of the soluble metabolites was present as conjugates. In soil mechanism (II) exceeded mechanism (I). At harvest of the wheat the 0.4 ppm radioactivity of the 0- to 30-cm soil layer was composed of 43% chlorotoluron, 36% N-mono- and 3% N-didemethylated chlorotoluron, as well as 13% 4-carboxyphenyl derivatives partly N-demethylated.  相似文献   

10.
Sulfonylurea resistance in Stellaria media [L.] Vill.   总被引：1，自引：1，他引：1  
A sulfonylurea resistant biotype of common chickweed (Stellaria media L. Vill.) was found in a field treated with chlorsulfuron or metsulfuron for eight consecutive years. In pot experiments the biotype was resistant to postemergence treatments with the following acetolactate synthase (ALS) inhibitors: chlorsulfuron, metsulfuron, tribenuron, triasulfuron, rimsulfuron, sulfometuron, flumetsulam and imazapyr. The level of resistance to chlorsulfuron and sulfometuron was higher than to the other sulfonylurea herbicides. Whereas the level of cross resistance to the triazolopyrimidine herbicide, flumetsulam was comparable to that of metsulfuron, that of imazapyr was significantly lower. In contrast to imazapyr the biotype was not resistant to imazethapyr, an other imidazolinone herbicide. ALS in vitro assays revealed that resistance was due to an ALS enzyme that was less sensitive to ALS inhibiting herbicides. Herbicides with different modes of action were equally effective on the susceptible and resistant biotypes.  相似文献   

11.
The mechanism of resistance to diclofop-methyl in three Italian populations of Lolium spp. (two resistant and one susceptible) was investigated. The major proportion of R-1 (Tuscania 1997) and R-2 (Roma 1994) plants (approximately 80%) survived after herbicide treatment by emitting new tillers from the crown. Both resistant (R-1 and R-2) and susceptible (Vetralla 1994) Lolium spp. populations were target-site sensitive. No difference in diclofop-methyl absorption by shoots of resistant and susceptible biotypes was observed. At the dose corresponding to 1× the recommended field rate, a relatively higher metabolism was found in R-2 biotype. In contrast, at the doses 2× and 10× the field rate no difference in herbicide metabolism between susceptible and resistant biotypes was observed. At all the three herbicide doses (1×, 2×, and 10× the field rate) 48 h after the treatment (HAT), the total amount of metabolites produced by wheat was more than three times higher than that produced by resistant and susceptible ryegrass biotypes. At the doses 1× and 2× the field rate, the herbicide translocation was different in the susceptible biotypes compared to resistant biotypes. The total amount of the radiolabel found 48 HAT in culm and root was approximately twice in susceptible biotype than in resistant biotypes. Susceptible and resistant ryegrass biotypes differed in the capability of their roots to acidify the external medium. Susceptible biotype acidified the external solution at approximately 6 times the rates of the resistant biotypes. In the present study, the mechanism responsible for resistance in the investigated resistant biotypes was not univocally identified. Indirect evidence supports the possible involvement of herbicide sequestration or immobilization.  相似文献   

12.
Primisulfuron‐resistant (AR and MR) and ‐susceptible (AS and MS) Bromus tectorum biotypes were collected from a Poa pratensis field at Athena, Oregon, and in research plots at Madras, Oregon. Studies were conducted to characterize the resistance of the B. tectorum biotypes. Whole plant bioassay and acetolactate synthase (ALS) enzyme assay revealed that the AR biotype was highly resistant to the sulfonylurea (SU) herbicides, primisulfuron and sulfosulfuron and to a sulfonylaminocarbonyltriazolinone (SCT) herbicide, propoxycarbazone‐sodium. However, the AR biotype was not resistant to imazamox, an imidazolinone (IMI) herbicide. Results of the whole plant bioassay studies showed that the MR biotype was moderately resistant to all ALS inhibitors tested. However, there were no differences in ALS sensitivities between the MR and MS biotypes. The nucleotide and amino acid sequence analysis of the als gene demonstrated a single‐point mutation from C to T, conferring the exchange of the amino acid proline to serine at position 197 in the AR biotype. However, this mutation was not found in the MR biotype. Results of this research indicate that: the resistance of the AR biotype to SU and SCT herbicides is based on an altered target site due to a single‐point mutation; resistance in the MR biotype is not due to a target site mutation.  相似文献   

13.
Pathways of chlortoluron metabolism were compared in excised leaves of four tolerant species, namely wheat (Triticum aestivum var Clement), Bromus sterilis, Galium aparine and Veronica persica. The herbicide was principally detoxified by hydroxylation of the ring methyl in wheat and by N-dealkylation in Veronica persica. Both pathways were involved in Bromus sterilis and Galium aparine. Kinetic study of the degradation showed that capacity to form non-toxic conjugates could, at least partially, explain the tolerance of these species to chlortoluron. In plants treated with 1-aminobenzotriazole, a cytochrome P-450 enzyme inactivator, N-dealkylation of chlortoluron was little or not affected, but ring methyl hydroxylation was strongly inhibited. This suggests that at least two distinct enzymatic systems could participate in this metabolism. Moreover, cytochrome P-450 enzymes could be involved in the ring methyl hydroxylating reaction.  相似文献   

14.
The ability of the white rot basidiomycete Phanerochaete chrysosporium to transform s-triazine herbicides has been investigated in laboratory experiments. The chlorinated metabolites formed during atrazine N-dealkylations were not further transformed by the fungus, whereas hydroxy-atrazine was converted to an unknown product. P. chrysosporium was also able to carry out the N-dealkylation of the herbicides simazine, propazine and terbuthylazine. Herbicide metabolism was not supported by purified peroxidases. The highest rates of herbicide N-dealkylation were obtained in liquid cultures maintained under moderate temperature allowing a long mycelium growing phase. Atrazine transformation was found to be supported by the mycelium, which contained significant amounts of microsomal cytochrome P450. Herbicide N-dealkylation was decreased in the presence of 1-aminobenzotriazole, in agreement with the involvement of P450 monooxygenases in atrazine metabolism. © 1997 SCI.  相似文献   

15.
In the Japanese quail, cytochrome P-450, A- and B-esterase, amidase, and glutathione S-aryl transferase were assayed in postmitochondrial centrifugal fractions, in microsomes, and supernatant fractions of liver, lungs, kidneys, and testes. Liver microsomes contained the highest A-esterase activity and P-450 levels. B-esterase was more generally distributed and higher in the microsomal tissue fractions. Microsomal amidase activity was highest in quail lung and kidney, and lowest in the liver (per mg protein). Very little difference in glutathione S-aryl transferase activity was noted among the tissues assayed. In vitro metabolism of carbaryl, phosphamidon, and chlorotoluron by the various centrifugal fractions revealed that the production of 1-naphthyl-N-hydroxymethylcarbamate and 1-naphthol, the major metabolites, was greatest in the postmitochondrial fraction of the liver. The major carbaryl metabolite in all other quail tissue fractions was 1-naphthol. Phosphamidon metabolism in postmitochondrial preparations of quail liver was higher than in the supernatant and microsomes. Chlorotoluron metabolism occurred only in the postmitochondrial fractions of quail liver. The major products were the oxidative metabolites, N-(3-chloro-4-methylphenyl)-N′-methylurea and N-(3-chloro-4-hydroxymethylphenyl)-N′-methylurea.  相似文献   

16.
Herbicide resistance poses a substantial threat to the agricultural industry throughout the world and during the past decade several reports regarding herbicide resistance have been published. Raphanus raphanistrum L., from two wheat farms located in the winter rainfall region of South Africa, showed indications of resistance to chlorsulfuron. Seeds from these suspected resistant biotypes as well as seeds from a susceptible biotype were collected and transported to the ARC-Small Grain Institute for herbicide resistance studies. Herbicides registered for R. raphanistrum control, i.e. chlorsulfuron, MCPA and bromoxynil, were used in this study. Significant differences in the degree of control were found between the susceptible and two resistant biotypes, when treated with chlorsulfuron. The LD50 values for the resistant biotypes (WR 1 & WR 2) were 45 and 11.3 g a.i. ha–1, respectively, whereas the LD50 value for the susceptible biotype was 5.6 g a.i. ha–1. The almost eightfold difference between the susceptible and resistant biotype (WR 1), indicated that resistance has developed to chlorsulfuron. Only twofold resistance was established between the other resistant biotype (WR 2) and the susceptible biotype. Significant differences between herbicide rates were also established with the MCPA and bromoxynil experiments. No significant difference could, however, be found between the susceptible and resistant biotypes when treated with MCPA and bromoxynil, indicating the importance of different modes of action of herbicide as a strategy to prevent herbicide resistance.  相似文献   

17.
A soybean cytochrome P450 monooxygenase, designated CYP71A10, catalyzes the oxidative N-demethylation or ring methyl hydroxylation of a variety of phenylurea herbicides. The ectopic expression of CYP71A10 in tobacco was previously shown to be an effective means of enhancing whole plant tolerance to the compounds linuron and chlortoluron. Because P450 enzymes require ancillary proteins to catalyze the transfer of electrons from NADPH to the functional heme group of the P450, it is possible that the endogenous levels of these companion proteins may be insufficient to support the optimal activation of a highly expressed recombinant P450. In the present report, we have generated transgenic tobacco that simultaneously express CYP71A10 and a soybean P450 reductase. Transformed plants that express both CYP71A10 and the P450 reductase demonstrated 20-23% higher metabolic activity against phenylurea herbicides than control plants expressing CYP71A10 alone. These results suggest that herbicide tolerance strategies based on the expression of P450 genes may require concomitant expression of a cognate electron transport partner to fully exploit the herbicide metabolic capacity of the P450.  相似文献   

18.
BACKGROUND: Quinclorac (3,7-dichloro-quinoline-carboxylic acid) is a selective herbicide widely used to control annual grasses and certain broadleaf weeds. Echinochloa phyllopogon (Stapf) Koss. is the most noxious grass weed in California rice fields and has evolved resistance to multiple herbicides with different modes of action. A quinclorac-resistant (R) E. phyllopogon biotype found in a Sacramento Valley rice field where quinclorac has never been applied was investigated. RESULTS: Resistant to susceptible (S) GR50 (herbicide rate for 50% growth reduction) ratios ranged from 6 to 17. The cytochrome P450 inhibitor malathion (200 mg L−1) caused R plants to become as quinclorac susceptible as S plants. Quinclorac rapidly (6 HAT) stimulated ethylene formation in S plants, but only marginally in R plants. Malathion pretreatment did not reduce ethylene formation by quinclorac-treated S and R plants. Activity of β-cyanoalanine synthase (β-CAS) in tissue extracts was 2-3-fold greater in R than in S plants, and incubation of shoot extracts with 1 mM malathion reduced β-CAS activity by 40% in both biotypes. CONCLUSION: Resistance to quinclorac in R E. phyllopogon involved at least two mechanisms: (a) insensitivity along the response pathway whereby quinclorac induces ethylene production; (b) enhanced β-CAS activity, which should enable greater HCN detoxification following quinclorac stimulation of ethylene biosynthesis. This unveils new resistance mechanisms for this multiple-resistant biotype widely spread throughout California rice fields. Copyright © 2011 Society of Chemical Industry  相似文献   

19.
The metabolic fate of 14C-labeled chlorotoluron, i.e., 1-(3-chloro-4-methyl[4C]-phenyl)-3,3-dimethyl urea, was followed in rats. After a single oral dose the radioactivity was preferably excreted with the urine. Nine of the eleven urinary metabolites isolated, were identified by spectroscopic and derivatization techniques, whereas the structure of the remaining two metabolites was only partially elucidated. N-Demethylation and stepwise oxidation of the ring methyl group to hydroxymethyl and carboxyl derivatives were found as the major metabolic mechanisms. Both mechanisms proceeded simultaneously so that the isolated metabolites showed all combinations of N-demethylation and ring methyl group oxidation in their structures. One of these metabolites was an N-formyl derivative, being probably an intermediate product of demethylation. In the urine of rats fed doses of [14C]chlorotoluron higher than 50 mg/kg three additional metabolites with different degrees of N-dealkylation were found, the ring methyl group of which was transformed to a methylthio methyl group. The metabolites identified in the faeces were of the same type as those found in the urine. Based on the structures of the metabolites elucidated, a metabolic pathway of chlorotoluron in the rat is presented.  相似文献   

20.
A population of Bromus tectorum infesting an olive grove at Córdoba (Spain) survived simazine use rates of 3.0 kg a.i. ha−1 over two consecutive years. Non‐tillage olive monoculture and two annual simazine applications had been carried out for 10 years. The resistant biotype showed a higher ED50 value (7.3 kg a.i. ha−1) than that of the susceptible control (0.1 kg a.i. ha−1), a 73‐fold increase in herbicide tolerance. The use of fluorescence, Hill reaction, absorption, translocation and metabolism assays showed that simazine resistance in this biotype was caused by a modification of the herbicide target site, since chloroplasts from the resistant biotype of B. tectorum were more than 300 times less sensitive to simazine than those from the susceptible biotype. In addition, non‐treated resistant plants of B. tectorum displayed a significant reduction in the QA to QB electron transfer rate when compared with the susceptible biotype, a characteristic that has been linked to several mutations in the protein D1 conferring resistance to PS II inhibiting herbicides. Resistant plants showed cross‐resistance to other groups of triazine herbicides with the hierarchy of resistance level being methoxy‐s‐triazines ≥chloro‐s‐triazines > methylthio‐s‐triazines > cis‐triazines. The results indicate a naturally occurring target‐site point mutation is responsible for conferring resistance to triazine herbicides. This represents the first documented report of target site triazine resistance in this downy brome biotype.  相似文献   

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