Metabolism of chlortoluron in tolerant species: possible role of cytochrome P-450 mono-oxygenases

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.     

Effect of ABT on the activity and rate of degradation of isoproturon in susceptible and resistant biotypes of Phalaris minor and in wheat

The effect of the monooxygenase inhibitor, 1-aminobenzotriazole (ABT) on isoproturon phytotoxicity and metabolism was studied in resistant (R) and susceptible (S) biotypes of Phalaris minor and in wheat (Triticum aestivum). Addition of ABT (2·5, 5 and 10 mg litre^-1) to isoproturon (0·25, 0·5, 1, 2 and 4 mg litre^-1) in the nutrient solution significantly enhanced the phytotoxicity of isoproturon against the R biotype. Isoproturon at 0·25 mg litre^-1 reduced the dry weight (DW) of the S biotype by 77%, whereas the R biotype required 4·0 mg litre^-1 for similar reduction. Addition of 10 mg litre^-1 of ABT to the 0·25 mg litre^-1 isoproturon caused 71 and 82% reduction in DW of R and S biotypes, respectively. Wheat was more sensitive to the mixture of isoproturon and ABT than the R biotype of P. minor. Reduced concentrations of ABT in the mixture from 10 to 2·5 mg litre^-1 increased the DW of the R biotype more than that of the S biotype. The R biotype metabolised [¹⁴C]isoproturon at a faster rate than the S biotype. ABT (5 mg litre^-1) inhibited the degradation of [¹⁴C]isoproturon in both biotypes of P. minor and in wheat. In the presence of ABT, about half of the applied [¹⁴C]isoproturon remained as parent herbicide in all the three species after two days. The metabolites were similar in the R and S biotypes and wheat as determined by co-chromatography with reference standards and mass spectroscopy (MS). ABT inhibited the appearance of the hydroxy and monomethyl metabolites and their conjugates in all the test plants. These results suggest that the activity of the enzymes responsible for the degradation of isoproturon is greater in the R than in the S biotype of P. minor, resulting in its rapid detoxification. Incorporation of the monooxygenase inhibitor ABT into the nutrient solution greatly inhibited the degradation of [¹⁴C]isoproturon in the R biotype and increased its phytotoxicity. Both hydroxylation and N-dealkylation reactions were found to be sensitive to ABT; inhibition of hydroxylation was greater than that of demethylation. Since ABT could not completely suppress isoproturon degradation, it is possible that more than one monooxygenase is involved. © 1998 SCI     

Influence of 1-aminobenzotriazole (ABT) derivatives on the toxicity of EPTC to monocotyledonous plants

The effect of l-aminobenzotriazole (ABT) and EPTC mixtures on plant growth was investigated in greenhouse experiments by presowing application in sand. The concentration of ABT for 50% shoot growth inhibition (I₅₀) was about 1 mM in the monoctyledons investigated. Both subtoxic and toxic doses of ABT were antagonists of EPTC in maize (Zea mays L.) However, subtoxic doses of ABT were syner-gists of EPTC in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and oat (Avena sativa L.). No correlation between ABT and EPTC sensitivity of monocotyledonous plants was found. Some of the C- and N-substituted ABT derivatives (e.g. N-methyl or N-benzyl-ABT) completely reversed the effect of toxic dose of EPTC in maize. These results are considered as indirect evidence for the hypothesis that oxidation by cytochrome P-450 enzymes is the biochemical target of EPTC and is involved in the mode of action of EPTC safeners.     

Mode of action of naphthalic anhydride as a safener for the herbicide AC 263222 in maize

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.     

除草剂安全剂对作物细胞色素P450及其他酶活性和水平的影响

综述了除草剂安全剂对作物中参与除草剂解毒作用的酶以及作为除草剂作用靶标位点酶水平与活性的影响。安全剂能增强细胞色素P450酶系统活性,诱导P450在除草剂降解中的作用;增加作物体内谷胱甘肽的含量,从而促进除草剂与谷胱甘肽的轭合而发挥解毒作用;降低由于除草剂对乙酰乳酸合成酶的抑制作用而引起的植物毒性等。     

Co-expression of a NADPH:P450 reductase enhances CYP71A10-dependent phenylurea metabolism in tobacco

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.     

Biochemical aspects of the fungitoxic and growth regulatory action of fenarimol and other pyrimidin-5-ylmethanols

The C-14 demethylation of the sterols, dependent on cytochrome P-450, and the C-22(23) desaturation of sterols are reactions in the ergosterol biosynthesis pathway that are regarded as primary target sites in the toxicity of fungicides of the pyrimidin-5-ylmethanol type. Currently, there is no evidence for target sites in other pathways of comparable sensitivity, although the failure of added ergosterol to reverse the fungitoxicity suggests the existence of such sites. The mitochondrial respiratory systems in Ustilago maydis and Aspergillus nidulans are insensitive to this type of fungicide and are not regarded as primary targets of fungitoxicity in these organisms. Appreciable evidence indicates that the primary targets in higher plant growth regulation are reactions, dependent on cytochrome P-450, that assist in the conversion of kaurene to kaurenoic acid in the gibberellin (GA) biosynthesis pathway. Growth retardation by high concentrations of pyrimidin-5-ylmethanols, which is not reversable by GA, apparently involves action at sites outside the GA biosynthesis pathway. The data derived from various studies of the mechanisms of fungitoxicity and growth regulation suggest that any undetected primary targets of the pyrimidin-5-ylmethanols are likely to be haem enzymes, similar to the cytochrome P-450 involved in sterol C-14 demethylation.     

Phytotoxicity and metabolism of chlortoluron in two wheat varieties

Varietal susceptibility of winter wheat to chlortoluron, 1-(3-chloro-4-methylphenyl)-3,3 dimethylurea, has been studied in two varieties, Corin (susceptible) and Clement (tolerant). After a 24-hr root absorption of the herbicide, phytotoxicity was estimated from growth measurements. When administered at 12 to 96 μM concentrations, the herbicide reduced the growth of both varieties. A significant selective effect was found at 96 μM. Measurements of chlorophyll fluorescence-induction kinetics allowed to discriminate between the two varieties treated with 12 to 48 μM chlortoluron. The metabolism of chlortoluron was studied following absorption of 24 μM solutions. Both varieties produced the same pattern of metabolites but the tolerant variety degraded the herbicide and the phytotoxic mono-N-demethylated metabolite at a slightly higher rate. An unexpected result was that the more susceptible variety possessed a very significant ability to metabolize chlortoluron. In conclusion, it appears that further studies are necessary before deciding whether the differences in susceptibility of the two varieties can be explained by the only metabolic factor.     

N-acyl-N-alkyl 1-aminobenzotriazoles: phytotoxicity, antagonism to EPTC and effects on maize cytochrome P-450

Eight amino-substituted derivatives of 1-amino-benzotriazole (ABT) were synthesized and tested. N-acetyl-N-methyl ABT (AcMeABT) was found to kill maize (Zea mays L.) hybrid Pioneer 3737 at 5 kg ha^?1 dose while ryegrass (Lolium perenne L.) was unaffected. The analogous N-propionyl-N-methyl ABT had an activity similar to AcMeABT whereas other N-acyl-N-alkyl derivatives had only moderate or no effects on maize. AcMeABT was considerably toxic to oats (Avena sativa L.) and sorghum (Sorghum bicolor L.) as well. Unlike ABT, AcMeABT did not show any in vitro inhibitory effect on cytochrome P-450 enzymes of maize microsomes. However, in vivo AcMeABT pre-treatment significantly decreased the microsomal cytochrome P-450 level of aetiolated maize seedlings. Moreover, a subtoxic dose of AcMeABT was a weak antagonist of a toxic dose of EPTC in maize in vivo. These data predict the involvement of cytochrome P-450 enzyme in the mode of action of AcMeABT. N-acyl-N-alkyl l-aminobenzotriazoles: phyto-toxicité, antagonisme avec l'EPTC et effets sur le cytochrome P-450 du maïe Huit dérivés sur le groupement amine de l'1-aminobenzotriazole (ABT) ont été synthétisés. Une dose 5 kg ha^?1 de N-acétyl-N-méthyl ABT (AcMeABT) détruisait le maïs (Zea mays L.) hy-bride Pioneer 3737 alors que le raygrass (Lolium perenne L.) n'était pas affecté. L'analogue N-propionyl-N-méthyl ABT avail une activité simi-laire à celle de l' AcMeABT, alors que les autres dérivés N-acyl-N-aklyl n'avaient que des effets nuls ou faibles surle maïs. L'AcMeABTétait très toxique à l'égard de l'avoine (Avena sativa L.) et du sorgho bicolore (Sorghum bicolor L.). Con-trairement à l'ABT, l'AcMeABT n'inhibait pas in vitro les enzymes P-450 des microsomes du maïs. Cependant, un pré-traitement in vivo a l'AcMeABT diminuait significativement le niveau de cytochromes P-450 microsomaux de jeunes plantes étiolées de maïs. En outre, une dose subtoxique d'AcMeABT avail sur maïs in vivo un effet antagoniste faible contre une dose loxique d'EPTC. Ces données suggèrent l'impli-cation d'enzymes cytochrome P-450 dans le mode d'action de l'AcMeABT. Phytotoxizität, Antagonismus gegenüber EPTC und Wirkungen auf das Cytochrom P-450 von N-Acyl-N-alkyl-1-aminobenztriazolen Bei Versuchen mit 8 Amino-subslituierten Deri-vaten von 1-Aminobenzotriazol (ABT) war N-Acetyl-N-methyl-1-aminobenztriazol (AcMeABT) mit 5 kg ha^?1 für Mais (Zea mays L.) ‘Pionier 3737’ phyloloxisch, für Deulsches Weidelgras (Lolium perenne L.) nicht. Das analoge N-Propionyl-N-methyl-1-aminobenztriazol war ähnlich wirksam wie AcBeABT, andere Deri-vale kaum oder gar nichte. AcMeABT war für Saat-Hafer (Avena sativa L.) und Sorghumhirse (Sorghum bicolor L.) erheblich phytotoxisch. Anders als ABT zeigte AcMeABT in vitro keine Hemmwirkung auf das Cytochrom P-450 in Mais-Mikrosomen, in vivo jedoch nahm der Cy-tochrom-P-450-Gehalt etiolierter Mais-keimpflanzen nach AcMeABT-Behandlungen signifikantab. Außerdem war eine subtoxische Dosis von AcMeABT schwach antagonistisch für eine für Mais toxische EPTC-Dosis. Diese Daten weisen auf die Beteiligung des Cytochrom-P-450-Enzyms an der Wirkungsweise des AcMeABT hin.     

吡唑解草酯对小麦细胞色素P450的诱导作用及其光谱特征

用吡唑解草酯浇灌小麦,试验结果表明,50μmol/L吡唑解草酯处理抗6号小麦,可使其细胞色素P450含量达到最大值108.18 pmol/mg蛋白质,为对照组的1.67倍;100μmol/L吡唑解草酯处理敏18号小麦,可使其细胞色素P450含量达到最大值80.97 pmol/mg蛋白质,为对照组的1.86倍。吡唑解草酯对两个小麦品种的细胞色素P450均有诱导作用,抗6号小麦更容易被诱导,这与两小麦品种的耐药性一致。室温(20±1)℃下扫描不同时间的细胞色素P450-CO结合光谱,结果表明,微粒体粗提液室温(20±1)℃保存200 min后,细胞色素P450完全转变为细胞色素P420。     

Responses to clomazone and 5-ketoclomazone by Echinochloa phyllopogon resistant to multiple herbicides in Californian rice fields

BACKGROUND: Late watergrass [Echinochloa phyllopogon (Stapf.) Koss.] is a major weed of Californian rice that has evolved P450-mediated metabolic resistance to multiple herbicides. Resistant (R) populations are also poorly controlled by the recently introduced herbicide clomazone. The authors assessed whether this cross-resistance was also P450 mediated, and whether R plants also had reduced sensitivity to photooxidation. Understanding mechanism(s) of resistance facilitates the design of herbicide management strategies to delay resistance evolution.RESULTS Ratios (R/S) of R to susceptible (S) GR(50) were near 2.0. [(14)C]Clomazone uptake was similar in R and S plants. Clomazone and its metabolite 5-ketoclomazone reduced chlorophyll and carotenoids in S more than in R plants. The P450 inhibitors disulfoton and 1-aminobenzo-triazole (ABT) safened clomazone in R and S plants. Disulfoton safened 5-ketoclomazone only in S plants, while ABT synergized 5-ketoclomazone mostly against S plants. Paraquat was more toxic in S than in R plants.CONCLUSION: Cross-resistance to clomazone explains failures to control R plants in rice fields, and safening by P450 inhibitors suggests that oxidative activation of clomazone is needed for toxicity to E. phyllopogon. Clomazone resistance requires mitigation of 5-ketoclomazone toxicity, but P450 detoxification may not significantly confer resistance, as P450 inhibitors poorly synergized 5-ketoclopmazone in R plants. Responses to paraquat suggest research on mechanisms to mitigate photooxidation in R and S plants is needed. Copyright (c) 2008 Society of Chemical Industry.     

Involvement of cytochrome P‐450 enzyme activity in the selectivity and safening action of pyrazosulfuron‐ethyl

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     

Effect of recent cropping history and herbicide use on the degradation rates of isoproturon in soils

Five soil samples were taken from each of five fields with different crop management histories. Three of the fields were in an arable rotation, the fourth field was temporary grassland, and the final field was under permanent grass. Of the three arable fields, two had been cropped with winter wheat in three of the preceding 6 years, and the third had last been cropped with winter wheat once only, 6 years previously. With one exception, the winter wheat had been sprayed with the herbicide isoproturon. The rate of isoproturon degradation in laboratory incubations was strongly related to the previous management practices. In the five soils from the field that had been treated most regularly with isoproturon in recent years, <2.5% of the initial dose remained after 14 days, indicating considerable enhancement of degradation. In the soils from the field with two applications of the herbicide in the past 6 years, residues after 27 days varied from 5% to 37% of the amount applied. In soils from the other three sites, residue levels were less variable, and were inversely related to microbial biomass. In studies with selected soils from the field that had received three applications of isoproturon in the previous 6 years, kinetics of degradation were not first‐order but were indicative of microbial adaptation, and the average time to 50% loss of the herbicide (DT₅₀) was 7.5 days. In selected soils from the field that had received just one application of isoproturon, degradation followed first‐order kinetics, indicative of cometabolism. Pre‐incubation of isoproturon in soil from the five fields led to significant enhancement of degradation only in the samples from the two fields that had a recent history of isoproturon application.     

细胞色素P450抑制剂对白腐菌Phlebia lindtneri降解氯丹的影响

为确定细胞色素P450酶系在白腐菌Phlebia lindtneri降解有机氯类农药氯丹中的作用,在液体培养条件下,以胡椒基丁醚和1-氨基苯并三唑作为细胞色素P450的抑制剂,分析了不同浓度抑制剂对氯丹的降解、代谢产物的生成以及中间代谢物降解的影响。结果表明:与对照相比,高浓度(1.0 mmol/L)胡椒基丁醚或1-氨基苯并三唑处理15 d后氯丹的降解率均下降了约60%,且其代谢产物中除七氯和二氯代六氯的检出量显著增加外,其余环氧化及羟基化代谢产物均未检出;此外,添加高浓度的抑制剂可导致白腐菌降解中间代谢产物七氯及二氯代六氯的降解率下降90%以上,氧化氯丹的降解率下降70%以上,但不影响环氧七氯的降解。表明细胞色素P450可能是氯丹降解的主要酶系,其参与催化了氯丹的初始羟基化和水解、七氯和二氯代六氯的环氧化及氧化氯丹的水解等多个反应过程。     

Effect of monooxygenase inhibitors on bentazon uptake and metabolism in maize cell suspension cultures

Maize “black Mexican sweet” (BMS) cell suspension cultures were used to study the effects of various cytochrome P450 monooxygenase inhibitors on the uptake and metabolism of the herbicide bentazon. Maize cells rapidly absorbed bentazon and metabolized it via aryl hydroxylation and glycosylation to a glycosyl conjugate of 6-hydroxybentazon. BMS cells accumulated bentazon to levels approximately 20-fold greater than those in the external medium. When BMS cells were incubated in an external medium containing 25 μM bentazon, the formation of the glycosyl conjugate (ca. 2 nmol/min/g fresh wt) was rate-limited by aryl hydroxylation. Tetcyclacis, a plant growth retardant, phenylhydrazine, a mechanism-based cytochrome P450 inhibitor, and piperonyl butoxide, an insecticide synergist, inhibited bentazon metabolism with I₅₀ values of approximately 0.1, 1.0, and 1.0 μM, respectively. Other mechanism-based cytochrome P450 inhibitors, 3(2,4-dichlorophenoxy)-1-propyne and aminobenzotriazole, also inhibited bentazon metabolism but were less effective. The results obtained with selected inhibitors are consistent with the hypothesis that aryl hydroxylation of bentazon is catalyzed by a cytochrome P450 monooxygenase.     

Expression of a wheat cytochrome P450 monooxygenase in yeast and its inhibition by glyphosate

Glyphosate is a non-selective herbicide which acts by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase. Wheat cytochrome P450 monooxygenase specifically catalyzes the metabolism of some sulfonylurea herbicides such as chlorsulfuron and triasulfuron. Here we report that glyphosate is an inhibitor of a wheat cytochrome (CYP71C6v1), the cDNA of which was amplified by RT-PCR and heterologously expressed in yeast. The microsomal fractions derived from this strain had a Soret peak at 502 nm in the reduced carbon monoxide difference spectrum, which is a typical spectral characteristic. The addition of glyphosate to the microsomal protein resulted in a Type II spectrum indicative of binding via the nitrogen group to haem of cytochrome P450 as a sixth ligand. A spectral dissociation constant, K(s) of 70 micromol litre(-1) was observed and an IC50 of 11 micromol litre(-1) was found for glyphosate inhibition of CYP71C6v1 P450 activity.     

基于生理指标早期诊断异丙隆对小麦药害的研究

为了早期诊断异丙隆对小麦的药害,采用室内生测的方法测定了不同剂量异丙隆作用下小麦株高、地上部鲜重、叶绿素、丙二醛(MDA)、可溶性糖、脯氨酸(Pro)含量和最大光量子产量(F_v/F_m)。结果表明,除可溶性糖外,叶绿素、MDA、Pro、F_v/F_m均能快速响应异丙隆对小麦的胁迫。当50%异丙隆WP施用剂量≥3 000 g/hm~2时,叶绿素a、b、总叶绿素含量和F_v/F_m明显下降,Pro含量明显上升,与清水对照相比均达显著水平,MDA含量随着施药剂量的增加而逐渐上升,当剂量≥2 250 g/hm~2时,小麦MDA含量显著高于清水对照处理;叶绿素、MDA、Pro、F_v/F_m可作为诊断异丙隆对小麦药害的敏感指标。     

Isolation and functional characterization in yeast of CYP72A18, a rice cytochrome P450 that catalyzes (ω-1)-hydroxylation of the herbicide pelargonic acid

Cytochrome P450 proteins play important roles in plant herbicide selectivity. Here, we demonstrate metabolism of the herbicide pelargonic acid by CYP72A18, a novel cytochrome P450 isolated from the rice Oryza sativa L. cv. Nipponbare. The CYP72A18 cDNA was cloned from rice and heterologously expressed in Saccharomyces cerevisiae AH22 cells from the alcohol dehydrogenase (ADH1) promoter. Microsomes isolated from recombinant yeast cells contained the CYP72A18, which was found to catalyze the (ω-1)-hydroxylation of the herbicide pelargonic acid. We also show that (ω-1)-hydroxypelargonic acid has reduced herbicide activity against rice seedlings. Based on these results, we suggest that CYP72A18 participates in the detoxification of the herbicide pelargonic acid in rice plants.     

The interaction between planting depth of four winter wheat cultivars, Alopecurus myosuroides Huds. and Bromus sterilis L. and their susceptibility to post-emergence applications of isoproturon and chlorotoluron

Seeds of four winter wheat cultivars, Slejpner, Galahad, Avalon and Penman, were sown at depths ranging from 6–75 mm in soil in pots, and isoproturon or chlorotoluron was then applied to the soil surface. For chlorotoluron-treated plants (both pre- and post-emergence) the dose required to produce a 50% effect (ED₅₀) was unaffected by depth of planting. In contrast, for isoproturon applied pre-emergence, the ED₅₀ for both Avalon and Slejpner was strongly affected by sowing depth. Although chlorotoluron was much more active in a second experiment when applied post-emergence to Slejpner wheat, the ED₅₀ for both herbicides increased with greater depth of sowing. Protection of wheat from isoproturon damage by deeper planting was enhanced if the adsorption capacity of the soil was raised from K_d 0.5 to 2.0 by incorporation of activated charcoal in the soil. Isoproturon entry into plants (as measured by the effect on rate of photosynthesis) was slower in those that had been sown deeper and were growing in more adsorptive soils, but there was no obvious relationship between these observations and isoproturon distribution in the soil profile. In nutrient culture the four wheat cultivars responded similarly to a range of doses of isoproturon. The chlorotoluron-sensitive cultivars, Slejpner and Galahad, were damaged by much lower doses of chlorotoluron than were Avalon and Penman. Bromus sterilis L. responded similarly to wheat with regard to its interaction with isoproturon and planting depth. Alopecurus myosuroides Huds., however, was less damaged by isoproturon when the zone above the seed was protected from the herbicide by growing the shoot through a plastic straw.     

Effect of heat-treating soil and straw on the subsequent adsorption of chlortoluron and atrazine

The adsorption of chlortoluron and atrazine was measured on a range wheat straw residues that had been heated for periods of 1–5 min at temperatures up to 700°C and and on three soils that had been heated at up to 250°C for 1–5 min. Adsorption of both compounds by straw ash increased with increasing temperature of pretreatment. Atrazine adsorption by any of the soils was not changed markedly chlortoluron absorption was increased with two of the three soils. The results are discussed in terms of the changes in organic functional groups, that are likely to have been produced by the treatment, in relation to the adsorption mechanisms thatmay operate on the two compounds. Adsorption of chlortoluron by mixtures of ash and soil was less than the sum of the adsorption capacities of each component on its own in the cases of two out of three soils. It is concluded that the effect of straw burning on herbicide performance in the field will be variable but could be substantial. Le chauffage du sol et du chaume: influence sur l’adsorption ultérieure de chlortoluron et de l'atrazine