The herbicidal activity of bromoxyside (3,5–dibromo-4–hydroxyphenyl methyl sulphide), its esters and related compounds

Studies on the herbicidal activity of 3,5–dibromo-4–hydroxybenzonitrile (bromoxynil) and 3,5–dibromo-4-hydroxyphenyl methyl sulphone (bromoxysone) have been extended to include 3,5–dibromo-4-hydroxyphenyl methyl sulphide (bromoxyside), its esters and other related compounds. Bromoxyside produced chlorotic effects in barley and wild oat seedlings, the herbicidal activity being similar to that of bromoxysone but quite different to that of bromoxynil. A range of aliphatic and aromatic esters of bromoxyside were prepared and examined as herbicides using barley and wild oat seedlings as test plants. The acetate and propionate were the most active against both species. No compound tested was more herbicidal to wild oat than to barley. Introduction of a further bromo or methyl ring-substituent into bromoxyside removed herbicidal activity. Spraying plants of differing age showed that the herbicidal effect was greater when the plants were small.     

Selectivity of diclofop-methyl between wheat and wild oat: growth and herbicide metabolism

Abstract Growth of the second leaf of susceptible wild oat (Avena fatua L.) was inhibited within 2 days after treatment with the herbicide, diclofop-methyl, in the 1-1/2 leaf stage. Leaf growth of resistant wheat (Triticum aestivum L.) was unaffected by diclofop-methyl. In wild oat. chlorosis developed 1 day after leaf growth was inhibited. Foliar absorption of diclofop-methyl was similar between wild oat and wheat with 67 and 61% of the recovered radioactivity from [¹⁴C]diclofop-methyl being absorbed by wild oat and wheat, respectively, after 4 days. Wild oal was equally sensitive to the methyl ester and acid forms of the herbicide when the compounds were injected into the stem. Wheat was unaffected by both forms when treated similarly. Very little diclofop-methyl and diclofop (combined total of 10 to 12% in wild oat and 5 to 7% in wheat) remained in plant tissues 2 days after leaf treatment in both susceptible and resistant plants. Therefore, the active form of the herbicide must inhibit growth of susceptible plants very rapidly and at relatively low concentrations. Diclofop-methyl was rapidly hydrolyzed to diclofop by wild oat and wheat. Wild oat predominantly conjugated diclofop to an ester conjugate but wheat hydroxylated the 2,4-dichlorophenyl ring and formed a phenolic conjugate. The formation of the different conjugates between wild oat and wheat was the most significant difference in metabolism between the two species. Nearly 60 and 70% of the methanol-soluble radioactivity was present as water-soluble conjugates in wild oat and wheat, respectively, 4 days after treatment.     

Cross-Resistance to Herbicides in Annual Ryegrass (Lolium rigidum): I. Properties of the Herbicide Target Enzymes Acetyl-Coenzyme A Carboxylase and Acetolactate Synthase

Lolium rigidum biotype SR4/84 is resistant to the herbicides diclofop-methyl and chlorsulfuron when grown in the field, in pots, and in hydroponics. Similar extractable activities and affinities for acetyl-coenzyme A of carboxylase (ACCase), an enzyme inhibited by diclofop-methyl, were found for susceptible and resistant L. rigidum. ACCase activity from both biotypes was inhibited by diclofop-methyl, diclofop acid, haloxyfop acid, fluazifop acid, sethoxydim, and tralkoxydim but not by chlorsulfuron or trifluralin. Exposure of plants to diclofop-methyl did not induce any changes in either the extractable activities or the herbicide inhibition kinetics of ACCase. It is concluded that, in contrast to diclofop resistance in L. multiflorum and diclofop tolerance in many dicots, the basis of resistance to diclofop-methyl and to other aryloxyphenoxypropionate and cyclohexanedione herbicides in L. rigidum is not due to the altered inhibition characteristics or expression of the enzyme ACCase. The extractable activities and substrate affinity of acetolactate synthase (ALS), an enzyme inhibited by chlorsulfuron, from susceptible and resistant biotypes of L. rigidum were similar. ALS from susceptible and resistant plants was equally inhibited by chlorsulfuron. Prior exposure of plants to 100 millimolar chlorsulfuron did not affect the inhibition kinetics. It is concluded that resistance to chlorsulfuron is not caused by alterations in either the expression or inhibition characteristics of ALS.     

Effects of diclofop and diclofop-methyl on membrane potentials in roots of intact oat, maize, and pea seedlings

Growth and electrophysiological studies in roots of intact diclofop-methyl susceptible and resistant seedlings were conducted to test the hypothesis that the herbicide acts primarily as a proton ionophore. The ester formulation of diclofop, at 0.2 micromolar, completely inhibited root growth in herbicide-susceptible oat (Avena sativa L.) after a 96 hour treatment, but induced only a delayed transient depolarization of the membrane potential in oat root cortical cells. Root growth in susceptible maize (Zea mays L.) seedlings was dramatically reduced by exposure to 0.8 micromolar diclofop-methyl, while the same diclofop-methyl exposure hyperpolarized the membrane potential within 48 hours after treatment. Furthermore, exposure of maize roots to the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP) (50 nanomolar), inhibited growth by only 31%, 96 hours after treatment, while the same CCCP exposure depolarized the resting potential by an average of 32 millivolts. Thus, the protonophore hypothesis cannot account for a differential membrane response to phytotoxic levels of diclofop-methyl in two susceptible species. From the results of others, much of the evidence to support the protonophore hypothesis was obtained using high concentrations of diclofop acid (100 micromolar). At a similar concentration, we also report a rapid (3 minute) diclofop-induced depolarization of the membrane potential in roots of susceptible oat and maize, moderately tolerant barley (Hordeum vulgare L.), and resistant pea (Pisum sativum L.) seedlings. Moreover, 100 micromolar diclofop acid inhibited growth in excised cultured pea roots. In contrast, 100 micromolar diclofop-methyl did not inhibit root growth. Since the membrane response to 100 micromolar diclofop acid does not correspond to differential herbicide sensitivity under field conditions, results obtained with very high levels of diclofop acid are probably physiologically irrelevant. The results of this study suggest that the effect of diclofop-methyl on the membrane potentials of susceptible species is probably unrelated to the primary inhibitory effect of the herbicide on plant growth.     

Effects of diclofop and diclofop-methyl on the membrane potentials of wheat and oat coleoptiles

Electrophysiological measurements were made on the mesophyll cells of wheat (Triticum aestivum L. cv Waldron) and oat (Avena sativa L. cv Garry) coleoptiles treated either with the herbicide diclofop-methyl (methyl 2-(4-(2′,4′-dichlorophenoxy)phenoxy)propanoate), or it's primary metabolite diclofop, (2-(4-(2′,4′-dichlorophenoxy)phenoxy)-propanoic acid). Application of a 100 micromolar solution of diclofop-methyl to wheat coleoptiles had little or no effect on the membrane potential (E_M), however in oat, E_M slowly depolarized to the diffusion potential (E_D). At pH 5.7, 100 micromolar diclofop rapidly abolished the electrogenic component of the membrane potential in both oat and wheat coleoptiles with half-times of 5 to 10 minutes and 15 to 20 minutes, respectively. The concentrations giving half-maximal depolarizations in wheat were 20 to 30 micromolar compared to 10 to 20 micromolar in oat. The depolarizing response was not due to a general increase in membrane permeability as judged from the E_M's response to changes in K⁺, Na⁺, Cl⁻, and SO₄²⁻, before and after treatment with diclofop and from its response to KCN treatment. In both plants, diclofop increased the membrane permeability to protons, making the E_M strongly dependent upon the external pH in the range of pH 5.5 to pH 8.5. The effects of diclofop can best be explained by its action as a specific proton ionophore that shuttles protons across the plasmalemma. The rapidity of the cell's response to both diclofop-methyl (15-20 minutes) and diclofop (2-5 minutes) makes the ionophoric activity a likely candidate for the earliest herbicidal event exhibited by these compounds.     

Aryl hydroxylation: A selective mechanism for the herbicides, diclofop-methyl and clofop-isohutyl, in gramineous species

Esters of substituted phenoxy-phenoxy propionic acid constitute a new class of herbicides that are effective against gramineous weed and crop species. Slight changes in chemical structure alter drastically the spectrum of weeds controlled by this class of herbicides. Wheat (Triticum aestivum L.) is resistant to diclofop-methyl (methyl 2-[4-(2′,4′-dichlorophenoxy)phenoxy] propanoate) (DM) and clofop-isobutyl (iso-butyl 2-[4-(4′-phenoxy)phenoxy] propanoate) (CI), oat (Avena sativa L.) and wild oat (Avena fatua L.) are susceptible to DM but resistant to CI, and corn (Zea mays L.) is susceptible to both compounds. The antagonism of IAA-induced elongation in the coleoptile straight growth test was determined to measure biological activity of the herbicides. The basis for the differential responses by gramineous species was related to the metabolism and deioxication of the herbicides in coleoptiles. Growth of wheat coleaptiles was relatively unaffected by both compounds, oat coleoptile growth was inhibited by DM but not by CI. but corn coleoptile growth was inhibited equally by both compounds. Coleoptiles and excised shoots of the three species rapidly hydrolyzed both compounds to their respective acids (diclofop, clofop). The acids were conjugated to a water-soluble ester conjugate or they were hydroxylated in the chlorine-substituted phenyl ring and conjugated as a phenolic conjugate. Aryl hydroxylation is a detoxication mechanism in resistant plants. Plants resistant to DM or CI contained low concentrations of the parent ester and the free or bound (ester conjugate) acid and a high concentration of free or bound (phenolic conjugate) aryl hydroxylated acid in coleoptile and shoot tissues, Differential responses by the three gramineous species to DM and CI axe due apparently to differences in their detoxication mechanism. The enzyme for aryl hydroxylation in oat appears to have a higher affinity for the 4-chloro- than for the 2,4-dichloro-substituted moiety. Therefore, oat hydroxylated clofop rapidly and was tolerant to CI but the limited ability of oat to hydroxylate diclofop resulted in oat being extremely susceptible to DM.     

Metabolism of diclofop-methyl (methyl-2-[4-(2',4'-dichlorophenoxy)phenoxy] propanoate) in cell suspensions of diploid wheat (Triticum monococcum)

The metabolism of the herbicide, diclofop-methyl (methyl-2-[4-(2′,4′-dichlorophenoxy)phenoxy]propanoate, in cell suspensions of resistant diploid wheat (Triticum monococcum L.) was determined 1, 8, and 24 h after treatment with ¹⁴C-diclofop-methyl. The ¹⁴C-labeled products were identified by thin layer chromatographic comparisons to appropriate standards. Eight hours after treatment with 5 μM diclofop-methyl in 0.8% acetone (neither of which were toxic to the cell suspensions) 87.2% (84.0% methanol soluble, 3.2% methanol insoluble) of the total ¹⁴C recovered (90.4%) was in the cells and 12.8% was in the medium. Major metabolites found in methanol extracts of the cells were diclofop (2-[4-(2′,4′-dichlorophenoxy)phenoxylpropionic acid), diclofop hydroxylated at an undetermined position on the 2,4-dichlorophenyl ring (ring-OH diclofop), and conjugates of ring-OH diclofop. Acid hydrolysis of the conjugated metabolite(s) yielded ring-OH diclofop and diclofop. Twenty-four hours after treatment 70–75% of the total ¹⁴C recovered was present as conjugated metabolites. With the exception of ring-OH diclofop, all metabolites present in the cells were also recovered from the medium. A metabolite found in low concentrations in the medium that yielded diclofop upon hydrolysis was identified as an ester conjugate. Toxic concentrations of diclofop-methyl (10 and 20μM) had no effect on the metabolism of the herbicide, although the rate of uptake was slower than for cells treated with 5 μM herbicide. The products of diclofop-methyl metabolism in cell suspensions of T. monococcum were compared to previous data from T. aestivum intact plant metabolism of diclofop-methyl.     

The absorption and translocation of diclofop-methyl and amitrole in wheat and oat roots

The absorption and translocation of diclofop-methyl (methyl 2-[4(2',4'-dichlorophenoxy)phenoxy]propanoate) was examined by using a specially designed treatment apparatus that separated excised roots or roots of seedlings into four zones. [¹⁴C]-Diclofop-methyl was absorbed along the entire root length of both wheat ( Triticum aestivum L.) and oat ( Avena sativa L.). In both species, absorption was greatest in the apical region of the root. Absorption by the apical region of wheat roots was more than three times greater than the basal portions, and more than twice as great as the apical region of oat roots. Less than 5% of the absorbed diclofop-methyl was translocated in both wheat and oat roots. Diclofop-methyl and diclofop(2-[4(2',4'-dichlorophenoxy)phenoxy]propanoic acid) were the predominant translocated forms. The absorption and translocation of amitrole (3-amino-1,2,4-triazole) were also examined. Amitrole was absorbed along the entire length of wheat roots and translocated primatily in the basipetal direction. The usefulness of the specially designed apparatus for biochemical and physiological studies is discussed.     

High survival frequencies at low herbicide use rates in populations of Lolium rigidum result in rapid evolution of herbicide resistance

The frequency of phenotypic resistance to herbicides in previously untreated weed populations and the herbicide dose applied to these populations are key determinants of the dynamics of selection for resistance. In total, 31 Lolium rigidum populations were collected from sites with no previous history of exposure to herbicides and where there was little probability of gene flow from adjacent resistant populations. The mean survival frequency across all 31 populations following two applications of commercial rates (375 g ha(-1)) of the acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicide, diclofop-methyl was 0.43%. Survivors from five of these populations were grown to maturity and seed was collected. Dose-response experiments compared population level resistance to diclofop-methyl in these selected lines with their original parent populations. A single cycle of herbicide selection significantly increased resistance in all populations (LD(50) R:S ratios ranged from 2.8 to 23.2), confirming the inheritance and genetic basis of phenotypic resistance. In vitro assays of ACCase inhibition by diclofop acid indicated that resistance was due to a non-target-site mechanism. Following selection with diclofop-methyl, the five L. rigidum populations exhibited diverse patterns of cross-resistance to ACCase and ALS-inhibiting herbicides, suggesting that different genes or gene combinations were responsible for resistance. The relevance of these results to the management of herbicide resistance are discussed.     

Diclofop-methyl increases the proton permeability of isolated oat-root tonoplast

Diclofop-methyl (methyl ester of 2-[4-(2′,4′-dichlorophenoxy)phenoxy]propionate; 100 micromolar) and diclofop (100 micromolar) inhibited both ATP- and PPi-dependent formation of H⁺ gradients by tonoplast vesicles isolated from oat (Avena sativa L., cv Dal) roots. Diclofop-methyl (1 micromolar) significantly reduced the steady-state H⁺ gradient generated in the presence of ATP. The ester (diclofop-methyl) was more inhibitory than the free acid (diclofop) at pH 7.4, but this relative activity was reversed at pH 5.7. Neither compound affected the rate of ATP or PPi hydrolysis by the proton-pumping enzymes. Diclofop-methyl (50, 100 micromolar), but not diclofop (100 micromolar), accelerated the decay of nonmetabolic H⁺ gradients established across vesicle membranes. Diclofop-methyl (100 micromolar) did not collapse K⁺ gradients across vesicle membranes. Both the (+)- and (−)-enantiomers of diclofop-methyl dissipated nonmetabolic H⁺ gradients established across vesicle membranes. Diclofop-methyl, but not diclofop (each 100 micromolar), accelerated the decay of H⁺ gradients imposed across liposomal membranes. These results show that diclofop-methyl causes a specific increase in the H⁺ permeability of tonoplast.     

The antagonism of IAA-induced hydrogen ion extrusion and coleoptile growth by diclofop-methyl

Diclofop-methyl (DM) (ester) was readily absorbed by peeled and unpeeled coleoptiles of wheat, Triticum aestivum L. cv. Waldron, and oat, Avena sativa L. cv. Garry. Substantial absorption of diclofop (acid) occurred only in peeled coleoptiles of the two species. IAA-induced acidification in peeled coleoptiles of both species was inhibited by 100 μ M DM or diclofop (acid) during a 3 to 4 h period. There was no recovery of acidification after DM or diclofop inhibition in oat coleoptiles; however, acidification in wheat coleoptiles recovered from inhibition by DM but not from diclofop. The recovery from DM inhibition may be due to a reduction in the diclofop pool derived from DM by efflux and metabolism (detoxification) in peeled wheat coleoptiles. Diclofop was not detoxified in oat coleoptiles. IAA-induced elongation of unpeeled oat coleoptiles was inhibited totally by 100 μ M DM but not by 100 μ M diclofop after 3.3 h of treatment. Wheat coleoptile elongation was relatively unaffected by either DM or diclofop. Basal elongation (no IAA) of both wheat and oat coleoptiles was inhibited by DM and diclofop. The inhibition by DM appeared to be irreversible, whereas the inhibition by diclofop was overcome by the addition of 10 μ M IAA.     

Tolerance of several wild oat herbicides by a range of winter wheat varieties

Screening tests exposed varietal differences in the tolerance of winter wheat to high doses of the wild oat herbicides difenzoquat and diclofop-methyl. Subsequent yield trials at recommended and higher doses confirmed the sensitivity of Score and Sportsman to difenzoquat and showed Atou, Bouquet and Hobbit to be less tolerant than some other varieties. In most varieties, earlier spraying proved safer than treatment at or beyond the beginning of jointing. Varietal differences with diclofop-methyl were smaller and inconsistent. Crops damage from these two herbicides was more severe in another experiment the following season, in which isoproturon at double the recommended dose also caused severe damage but flamprop-methyl was fairly well tolerated by all varieties.     

小桐子提取物除草活性的生物测定

为全面了解小桐子(Jatropha curcas L. )提取物的除草活性,以萝卜(Raphanus sativus L. )、苋(Amaranthus tricolor L. )、苏丹草[Sorghum sudanense (Piper) Stapf]和黑麦草(Lolium perenne L. )为实验材料,对小桐子果壳和枝叶的水、乙醇(体积分数95%)、正丁醇、乙酸乙酯、氯仿和石油醚粗提物的除草活性进行了生物测定,并从中筛选出抑制作用最强的水粗提物进行进一步的活性组分分离及其除草活性的生物测定.测定结果显示,小桐子果壳和枝叶的6种溶剂提取物(10 g·L~(-1))对供试的4种植物幼苗的根长和茎高均有不同程度的抑制作用,其中水粗提物和乙醇(体积分数95%)粗提物的抑制作用较强,且水粗提物对供试的4种植物幼苗的根长和茎高的抑制作用均在75%以上,显著高于其他溶剂粗提物(P<0.05);石油醚粗提物的抑制作用最小,均在10%以下.小桐子果壳和枝叶水粗提物的石油醚、氯仿、乙酸乙酯、正丁醇和水萃取物(10 g·L~(-1))对萝卜和苏丹草幼苗的根长和茎高均有不同程度的抑制作用,其中水、正丁醇和乙酸乙酯萃取物的抑制作用显著高于氯仿和石油醚萃取物,抑制率均在70%以上;水萃取物的抑制作用最强,抑制率均在80%以上;石油醚萃取物的抑制作用最小,抑制率均在10%以下.研究结果表明,小桐子果壳和枝叶的水粗提物具有一定的除草活性,其有效成分为极性较大的组分.     

Effect of drought stress,abscisic acid,and abscisic acid analogues on the efficacy of diclofop-methyl and tralkoxydim

The effects of drought stress, abscisic acid (ABA), and four ABA analogues on diclofop-methyl and tralkoxydim efficacy were investigated in oat (Avenu sativa). Drought stress conditions (6% soil moisture content) reduced the efficacy of diclofop-methyl at 350 g ha^–1, but not at 700 g ha^–1. Similarly, tralkoxydim efficacy was reduced by drought stress at 62.5 and 125 g ha^–1, but not at 250 g ha^–1. ABA (100 m), applied as a root drench 2 days before the herbicide, protected oat plants against all rates of diclofop-methyl and against low rates of tralkoxydim. Two ABA analogues protected oat plants from diclofop-methyl injury, whereas two others had no effect. Foliage applications of ABA were much less effective than root applications in protecting against herbicide injury. Protection by ABA and the two active analogues was dependent on the relative time of application with respect to the herbicides. Optimal protection by ABA and analogue I was obtained when they were applied between 2 days before and 1 day after diclofop-methyl application. Analogue IV protected plants when applied between 3 days before and 1 day after diclofop-methyl application. Partial protection against tralkoxydim activity by ABA was observed when it was applied between 1 day before and 1 day after herbicide application. Analogue I did not afford any protection against tralkoxydim, and analogue IV afforded partial protection when applied the same day or 1 day after tralkoxydim. The results indicate that protection against these postemergence herbicides, similar to that conferred by water stress, can be induced by ABA and structural analogues that apparently mimic the action of ABA.Abbreviations SMC soil moisture content - ABA abscisic acid     

Effect of diclofop-methyl and 2,4-D on transmembrane proton gradient: a mechanism for their antagonistic interaction

The site of action of the postemergence graminicide, diclofop-methyl (DM), in susceptible plants is possibly the plasmalemma. Indole-acetic acid (IAA)- and fusicoccin (FC)-induced net proton excretion in Avena coleoptiles was inhibited by the free acid, diclofop. However, net proton excretion recovered within 2 h when 2,4-dichlorophenoxy acid (2,4-D) was added simultaneously with diclofop. Diclofop depolarized the membrane potential (Em) within 12 min but the Em recovered within 30 min when diclofop was removed and replaced with either IAA or 2,4-D. The inhibition of IAA-induced coleoptile growth by DM and the membrane effects of its acid, diclofop, were partially reversed by 2,4-D if it was added shortly after treatment of the tissue. These results are consistent with the reversal of DM injury in whole plants with 2,4-D.     

Recurrent selection with reduced herbicide rates results in the rapid evolution of herbicide resistance in Lolium rigidum

There has been much debate regarding the potential for reduced rates of herbicide application to accelerate evolution of herbicide resistance. We report a series of experiments that demonstrate the potential for reduced rates of the acetyl-co enzyme A carboxylase (ACCase)-inhibiting herbicide diclofop-methyl to rapidly select for resistance in a susceptible biotype of Lolium rigidum. Thirty-six percent of individuals from the original VLR1 population survived application of 37.5 g diclofop-methyl ha^–1 (10% of the recommended field application rate). These individuals were grown to maturity and bulk-crossed to produce the VLR1 low dose-selected line VLR1 (0.1). Subsequent comparisons of the dose-response characteristics of the original and low dose-selected VLR1 lines demonstrated increased tolerance of diclofop-methyl in the selected line. Two further rounds of selection produced VLR1 lines that were resistant to field-applied rates of diclofop-methyl. The LD₅₀ (diclofop-methyl dose required to cause 50% mortality) of the most resistant line was 56-fold greater than that of the original unselected VLR1 population, indicating very large increases in mean population survival after three cycles of selection. In vitro ACCase inhibition by diclofop acid confirmed that resistance was not due to an insensitive herbicide target-site. Cross-resistance studies showed increases in resistance to four herbicides: fluazifop-P-butyl, haloxyfop-R-methyl, clethodim and imazethapyr. The potential genetic basis of the observed response and implications of reduced herbicide application rates for management of herbicide resistance are discussed.     

Penoxsulam—Structure–activity relationships of triazolopyrimidine sulfonamides

The discovery of the sulfonamide herbicides, which inhibit the enzyme acetolactate synthase (ALS), has resulted in many investigations to exploit their herbicidal activity. One area which proved particularly productive was the N-aryltriazolo[1,5-c]pyrimidine sulfonamides, providing three commercial herbicides, cloransulam-methyl, diclosulam and florasulam. Additional structure–activity investigations by reversing the sulfonamide linkage resulted in the discovery of triazolopyrimidine sulfonamides with cereal crop selectivity and high levels of grass and broadleaf weed control. Research efforts to exploit these high levels of weed activity ultimately led to the discovery of penoxsulam, a new herbicide developed for grass, sedge and broadleaf weed control in rice. Synthetic efforts and structure–activity relationships leading to the discovery of penoxsulam will be discussed.     

Effects of Acetyl-Coenzyme A Carboxylase Inhibitors on Root Cell Transmembrane Electric Potentials in Graminicide-Tolerant and -Susceptible Corn (Zea mays L.)

Herbicidal activity of aryloxyphenoxypropionate and cyclohexanedione herbicides (graminicides) has been proposed to involve two mechanisms: inhibition of acetyl-coenzyme A carboxylase (ACCase) and depolarization of cell membrane potential. We examined the effect of aryloxyphenoxypropionates (diclofop and haloxyfop) and cyclohexanediones (sethoxydim and clethodim) on root cortical cell membrane potential of graminicide-susceptible and -tolerant corn (Zea mays L.) lines. The graminicide-tolerant corn line contained a herbicide-insensitive form of ACCase. The effect of the herbicides on membrane potential was similar in both corn lines. At a concentration of 50 [mu]M, the cyclohexanediones had little or no effect on the membrane potential of root cells. At pH 6, 50 [mu]M diclofop, but not haloxyfop, depolarized membrane potential, whereas both herbicides (50 [mu]M) dramatically depolarized membrane potential at pH 5. Repolarization of membrane potential after removal of haloxyfop and diclofop from the treatment solution was incomplete at pH 5. However, at pH 6 nearly complete repolarization of membrane potential occurred after removal of diclofop. In graminicide-susceptible corn, root growth was significantly inhibited by a 24-h exposure to 1 [mu]M haloxyfop or sethoxydim, but cell membrane potential was unaffected. In gramincide-tolerant corn, sethoxydim treatment (1 [mu]M, 48 h) had no effect on root growth, whereas haloxyfop (1 [mu]M, 48 h) inhibited root growth by 78%. However, membrane potential was the same in roots treated with 1 [mu]M haloxyfop or sethoxydim. The results of this study indicate that graminicide tolerance in the corn line used in this investigation is not related to an altered response at the cell membrane level as has been demonstrated with other resistant species.     

Cross-Resistance to Herbicides in Annual Ryegrass (Lolium rigidum) : III. On the Mechanism of Resistance to Diclofop-Methyl

Annual ryegrass (Lolium rigidum) biotype SLR 31 is resistant to the postemergent graminicide methyl-2-[4-(2,4-dichlorophenoxy)phenoxy]-propanoate (diclofop-methyl). Uptake of [¹⁴C](U-phenyl)diclofop-methyl and root/shoot distribution of radioactivity in susceptible and resistant plants were similar. In both biotypes, diclofop-methyl was rapidly demethylated to the biocidal metabolite diclofop acid which, in turn, was metabolized to ester and aryl-O-sugar conjugates. Susceptible plants accumulated 5 to 15% more radioactivity in dicloflop acid than did resistant plants. Resistant plants had a slightly greater capacity to form nonbiocidal sugar conjugates. Despite these differences, resistant plants retained 20% of ¹⁴C in the biocidal metabolite diclofop acid 192 hours after treatment, whereas susceptible plants, which were close to death, retained 30% in diclofop acid. The small differences in the pool sizes of the active and inactive metabolites are by themselves unlikely to account for a 30-fold difference in sensitivity to the herbicide at the whole plant level. Similar high-pressure liquid chromatography elution patterns of conjugates from both susceptible and resistant biotypes indicated that the mechanisms and the products of catabolism in the biotypes are similar. It is suggested that metabolism of diclofop-methyl by the resistant biotype does not alone explain resistance observed at the whole-plant level. Diclofop acid reduced the electrochemical potential of membranes in etiolated coleoptiles of both biotypes; 50% depolarization required 1 to 4 μm diclofop acid. After removal of diclofop acid, membranes from the resistant biotype recovered polarity, whereas membranes from the susceptible biotype did not. Internal concentrations of diclofop acid 4 h after exposing plants to herbicide were estimated to be 36 to 39 micromolar in a membrane fraction and 16 to 17 micromolar in a soluble fraction. Such concentrations should be sufficient to fully depolarize membranes. It is postulated that differences in the ability of membranes to recover from depolarization are correlated with the resistance response of biotype SLR 31.     

Influence of Drought on Graminicide Phytotoxicity in Wild Oat (Avena fatua) Grown under Different Temperature and Humidity Conditions

Controlled environmental experiments were carried out to determine the phytotoxicity of several graminicides on wild oat (Avena futua L.) as influenced by combination of drought and temperature stress or drought and low relative humidity. Compared with unstressed conditions (20/15°C plus adequate soil moisture), imazamethabenz phytotoxicity to wild oat was reduced significantly when plants were exposed to a combination of drought and high temperature (30/20°C) stress. Imazamethabenz phytotoxicity was reduced almost as much by high temperature stress alone as by a combined temperature and drought stress. When herbicides were applied to wild oat plants subjected to drought alone or to drought plus high temperature, the observed reduction in phytotoxicity from greatest to least was: fenoxaprop = diclofop > flamprop > imazamethabenz. Fenoxaprop performance was most inhibited by the combination of drought plus high temperature, although drought alone and to a lesser degree, high temperature alone, inhibited fenoxaprop action. High temperature had an adverse effect on the efficacy of fenoxaprop at lower application rates. Raising fenoxaprop application rates to 400 g ha⁻¹ overcame the inhibition caused by high temperature alone but only partially alleviated the effect of drought combined with high temperature. When plants were grown under a low temperature regimen the imposition of drought stress had little effect on imazamethabenz phytotoxicity but did reduce fenoxaprop phytotoxicity. At 25/15°C drought reduced the phytotoxicity of fenoxaprop and diclofop greatly but had no significant impact on the performance of any of the herbicides examined, regardless of soil moisture regimen. Received April 14, 1997; accepted September 22, 1997