Tribenuron antagonized diclofop control of wild oat in greenhouse studies. Wild oat shoot fresh weight was reduced 79% when plants were treated with diclofop at 1.12 kg ha−1, but only 46% when treated with diclofop at 1.12 kg ha−1plus tribenuron at 18 g ha−1. Increasing diclofop rate increased control of wild oat but did not overcome the antagonism. Separating the applications of diclofop and tribenuron by as little as 12 s reduced the antagonism by 40%, indicating the importance of close proximity of the two herbicides on the leaf surface for the antagonistic response. The inhibitory effect of diclofop at its target site (acetylCoA carboxylase) was not reduced by tribenuron. Metabolism of 14C-diclofop by wild oat leaves was not altered quantitatively or qualitatively by tribenuron. Tribenuron had no effect on spray retention or absorption of 14C-diclofop by wild oat leaves, and only slightly decreased the total amount of radiolabel translocated out of the treated zone on the leaf. However, tribenuron decreased basipetal translocation of diclofop from the treated zone by approximately 20%. The ability of tribenuron to reduce basipetal translocation of diclofop to meristematic regions of wild oat may be a factor in antagonism.

Field trials indicated that addition of ammonium sulfate to imazethapyr plus nonionic surfactant increased quackgrass control, especially at low imazethapyr rates. In greenhouse experiments, approximately twice as much imazethapyr was absorbed by quackgrass leaves when the herbicide was applied in combination with nonionic surfactant plus ammonium sulfate than when the herbicide was applied with nonionic surfactant alone. Black Mexican Sweet maize (BMS) suspension-cultured cells were used to evaluate the effects of ammonium sulfate and nonionic surfactant on cellular absorption of imazethapyr in the absence of a cuticular barrier. Imazethapyr absorption by BMS cells was diffusion-mediated, energy-dependent, and exhibited a pH optimum of approximately 3. Over the concentration range of 0.1 to 10.0 μM, the equilibrium concentration of imazethapyr in BMS cells was a linear function of the external concentration. Addition of ammonium sulfate to the external medium of BMS cells enhanced both the rate of imazethapyr uptake and medium acidification. There was a linear correlation between the ability of ammonium sulfate (0.5 to 10 mM) to promote medium acidification and imazethapyr uptake by BMS cells. The ammonium sulfate-induced stimulation of imazethapyr absorption in BMS cells was sensitive to plasma membrane adenosine triphosphatase inhibitors (sodium vanadate, diethylstilbestrol), the uncoupler carbonyl cyanide m-chlorophenylhydrazone, and energy metabolism inhibitors (sodium azide, nitrogen gas), demonstrating that this effect was dependent on ATP production and the functioning of the plasma membrane ATPase. It is hypothesized that cytoplasmic acidification in BMS cells due to ammonium assimilation stimulates the plasma membrane ATPase to pump protons across the plasma membrane which in turn acidifies the cell wall promoting cellular accumulation of imazethapyr by ion-trapping. Cell wall acidification due to ammonium assimilation may contribute to the ability of ammonium sulfate to enhance the efficacy of imazethapyr and other foliar-applied herbicides.

A diclofop-methyl-resistant biotype of Italian ryegrass was characterized to determine the expression and inheritance of herbicide resistance and whether this trait was due to the presence of a diclofop-insensitive form of acetyl-coenzyme A carboxylase (ACCase). At the whole plant level, the resistant biotype was > 93-fold more resistant to diclofop-methyl than the susceptible biotype. Crosses of diclofop-resistant and –susceptible plants were performed to produce F1 plants. No maternal effects were evident in responses of reciprocal F1 plants to diclofop. GR50 diclofop rates determined for resistant, F1, and susceptible plants were 15, 6.3, and 0.16 kg ha−1, respectively. F2 populations treated with a 7.5 kg ha−1 rate of diclofop exhibited three injury response phenotypes 3 wk after treatment: a susceptible (S) phenotype which was killed, an intermediate resistance (I) phenotype with severe injury, and a resistant (R) phenotype with little or no injury. Testcross progeny exhibited only I and S phenotypes. Observed segregation of phenotypes in F2 and testcross populations conformed to segregation ratios predicted for a trait with inheritance controlled by a single partially dominant nuclear gene. ACCase activity determined in crude cell-free extracts of resistant, F1, and susceptible biotypes exhibited I50 values of 50, 20, and 0.7 μM diclofop, respectively. A positive relationship between the injury response phenotype and site of action (ACCase) response to diclofop was evident in both F1 and F2 populations. In extracts from R, I, and S phenotype F2 plants, 20 μM diclofop acid inhibited ACCase-mediated incorporation of 14C by 27.1, 45.1, and 78.9%, respectively. The ACCase data are consistent with the hypothesis that diclofop resistance in Italian ryegrass is conferred by a diclofop-insensitive form of ACCase.

CGA-154281 and temperature effects on metolachlor absorption and metabolism were studied in corn seedlings grown in untreated soil or soil treated with metolachlor, CGA-154281, or both. Seedlings were grown under a cool (21/13 C, 16-h day) or warm (30/21 C, 16-h day) temperature regime and exposed to 14C-metolachlor for 10 min at either 21 or 30 C. Corn grown under the cool temperature regime absorbed slightly more 14C-metolachlor than corn grown under the warm temperature regime. Corn held at 21 C during a 10-min 14C-metolachlor exposure period metabolized metolachlor more slowly than corn held at 30 C. Decreased corn tolerance to metolachlor observed at lower temperatures may be due in part to slower metolachlor metabolism. Corn grown in the presence of metolachlor plus CGA-154281 metabolized 14C-metolachlor to the glutathione conjugate twice as fast as corn grown in the presence of metolachlor alone. The increase in metabolism rate was due to a fivefold increase in glutathione-S-transferase (GST, EC 2.5.1.18) activity and not to an increase in glutathione (GSH) content. Results are consistent with the hypothesis that CGA-154281 protects corn from metolachlor injury by enhancing GST activity, which accelerates metolachlor detoxification via GSH conjugation.

In greenhouse studies, the calculated I50 (herbicide application resulting in 50% inhibition of shoot regrowth) in tall fescue was approximately 0.004 kg/ha for both sethoxydim and haloxyfop. In red fescue, the I50 for sethoxydim and haloxyfop was 9.4 kg/ha and 0.04 kg/ha, respectively. As measured in crude cell extracts of tall fescue, incorporation of 14C-acetyl-coenzyme A into fatty acids was inhibited 62 and 71% by 10 μM sethoxydim and 10 μM haloxyfop, respectively. In red fescue, 10 μM haloxyfop inhibited 14C-acetyl-CoA incorporation into fatty acids by 29%, whereas 10 μM sethoxydim had no effect. The I50 for inhibition of acetyl-coenzyme A carboxylase activity in tall fescue with sethoxydim and haloxyfop was 6.9 and 5.8 μM, respectively. In red fescue the I50 for haloxyfop was 118 μM. Sethoxydim concentrations as high as 1 mM had little effect on acetyl-coenzyme A carboxylase activity in red fescue. These results suggest that acetyl-coenzyme A carboxylase is a sensitive site of action for sethoxydim and haloxyfop in tall fescue, and that tolerance to these herbicides in red fescue is due to the presence of a more tolerant form of the enzyme.

Previous reports have suggested that bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] tolerance among soybean genotypes is the result of differential translocation or metabolism. The basis for tolerance was reexamined using susceptible and tolerant genotypes. Tolerant genotypes (‘Hill’ and ‘Clark 63’) were found to tolerate 100- to 300-fold more bentazon than susceptible genotypes (‘L78–3263’, ‘Hurrelbrink’, and ‘PI 229.342’). Minor differences in absorption and translocation occurred among the genotypes but they did not correlate with tolerance. Tolerant genotypes metabolized 80 to 90% of absorbed bentazon within 24 h, while susceptible genotypes metabolized only 10 to 15%. Two major metabolites, the glycosyl conjugates of 6- and 8-hydroxybentazon, were formed in tolerant genotypes. Susceptible genotypes did not form the hydroxybentazon conjugates but instead produced relatively low levels of two unidentified metabolites. It is concluded that differential bentazon tolerance among soybean genotypes is linked to the ability to form both the 6- and 8-hydroxybentazon conjugates.

Sorghum [Sorghum bicolor (L.) Moench. ‘G-623 GBR’] bioassays indicated that shoot growth was more susceptible to metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] and more responsive to the antidote CGA-92194 {α-[(1,3-dioxolan-2-yl-methoxy)-imino] benzeneacetonitrile} than root growth. Seed treatment with CGA-92194 increased seedling shoot tolerance to metolachlor approximately tenfold. CGA-92194 seed treatment enhanced shoot absorption of 14C-metolachlor approximately twofold. Metolachlor was initially metabolized to the glutathione conjugate in untreated shoots and those treated with CGA-92194. However, CGA-92194 seed treatment caused accelerated metolachlor metabolism in the shoot, decreasing metolachlor content and increasing formation of the glutathione conjugate. Cyometrinil {(Z)-α[(cyanomethoxy)imino] benzeneacetonitrile}, flurazole [phenylmethyl 2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylate], naphthalic anhydride (1H,3H-naphtho[1,8-cd]-pyran-1,3-dione), and dichlormid (2,2-dichloro-N,N-di-2-propenylacetamide) also protected sorghum from metolachlor injury and enhanced metolachlor absorption and metabolism. The degree of protection conferred by a particular antidote was correlated with its ability to enhance metabolism of metolachlor in shoot tissue. These results are consistent with the hypothesis that the above antidotes protect sorghum from metolachlor injury by inducing rapid detoxification of metolachlor through conjugation with glutathione.

An accession of velvetleaf (Abutilon theophrasti Medik. # ABUTH) known to be highly tolerant of atrazine [6-chloro-N-ethyl-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and a normal susceptible accession were crossed reciprocally. F1 plants were intermediate in response to atrazine, but they were more like the tolerant parent than the susceptible parent. The response of F1 plants was the same for each reciprocal cross, indicating that atrazine tolerance was not cytoplasmically inherited. Response of F2 plants to atrazine was consistent with a ratio of one tolerant: two intermediate: one susceptible. These results and further evaluations of F3 plants indicated that atrazine tolerance was controlled by a single, partially dominant gene.

The effects of haloxyfop {2-[4-[[3-chloro-5(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and haloxyfop-methyl on elongation and respiration of primary roots of corn [Zea mays L. ‘B37 X Oh43’] and soybean [Glycine max (L.) Merr. ‘Hodgson 78’] were examined. Intact roots of etiolated seedlings were exposed to both forms of the herbicide. At a concentration of 10-6 M, neither form of the herbicide had an effect on elongation or respiration of soybean roots. In contrast, elongation of corn roots was completely inhibited within 24 h of exposure to 10-6 M haloxyfop. This treatment reduced the respiration rate of corn root apices by 30% but had little effect on ATP content. After 72 h of exposure to haloxyfop [10-6 M], both ATP content and respiration rate of corn root apices had declined 35%. Haloxyfop-methyl produced equivalent effects on elongation and respiration of corn roots. Studies with isolated corn root mitochondria indicated that haloxyfop inhibits electron transport at relatively high concentrations (I50 = approximately 2 mM for various substrates). Because significant reductions in ATP content and respiration rate of corn root apices were not detected during the period when elongation was inhibited (initial 24 h of exposure), it is concluded that impairment of respiration is a secondary response to the herbicide.

Five classes of herbicides (carbamothioates, chloroacetamides, substituted pyridazinones, cyclohexanediones, and aryloxyphenoxypropionic acids) have been reported to inhibit lipid biosynthesis in higher plants. Carbamothioates impair the synthesis of surface lipids (waxes, cutin, suberin). These effects have been attributed to the ability of this herbicide class to inhibit one or more acyl-CoA elongases. Though as yet poorly characterized, these enzymes are associated with the endoplasmic reticulum and catalyze the condensation of malonyl-CoA with fatty acid acyl-CoA substrates to form very long-chain fatty acids used in the synthesis of surface lipids. There is contradictory evidence regarding the effects of chloroacetamide herbicides on de novo fatty acid biosynthesis. Selected substituted pyridazinones decrease the degree of unsaturation of plastidic galactolipids. This effect is attributed to the ability of selected members of this herbicide class to inhibit fatty acid desaturases which are thought to be located in the chloroplast envelope. Aryloxyphenoxypropionic acid and cyclohexanedione herbicides inhibit de novo fatty acid biosynthesis in grasses. The target site for these herbicide classes is the enzyme acetyl-CoA carboxylase which is found in the stroma of plastids. In most cases, selectivity between grasses and dicots is expressed at this site. Aryloxyphenoxypropionic acids and cyclohexanediones are reversible, linear, noncompetitive inhibitors of acetyl-CoA carboxylase from grasses. Both classes are also mutually exclusive inhibitors of grass acetyl-CoA carboxylase which suggests that they bind at a common domain on the enzyme.

Growth chamber and field experiments were conducted to assess the potential of Pseudomonas syringae pv. tagetis (Pst) as a biocontrol agent for Canada thistle. Silwet L-77, an organosilicone surfactant, was required to facilitate Pst penetration into Canada thistle leaves. Growth chamber experiments indicated that maximum Pst populations inside leaves were obtained with a Silwet L-77 concentration of 0.3% (v/v) or greater. High Pst populations (109 colony-forming units [cfu] per gram fresh weight) were found in leaves 48 h after treatment with 108 or 109 cfu ml−1 Pst plus Silwet L-77 (0.3%, v/v). In growth chamber experiments, foliar application of Pst (109 cfu ml−1) plus Silwet L-77 (0.3%, v/v) on 4- to 5-wk-old Canada thistle reduced shoot dry weight by 52% (measured 14 d after treatment) and chlorophyll content of emerging leaves by 92% (measured 10 d after treatment). In field trials conducted in 1999 and 2000, Pst (109 cfu ml−1) plus Silwet L-77 (0.3%, v/v) were applied at 700 L ha−1, and the method of application (paint gun, backpack sprayer, boom) and the number of applications (one or two separated by 14 d) were examined. Averaged over 2 yr, two applications with a backpack sprayer resulted in 67% disease incidence (apical chlorosis) of treated plants measured 4 wk after the initial treatment (WAIT). At the time of flower bud formation (8 WAIT), there was little or no disease incidence, 31% reduction in plant height, 81% reduction in number of flower buds, and 20% reduction in shoot survival during 1999 but no effect on survival in 2000.

A Lolium multiflorum Lam. biotype resistant to diclofop-methyl was found in a Triticum aestivum field in France (Normandy) that had been treated for several years with diclofop-methyl. Based on plant survival evaluated 21 d after herbicide application, the biotype exhibited a high level of resistance to diclofop-methyl and moderate resistance to CGA-184927-propargil and PP-604. The resistant biotype exhibited a small increase in tolerance to haloxyfop-methyl, quizalofop-ethyl, sethoxydim, and BAS-517-H, but was controlled by recommended field rates for these herbicides. The mechanism of resistance was investigated for diclofop-methyl. There was little or no difference in diclofop-methyl absorption by leaves of resistant and susceptible biotypes measured 48 h after treatment. For both biotypes, less than 1% of absorbed radiolabel was translocated during 48 h following foliar application of 14C-diclofop-methyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rapidly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional acetyl coenzyme A carboxylase isoforms (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Both isoforms exhibited a subunit molecular mass of 203 kDa. For both resistant and susceptible biotypes, ACCase I constituted approximately 80% of total ACCase activity. Graminicide concentrations required to inhibit ACCase activity by 50% (I50 values) were determined for both ACCase isoforms from resistant and susceptible biotypes. The ACCase II isoform was highly resistant to graminicides in both biotypes. In contrast, the I50 value for diclofop inhibition of ACCase I was 19-fold greater for the enzyme isolated from the resistant biotype compared with the susceptible biotype. It is concluded that diclofop resistance in the L. multiflorum biotype from Normandy is caused by the presence of a resistant form of the ACCase I isoform.

The effects of the fungal protein Nep1 and Pseudomonas syringae pv. tagetis (Pst) applied separately or in combination on Canada thistle, common ragweed, and common dandelion were examined in growth chamber experiments. Experiments examined five treatments: (1) untreated control, (2) Silwet L-77 (0.3%, v/v) control, (3) Nep1 (5 μg ml−1) plus Silwet L-77 (0.3%, v/v), (4) Pst (109 colony-forming units [cfu] ml−1) plus Silwet L-77 (0.3%, v/v), and (5) Pst (109 cfu ml−1) and Nep1 (5 μg ml−1) plus Silwet L-77 (0.3%, v/v). Foliar treatments were applied at 28, 26, and 21 d after planting for Canada thistle, common dandelion, and common ragweed, respectively. For all three species, foliar application of Nep1 alone or in combination with Pst caused rapid desiccation and necrosis of leaves, with the greatest effect on recent, fully expanded (RFE) leaves. Within 4 to 8 h after treatment (HAT), 60 to 80% of RFE leaves of all three species were necrotic. Measured 72 HAT, Pst populations in Canada thistle leaves treated with Nep1 plus Pst were approximately 105 cfu cm−2 compared with 107 cfu cm−2 for leaves treated with Pst alone. Measured 2 wk after treatment, foliar application of Nep1 reduced shoot dry weight of the three weeds by 30 to 41%. Treatment with Pst reduced shoot growth of common ragweed, Canada thistle, and common dandelion by 82, 31, and 41%, respectively. The large suppression of common ragweed shoot growth caused by Pst treatment was associated with a high percentage (60%) of leaf area exhibiting chlorosis. Treatment with Pst plus Nep1 did not result in significant decreases in shoot dry weight for Canada thistle and common dandelion compared with either treatment alone. For common ragweed, shoot growth reduction caused by applying Pst and Nep1 together was not greater than that caused by Pst alone.