Two mechanisms of action have been proposed for aryloxyphenoxypropanoate (APP) and cyclohexanedione (CHD) herbicides: a biochemical mechanism involving the inhibition of acetyl-CoA carboxylase and subsequent fatty acid biosynthesis in plastids, and a biophysical mechanism involving the perturbation of the transmembrane proton gradient across the plasma membrane. The synthesis of malonyl-carboxylase catalyzed by acetyl-coenzyme A carboxylase is the first committed step in fatty acid biosynthesis. Most APP and CHD herbicides are particularly active on annual grass weeds, with little activity on perennials. Diclofop-methyl and other APP and CHD compounds were initially shown to inhibit lipid biosynthesis. All plant cells establish and maintain a transmembrane proton gradient that is vital to their growth and development. The biochemical and physiological interactions between the two mechanisms in the cause of plant death are not clearly understood. Haloxyfop-methyl was rapidly converted to haloxyfop in soybean, Setaria glauca, and Sorghum bicolor, with no apparent differences between species in intact plants.

The aryloxyphenoxypropionic acid (AOPP) and cyclohexanedione (CHD) herbicides inhibit the first committed enzyme in fatty acid biosynthesis, acetyl CoA carboxylase (ACCase). The frequent use of AOPP and CHD herbicides has resulted in the development of resistance to these herbicides in many grass weed species. New herbicides that inhibit both the susceptible and resistant forms of ACCase in grass weeds would have obvious commercial appeal. In the present study, an attempt was made to identify molecules that target both the herbicide-sensitive and -resistant forms of ACCase. Seven experimental compounds, either CHD-like or AOPP-CHD hybrids, were synthesized and assayed against previously characterized susceptible and resistant forms of ACCase. All seven compounds inhibited ACCase from sensitive biotypes of Setaria viridis and Eleusine indica (I50 values from 6.4 to >100 microM) but were not particularly potent compared to some commercialized herbicides (I50 values of 0.08-5.6 microM). In almost all cases, the I50 values for each compound assayed against the resistant ACCases were higher than those against the corresponding sensitive ACCase, indicating reduced binding to the resistant ACCases. One compound, a CHD analogue, was almost equally effective against the resistant and susceptible ACCases, although it was not a very potent ACCase inhibitor per se (I50 of 51 and 76 microM against susceptible ACCase from S. viridis and E. indica, respectively). The AOPP-CHD hybrid molecules also inhibited some of the resistant ACCases, with I50 values ranging from 6.4 to 50 microM. These compounds may be good leads for developing ACCase inhibitors that target a wider range of ACCase isoforms, including those found in AOPP- and CHD-resistant weed biotypes.

Herbicide-tolerant (HT) canola (Brassica napus) was introduced commercially in Canada in 1995. Since then, canola varieties or hybrids resistant to glufosinate, glyphosate, imidazolinones or bromoxynil have been introduced. These HT varieties/hybrids have grown in market share such that they now occupy approximately 70% of the seeded area in western Canada. Growers have adopted HT canola for several reasons, including easier and better weed control, higher seed yield, and higher financial returns and profits, based primarily on the higher yield, reduced dockage and lower herbicide costs. Significant benefits also include reduced herbicide and fuel use. Although growing HT canola does involve critical management decisions, including how best to fit these cultivars into typical crop rotations and management of crop volunteers in subsequent years, their widespread adoption, sustained after 6 years, strongly suggests a net benefit to producers.

Over 200 distinct herbicide resistant weed biotypes have evolved worldwide. In most of these, resistance is conferred by an altered target site, i.e. a modified target protein with reduced affinity for the herbicide(s) in question. This has been documented for herbicides that target most major known sites of action, including those that inhibit photosynthetic electron transfer at photosystem II, acetyl-CoA carboxylase, acetolactate synthase, and tubulin polymerization. Patterns of cross-resistance to structurally similar herbicides and those from Ether chemical classes that target the same site vary, depending on the mutation and its effect on protein steric and electronic properties. Mechanisms of target site-based herbicide resistance are reviewed, with emphasis on the biochemical and molecular basis for resistance. (C) 2000 Elsevier Science Ltd. All rights reserved.

A false cleavers population that survived treatment with triasulfuron/bromoxynil in 1996 was identified in central Alberta, Canada, in a field that had been treated with acetolactate synthase (ALS) inhibitors in 3 of the previous 6 yr. In greenhouse studies, this biotype was highly resistant to the ALS inhibitors triasulfuron, thifensulfuron/tribenuron, and sulfometuron and moderately resistant to imazethapyr; GR 50 , values were > 16, > 5, > 1.0, and 9.9, respectively. In addition, cross-resistance was identified to the auxin-type herbicide quinclorac (GR 50 value > 6.7) but not to fluroxypyr (GR 50 value 1) or MCPA/mecoprop/dicamba. Quinclorac had not been used previously in this field. Analysis of ALS extracted from the resistant biotype and a susceptible biotype from a nearby location indicated that resistance to ALS inhibitors was due to an altered target site with reduced sensitivity to a broad range of ALS inhibitors. The ALS I 50 values for triasulfuron, metsulfuron, chlorsulfuron, thifensulfuron, and imazethapyr were 36, 34, 92, 96, and 14 times higher, respectively, for the resistant compared to the susceptible biotype. The mechanism of resistance to quinclorac is unknown. This is the first report of high-level herbicide resistance in this weed species.

Previous results indicated that resistance to acetyl-CoA carboxylase-inhibiting herbicides in the wild oat biotype UM1 was not due to an insensitive form of acetyl-CoA carboxylase (ACCase). However, reanalysis of ACCase extracted under a variety of different buffer conditions indicated that resistance in this biotype is due to an altered form of the enzyme with reduced herbicide sensitivity. Under optimal conditions, ACCase from UM1 was very resistant to sethoxydim (I50= 398 μM; R/SI50ratio = 105) and resistant, although to a lesser extent, to fenoxaprop, diclofop, and tralkoxydim (R/SI50ratios ca. 10). Use of the optimum extraction buffer for this biotype did not indicate an ACCase-based resistance mechanism for a second resistant wild oat biotype, UM33. Fenoxaprop and diclofop were metabolized at equal rates in UM33 and a susceptible biotype, indicating that enhanced herbicide metabolism was not responsible for resistance in this biotype. Further modification of the ACCase extraction buffer revealed that resistance in UM33 was also conferred by a target site alteration. These results suggest that certain ingredients in the ACCase extraction buffers can result in the apparent loss of resistance to herbicides and that the optimum buffer may vary for biotypes within a given species. The results also suggest that careful analysis of the sensitivity of ACCase to herbicides is required before concluding that resistance is not based on a target site alteration.

Resistance to acetyl-coenzyme A carboxylase (ACCase) inhibitors has developed in at least 10 grass weed species in recent years. In most instances, resistance is conferred by an ACCase alteration in the resistant biotypes that reduces sensitivity to aryloxyphenoxypropionate (AOPP) and cyclohexanedione (CHD) herbicides. Analysis of ACCase from many of these resistant weed biotypes suggests the presence of different mutations, each conferring a different pattern and level of resistance to various AOPP and CHD herbicides. In all cases analyzed to date, resistance is controlled by a single dominant or semidominant nuclear gene. In several weed biotypes, resistance is conferred by enhanced herbicide detoxification, primarily through elevated expression or activity of cytochrome P450 monooxygenase(s). This mechanism can confer cross-resistance to herbicides from other chemical classes with different modes of action. Finally, multiple herbicide resistance, i.e. the acquisition of several different resistance mechanisms, has been reported in some weed biotypes.

Plasma membrane lipid composition of herbicide-resistant (R) and -susceptible (S) wild oat biotypes was analyzed to determine the basis for the differential effect of diclofop on the transmembrane electrogenic potential between the two biotypes and reduced herbicide uptake into protoplasts of the R biotype. In addition, Lipoxygenase (LOX) activity was examined in herbicide-treated and untreated R and S plants to determine its involvement in herbicide action and resistance. Overall, no significant differences in lipid composition were found between the two biotypes. Glycolipids represented 41 and 36%, phospholipids 29 and 37%, and free sterols 30 and 27% of the total plasma membrane lipid in the R and S biotypes, respectively No differences in LOX activity were observed between the herbicide-treated and untreated wild oat biotypes. It was concluded that differences in membrane transport of diclofop and its effect on plasma membrane potential in the R and S biotypes are not related to differences in membrane lipid composition or to differential effects of herbicides on LOX activity in the two biotypes.

Sethoxydim-resistant biotypes of Setaria faberi Herrm. (giant foxtail) and Setaria viridis L. Beauv. (green foxtail) were identified in Iowa, USA in 1994 and Manitoba, Canada in 1991, respectively. Sethoxydim and related herbicides inhibit acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) in sensitive grass species. ACCase from susceptible biotypes of the two species was very sensitive to sethoxydim and a selection of other ACCase inhibitors, including clethodim, fenoxaprop and quizalofop, with I 50 values of 0.3-6.5 μM and 2.1-7.7 μM for S. faberi and S. viridis, respectively. In contrast, ACCase from the resistant biotypes was very resistant to sethoxydim (I 50 of 1250 and 3260 μM for S. faberi and S. viridis, respectively) and resistant, although to a lesser extent, to the other herbicides tested. The results indicate that resistance to ACCase inhibitors in these biotypes is due to an alteration in the target enzyme, ACCase, that confers a very high level of resistance to sethoxydim and lower levels of resistance to other ACCase-inhibiting herbicides.

Knowledge of fecundity is essential to understanding population dynamics, yet accurate information on reproductive effort under field conditions is not available for many weed species. Impact of crops on weed fecundity must be known in order to predict population dynamics in managed ecosystems. Echinochloa crus-galli seed production was less than 3,500 seeds per plant when grown in maize (Zea mays) or grain sorghum (Sorghum vulgare), but in low-competitive crops like sugarbeets (Beta vulgaris) E. crus-galli produced over 80,000 seeds per plant. A technique permitting seed collection under rain-fed field conditions is described. Seed production by Capsella bursa-pastoris varied from 30,000 to 150,000 seeds per plant in the absence of competition. A biotype from California produced from 2- to 3-fold more seed than a biotype from England under common garden conditions.

This study was initiated to determine net photosynthesis, respiration, and stomatal conductance for early-flowering (EF) and late-flowering (LF) strains of Thlaspi arvense L., and to determine the relative impact of temperature on these processes in the two strains. The strains have been shown to differ by a single gene. Vernalized EF and LF plants were grown under controlled environments at day temperatures of 15 degrees C, 20 degrees C, and 25 degrees C (16-h photoperiod) each, with 15 degrees C nights. During the period of floral initiation for the EF strain, net photosynthesis, respiration, and stomatal conductance were determined in both strains. Plants of the LF strain had higher net photosynthesis rates and lower respiration rates. Net photosynthesis was most efficient at 20 degrees C days, while respiration rates increased with increasing temperature. Stomatal conductance was greater in the EF strain, which also had Larger stomata than plants of the LF strain. Generally, EF plants grew more quickly in terms of shoot dry weight than LF plants. However, at 25 degrees C, the LF plants had significantly more shoot dry weight. We suggest that differential net photosynthesis and respiration rates of the EF and LF strains of T. arvense may influence the relative amounts of carbohydrates and nitrogen available to the shoot apices and therefore affect the time to floral transition.

A biotype of Eleusine indica L. Gaertn. from Malaysia, which had a field history of two or three applications of fluazifop-p-butyl per year over 4-5 years, was previously found to show 100-fold resistance to the herbicide under field conditions compared with a susceptible population. This biotype is cross-resistant to other aryloxyphenoxypropanoate and cyclohexanedione herbicides. Previous research has shown no evidence for differential uptake, translocation, or metabolism of fluazifop-butyl between the R and S biotypes. Fluazifop acid reduced [14C]acetate incorporation into the lipid fraction of leaf disks of the S biotype but not of the R biotype. Acetyl coenzyme A carboxylase (ACCase) from the S biotype was sensitive to fluazifop, fenoxaprop, sethoxydim, and clethodim, with I50 values ranging from 1.0 to 5.6 μM. In contrast, ACCase from the R biotype was much less sensitive to fluazifop, fenoxaprop, and sethoxydim, with I50 values of >500, 25, and 77 μM, respectively. ACCase from the R biotype showed only a low level of resistance to clethodim, with an I50 of 6.6 μM. The close correlation between the whole plant and ACCase sensitivities to various ACCase inhibitors suggests that herbicide resistance in this biotype is conferred by a mutation to the herbicide target site, ACCase.

Herbicide resistance was first recognized as a problem on the Canadian Prairies in 1988 when trifluralin-resistant green foxtail ( Setaria viridis ) was reported in Manitoba, and chlorsulfuron-resistant chickweed ( Stellaria media ) and koehia ( Kochia scoparia ) in Alberta and Saskatchewan, respectively. Since then, the number of resistant weeds has increased to include wild oats ( Avena fatua ) resistant to triallate and to aryloxyphenoxypropionate and cyclohexanedione (group 1) herbicides, green foxtail to group 1 herbicides, Russian thistle ( Salsola pestifer ) and wild mustard ( Sinapis arvensis ) to sulfonylurea and imidazolinone (group 2) herbicides, and wild mustard to growth regulator (group 4) herbicides. The levels and patterns of cross-resistance to chemicals in groups 1 and 2 vary widely among different populations, with resistance factors [resistant to susceptible (R:S) ratios] derived from dose response curves typically ranging from < 2 to > 150. Group 1 resistance in green foxtail and group 2 resistance in chickweed and kochia populations are due to reduced sensitivities of the target enzymes, acetyl coenzyme-A carboxylase (ACCase) and acetolactate synthase (ALS), respectively. The mechanisms of resistance in the other species including wild oats resistant to ACCase inhibitors (group 1 ) and to triallate/difenzoquat (group 8) are unclear. At present, the only instance of multiple resistance in western Canada is green foxtail resistant to chemicals in both groups 1 and 3 (ACCase inhibitors and dinitroanilines). Future concerns focus mainly on the increasing seriousness of group 1 and 8 resistance across the Prairies, and on the possibility of selecting for multiple resistance in weeds such as green foxtail for which there are few remaining effective control options.

Aryloxyphenoxypropanoate (APP) herbicides, such as diclofop, depolarize membranes in parenchyma cells of coleoptiles and root tips, and isolated tonoplast or plasma membrane vesicles from a variety of plant species. Some APP-resistant biotypes of rigid ryegrass and wild oat repolarize membranes after removal of herbicide from a bathing medium. The repolarization ability does not require presence of either APP-insensitive acetyl coenzyme A carboxylase or an increased capacity for herbicide detoxification. The kinetics of depolarization and repolarization depend upon the herbicide, the herbicide concentration, the biotype, and the pH of the bathing solution. For rigid ryegrass, depolarization in the presence of diclofop acid is more rapid than in the presence of diclofop-methyl, and 50% depolarization required about 4 muM diclofop acid. Both the nonherbicidal S(-) and the herbicidal R(+) enantiomers of diclofop acid depolarized membranes in susceptible and resistant ryegrass. Susceptible biotypes regenerated transmembrane potentials following removal of the S(-) but not the R(+) enantiomer, whereas resistant biotypes repolarized following exposure to either enantiomer or a mixture of the two. The herbicide 2,4-D affected, in a complex manner, the ability of both susceptible and resistant ryegrass biotypes to depolarize and repolarize. It is postulated that the intracellular concentration of diclofop acid in susceptible and resistant plants is not the same due to differences in the partitioning of diclofop acid between the extracellular spaces and the cytoplasm. The mechanism producing the postulated difference is unknown, but observations on the proton extrusion capacity of both ryegrass and wild oats, the responses of ryegrass to [K+] and PCMBS, and the single-gene inheritance pattern of resistance in wild oats indicate that changes in the diclofop sensitivity of a plasma membrane protein involved in the generation of proton or ion gradients may be involved.

The herbicide chlorsulfuron is not translocated readily in plants because of an inhibitory effect on phloem translocation. More chlorsulfuron was translocated in a chlorsulfuron-resistant (R) biotype of Arabidopsis thaliana than in a susceptible (S) biotype, indicating that the effect on translocation is secondary to inhibition of ALS, the primary site of action of the herbicide. The R biotype did not differ from the S biotype in its ability to translocate exogenously applied sucrose; however, translocation of exogenously applied sucrose following chlorsulfuron treatment was greater in the R biotype than in the S biotype. Chlorsulfuron pretreatment inhibited rapid sucrose uptake into leaf discs by 41% in the S biotype but by only 17% in the R biotype. This result suggests that chlorsulfuron inhibits phloem transport by restricting sucrose uptake into the phloem. Purified plasma membrane preparations extracted from the two biotypes following chlorsulfuron treatment did not differ in H+-ATPase activity or total plasmalemma protein content. Possible alternative mechanisms by which chlorsulfuron may inhibit phloem transport are discussed.