Weak Acid Herbicides Research Articles

Spray water quality and herbicide performance: a review

AbstractWater is the primary carrier for herbicide applications. Spray water qualities such as pH, hardness, temperature, or turbidity can influence herbicide performance and may need to be amended for optimum weed control. Water quality factors can affect herbicide activity by reducing solubility, enhancing degradation in the spray tank, or forming herbicide-salt complexes with mineral cations, thereby reducing the absorption, translocation, and subsequent weed control. The available literature suggests that the effect of water quality varies with herbicide chemistry and weed species. The efficacy of weak-acid herbicides such as glyphosate, glufosinate, clethodim, sethoxydim, bentazon, and 2,4-D is improved with acidic water pH; however, the efficacy of sulfonylurea herbicides is negatively impacted. Hard-water antagonism is more prevalent with weak-acid herbicides, and trivalent cations are the most problematic. Spray solution temperature between 18 C and 44 C is optimum for some weak-acid herbicides; however, their efficacy can be reduced at relatively low (5 C) or high (56 C) water temperature. The effect of water turbidity is severe on cationic herbicides such as paraquat and diquat, and herbicides with low soil mobility such as glyphosate. Although adjuvants are recommended to overcome the negative effect of spray water hardness or pH, the response has been inconsistent with the herbicide chemistry and weed species. Moreover, information on the effect of spray water quality on various herbicide chemistries, weed species, and adjuvants is limited; therefore, it is difficult to develop guidelines for improving weed control efficacy. Further research is needed to determine the effect of spray water factors and develop specific recommendations for improving herbicide efficacy on problematic weed species.

Weed control by 2,4-D dimethylamine depends on mixture water hardness and adjuvant inclusion but not spray solution storage time

AbstractHerbicides are an important tool in managing weeds in turf and agricultural production. One of the earliest selective herbicides, 2,4-D, is a weak acid herbicide used to control broadleaf weeds. Water-quality parameters, such as pH and hardness, influence the efficacy of weak acid herbicides. Greenhouse experiments were conducted to evaluate how varying water hardness level, spray solution storage time, and adjuvant inclusion affected broadleaf weed control by 2,4-D dimethylamine. The first experiment evaluated a range of water-hardness levels (from 0 to 600 mg calcium carbonate [CaCO3] L−1) on efficacy of 2,4-D dimethylamine applied at 1.60 kg ae ha−1 for dandelion and horseweed control. A second experiment evaluated dandelion control from spray solutions prepared 0, 1, 4, 24, and 72 h before application. Dandelion and horseweed control by 2,4-D dimethylamine was reduced when the CaCO3 level in water was at least 422 or at least 390 mg L−1, respectively. Hard-water antagonism was overcome by the addition of 20 g L−1 ammonium sulfate (AMS) into the mixture. When AMS was included in spray mixtures, no differences were observed at 600 mg CaCO3 L−1, compared with distilled water. Spray solution storage time did not influence dandelion control, regardless of water-hardness level or adjuvant inclusion. To prevent antagonism, applicators should use a water-conditioning agent such as AMS when applying 2,4-D dimethylamine in hard water.

Mechanism of the effect of pH and biochar on the phytotoxicity of the weak acid herbicides imazethapyr and 2,4-D in soil to rice (Oryza sativa) and estimation by chemical methods

The existing form of an ionizable organic compound can simultaneously affect its soil adsorption and plant bioactivity. In this experiment, the adsorption and bioactivity of two weak acid herbicides (WAHs), imazethapyr and 2,4-D, were studied to explore the predominant mechanism by which the soil pH and the addition of biochar can influence the phytotoxicity of WAHs in soil. Then, the WAH concentration extracted by hollow fiber-based liquid-phase microextraction (CHF-LPME), the in situ pore water concentration (CIPW) and the added concentration (CAC) were employed to estimate the phytotoxicity. The results showed that with increased pH from 5.5 to 8.5, the phytotoxicity of the WAHs to rice increased about 1-fold in the soil, but decreased in aqueous solutions, the IC50 values for imazethapyr and 2,4-D at pH 5.0 were 3- and 2-fold higher than that at pH 8.0. In addition, the soil adsorption decreased, indicating that the adsorption process was the dominant factor for the variation of the phytotoxicity of the WAHs in the tested soil instead of the decreasing bioactivity. The concentration that inhibits plant growth by 50% (IC50) calculated by the CAC in different pH and biochar soils ranged from 0.619 to 3.826 mg/kg for imazethapyr and 1.871–72.83 mg/kg for 2,4-D. The coefficient of variation (CV) of the IC50 values reached 65.61% for imazethapyr and 130.0% for 2,4-D. However, when IC50 was calculated by CIPW and CHF-LPME, the CVs of the IC50 values decreased to 23.51% and 36.23% for imazethapyr and 40.21% and 50.93% for 2,4-D, respectively. These results suggested that CIPW and CHF-LPME may be more appropriate than CAC for estimating the phytotoxicity of WAHs.

Pesticide sorption and leaching potential on three Hawaiian soils

On the Hawaiian Islands, groundwater is the principal source of potable water and contamination of this key resource by pesticides is of great concern. To evaluate the leaching potential of four weak acid herbicides [aminocyclopyrachlor, picloram, metsulfuron-methyl, biologically active diketonitrile degradate of isoxaflutole (DKN)] and two neutral non-ionizable herbicides [oxyfluorfen, alachlor], their sorption coefficients were determined on three prevalent soils from the island of Oahu. Metsulfuron-methyl, aminocylcopyrachlor, picloram, and DKN were relatively low sorbing herbicides (Koc = 3–53 mL g−1), alachlor was intermediate (Koc = 120–150 mL g−1), and oxyfluorfen sorbed very strongly to the three soils (Koc > 12,000 mL g−1). Following determination of Koc values, the groundwater ubiquity score (GUS) indices for these compounds were calculated to predicted their behavior with the Comprehensive Leaching Risk Assessment System (CLEARS; Tier-1 methodology for Hawaii). Metsulfuron-methyl, aminocyclopyrachlor, picloram, and DKN would be categorized as likely leachers in all three Hawaiian soils, indicating a high risk of groundwater contamination across the island of Oahu. In contrast, oxyfluorfen, regardless of the degradation rate, would possess a low and acceptable leaching risk due to its high sorption on all three soils. The leaching potential of alachlor was more difficult to classify, with a GUS value between 1.8 and 2.8. In addition, four different biochar amendments to these soils did not significantly alter their sorption capacities for aminocyclopyrachlor, indicating a relatively low impact of black carbon additions from geologic volcanic inputs of black carbon. Due to the fact that pesticide environmental risks are chiefly dependent on local soil characteristics, this work has demonstrated that once soil specific sorption parameters are known one can assess the potential pesticide leaching risks.

Overcoming the Antagonistic Effect from Spray Carrier Minerals on Imazethapyr Activity

Minerals in spray carrier water can adversely influence the activity of weak acid herbicides, thus overcoming them has been considered a main research priority. A series of dose–response pot experiments were conducted to compare the water softening chemical methods [(NH4)2SO4, NH4NO3, C6H8O7, and KHPO4] and a new water softening physical method (passing through a magnetic field of 0.7 T) for overcoming antagonism from the carriers containing 0.5 g L−1 of CaCO3, MgCO3, Na2CO3, K2CO3, or Fe2(CO3)3 on imazethapyr (2‐[4,5‐dihydro‐4‐methyl‐4‐(1‐methylethyl)‐5‐oxo‐1H‐imidazol‐2‐yl]‐5‐ethyl‐3‐pyridinecarboxylic acid) activity on jimsonweed (Datura stramonium L.). The imazethapyr activity was significantly increased by all methods when distilled water was used as the carrier. The (NH4)2SO4 (relative potency [RP] = 3.52) was the most effective method, followed by C6H8O7 and NH4NO3 (RP = 2.58), a magnetized carrier (RP = 1.83), and then KHPO4 (RP = 1.48) to improve imazethapyr activity. The imazethapyr activity was clearly antagonized when carbonate salts were added to distilled water. The intensity of the antagonistic effects depended on the cation's valence and ranked as Fe2(CO3)3 (RP = 0.47) > MgCO3 (RP = 0.52) = CaCO3 (RP = 0.53) > K2CO3 (RP = 0.69) = Na2CO3 (RP = 0.66). The application of a magnetized carrier was not as effective a method for overcoming this antagonism as the addition of (NH4)2SO4 or C6H8O7, but it was more effective than the addition of NH4NO3 or KHPO4. Nonetheless, from different points of view—agronomic, environmental, and economic—applying a magnetized carrier is recommended as a technique because it not only needs no chemical but it also causes no side effects.

Overcoming Hard Water Antagonistic to Glyphosate or Imazethapyr with Water Conditioners

Carrier water quality may affect the activity of weak acid herbicides when concentrations of some cations are high. A dose-response experiment on glyphosate and imazethapyr activity, which were carried by the carrier types of distilled water and hard water, against jimsonweed were conducted to compare the water conditioning chemicals ammonium sulfate, ammonium nitrate, citric acid and potassium phosphate, with magnetized carrier as a new method. A magnetic field of 0.7 Tesla was applied to prepare the magnetized carrier. With the exception of potassium phosphate with imazethapyr, the activity of glyphosate and imazethapyr was significantly increased in the presence of the water conditioning methods when distilled water was used as the carrier. Ammonium sulfate was the most effective method. The activity of both herbicides was decreased when applied with hard water carrier. Potassium phosphate was not effective at reducing the antagonism of cations in the hard water carrier. In glyphosate, the performance of water conditioning methods in softening hard water carrier could be ranked as follows: ammonium sulfate (2.52-fold) > magnetized carrier (2.12-fold) ≥ citric acid (1.64-fold) ≥ ammonium nitrate (1.39-fold) > potassium phosphate (0.96-fold). In imazethapyr, this order was as follows: ammonium sulfate (2.99-fold) > ammonium nitrate (2.66-fold) > magnetized carrier (1.81-fold) ≥ citric acid (1.64-fold) > potassium phosphate (1.10-fold).

Overcoming Hard Water Antagonistic to Glyphosate or Imazethapyr with Water Conditioners

Carrier water quality may affect the activity of weak acid herbicides when concentrations of some cations are high. A dose-response experiment on glyphosate and imazethapyr activity, which were carried by the carrier types of distilled water and hard water, against jimsonweed were conducted to compare the water conditioning chemicals ammonium sulfate, ammonium nitrate, citric acid and potassium phosphate, with magnetized carrier as a new method. A magnetic field of 0.7 Tesla was applied to prepare the magnetized carrier. With the exception of potassium phosphate with imazethapyr, the activity of glyphosate and imazethapyr was significantly increased in the presence of the water conditioning methods when distilled water was used as the carrier. Ammonium sulfate was the most effective method. The activity of both herbicides was decreased when applied with hard water carrier. Potassium phosphate was not effective at reducing the antagonism of cations in the hard water carrier. In glyphosate, the performance of water conditioning methods in softening hard water carrier could be ranked as follows: ammonium sulfate (2.52-fold) > magnetized carrier (2.12-fold) ≥ citric acid (1.64-fold) ≥ ammonium nitrate (1.39-fold) > potassium phosphate (0.96-fold). In imazethapyr, this order was as follows: ammonium sulfate (2.99-fold) > ammonium nitrate (2.66-fold) > magnetized carrier (1.81-fold) ≥ citric acid (1.64-fold) > potassium phosphate (1.10-fold).

Effect of soil properties on pure and formulated mesotrione adsorption onto vertisol (Limagne plane, Puy-de-Dôme, France)

The fate of ionisable pesticides in the environment is complex as it is importantly related to many soil properties: pH, mineralogy, organic matter content and other soil characteristics. The adsorption of a weak acid herbicide, mesotrione, was studied in detail on whole and peroxide-treated vertisol topsoil and also on its granulometric fractions (clay, silt, sand) to evaluate the role of mineralogy and different organic matter pools. The soil studied is alkaline silty loam with smectite as the main clay mineral. It contains 1.7% organic carbon, mostly stabilized as a complex with smectite. Humus is of fulvic type. Mesotrione adsorption occurs on both mineral and organic constituents. Adsorption is weak and mesotrione can be easily and totally desorbed. As shown with 13C NMR experiments, adsorption best correlates with the alkyl and carboxylic carbon content, and occurs on both bound and free organic matter. No difference of mesotrione sorption was observed with the formulation Callisto®.

A Test Method for Evaluating Water Conditioning Adjuvants

Abstract Glyphosate is a weak acid herbicide and can bind with antagonistic salts in the spray carrier. Diammonium sulfate (AMS) is commonly used as an adjuvant with glyphosate to enhance activity and overcome antagonistic salts. Water conditioning (WC) adjuvants are used at low rates and are used as AMS replacement adjuvants. However, an ASTM approved method for testing WC adjuvants has not been established. Appropriate research parameters were used to test the water conditioner definition. Glyphosate plus NIS and glyphosate plus AMS and nonionic surfactant (NIS) were used as standards from comparing WC adjuvants. WC were compared to NIS and AMS plus NIS to determine the potential to reduce or eliminate antagonism between glyphosate and antagonistic cations. Velvetleaf was a good indicator to show antagonism, the ability for AMS and WC adjuvants to overcome antagonism, and also show greater bioefficacy. Velvetleaf, a mixture of broadleaf species, and a mixture of grass species all showed similar response to WC adjuvants. Studies to test water conditioning adjuvants were conducted at multiple locations provided a high degree of precision.

Effect of Hard Water and Ammonium Sulfate on Weak Acid Herbicide Activity

Abstract Glyphosate is a weak acid herbicide and can bind with calcium in the spray carrier. Diammonium sulfate is commonly used as an adjuvant with glyphosate to enhance phytotoxicity and overcome antagonistic effect of these cations. Most postemergence herbicides are also weak acid herbicides. Data is limited for other weak acid herbicides and the effect of diammonium sulfate in enhancing herbicide activity and overcoming antagonism. Field studies were conducted with aminopyralid, tembotrione, dicamba plus diflufenzopyr, and glufosinate to determine if (1) these weak acid herbicides are enhanced by ammonium, (2) if they are antagonized by calcium and magnesium in the spray solution, (3) if diammonium sulfate overcomes salt antagonism, and (4) if a previously published equation for the amount of ammonium sulfate required to overcome salt antagonism of glyphosate based on cation concentration in spray water correctly predicts to other weak acid herbicides. The activity of the four weak acid herbicides increased with the addition of ammonium to the spray solution, all were antagonized by calcium and magnesium, and diammonium sulfate overcame the antagonism. The previously published equation to calculate the amount the diammonium sulfate needed to overcome 500 and 1000 ppm hardness was accurate and can be used for these herbicides and possibly other weak acid herbicides.

Sorption of Acetochlor, Atrazine, 2,4-d, Chlorotoluron, MCPA, and Trifluralin in Six Soils From Slovakia

Sorption of the herbicides (acetochlor, atrazine, 2,4-D: , chlorotoluron, MCPA and trifluralin) in their commercially available formulations was characterized on six agricultural soils from Slovakia. Weak acid herbicides (2,4-D: and MCPA) were the least sorbed, whereas weak base such as atrazine and nonionic herbicides were the most sorbed in the order: atrazine < acetochlor approximately chlorotoluron < trifluralin, which was closely related to the hydrophobicity of herbicides expressed as logKOW. The KF values positively correlated with soil organic carbon content for all herbicides. Moreover, all herbicides were the most sorbed by Vertisol with relatively high smectite clay content in proportion to organic carbon content. The KOC values for each herbicide showed a smaller variation among soils (excluding Vertisol) than the KF and were used to compare soil mobility potential of herbicides.

Laboratory study of retention and release of weak acid herbicide MCPA by soils and sediments and leaching potential of MCPA

MCPA sorption and desorption in five surface soils (denoted as A1-5), three bottom sediments (S1-3), two river sediments (L1-2) and one subsurface soil (SS) at two initial concentrations in aqueous solution &amp;ndash; C&lt;sub&gt;0&lt;/sub&gt; = 0.5 and 10&amp;nbsp;mg/l were studied. No significant effect of the initial concentration on MCPA equilibrium distribution between soil/sediment and aqueous solution was observed. The difference between distribution coefficient K&lt;sub&gt;D&lt;/sub&gt; at C&lt;sub&gt;0&lt;/sub&gt; = 0.5&amp;nbsp;mg/l and K&lt;sub&gt;D&lt;/sub&gt; at C&lt;sub&gt;0&lt;/sub&gt; = 10 mg/l was found only in the case of one bottom sediment (S2). A simple regression analysis between K&lt;sub&gt;D&lt;/sub&gt; at C&lt;sub&gt;0&lt;/sub&gt; = 0.5 and 10 mg/l and soil/sediment properties indicated that the most important property which determined the variation in MCPA sorption is organic carbon (r = 0.886*** and r = 0.926***, respectively). Similarly, desorption of MCPA was inversely proportional to organic carbon content of the soils and sediments used (r = &amp;ndash;0.862* and r = &amp;ndash;0.842**). These observations showed that MCPA sorption and desorption in soils and sediments were primarily controlled by organic components of the geosorbents used. Overall, the percentage of MCPA sorption in soils and sediments was low (P&lt;sub&gt;sorp&lt;/sub&gt; &amp;asymp; 3&amp;ndash;53%; K&lt;sub&gt;D&lt;/sub&gt; = 0.077&amp;ndash;2.827 l/kg) and the percentage of MCPA desorbed was relatively high (P&lt;sub&gt;des&lt;/sub&gt; &amp;asymp; 11&amp;ndash;70%), especially in the soils and sediments with lower organic carbon content. The experimental results and calculated values of groundwater ubiquity score GUS and relative leaching potential index RLPI imply that MCPA is very mobile in all the surface soils and has a potential to contaminate groundwater.

Increasing the Biological Activity of Weak Acid Herbicides by Increasing and Decreasing the pH of the Spray Mixture

Abstract The pH of the spray mixture controls the solubility and ionic state of weak acid herbicides and thus influences their uptake and biological activity. When the pH of the spray water is below the pKa of the herbicide, increasing pH can increase solubility and improve activity when herbicide solubility limits uptake. However, raising the pH above the pKa makes the weak acid anionic and thus may make it more difficult to penetrate into the lipophilic cuticle and the negatively charged membrane and cell wall. Decreasing the pH below the pKa converts the weak acid into a neutral or unionized form and thus makes it easier to penetrate these lipophilic and negatively charged barriers. pH also influences other herbicide properties including chemical stability, volatility, and chemical compatibility. Thus, manufacturers need to balance a number of properties when they adjust the pH of their adjuvant and herbicide formulations. These studies show significant differences in the biological activity of several herbicides when the pH is increased and decreased with a range of surfactant types. These results support the concept that the physicochemical properties of the herbicide and adjuvant should be matched for optimum activity.

Increasing and Decreasing pH to Enhance the Biological Activity of Nicosulfuron

Increasing the pH of the spray water to solubilize the weak acid herbicide nicosulfuron and then decreasing pH below its pKaso that it converts into a neutral form enhances biological activity under some conditions. The water-dispersible granule formulation of nicosulfuron starts as dispersed particles. Adding 1% wt/wt K3PO4solubilizes nicosulfuron and increases its activity compared to its dispersion without base. The type of buffer and the surfactant HLB or hydrophilic lipophilic balance, a measure of the molecular balance of the hydrophilic and lipophilic groups, altered the activity of nicosulfuron. Adding 1% wt/wt K3PO4increases the pH, and the optimum HLB ranged from 13 to 17 on large crabgrass. Adding 1% wt/wt H3PO4reduces the pH and lowers the optimum HLB range from 10 to 14 on large crabgrass. Adding the acidic buffer converts the solubilized nicosulfuron into its neutral form and increases activity under some surfactant conditions. Thus, neutral nicosulfuron is more active with lipophilic surfactants, while ionic nicosulfuron is more active with hydrophilic surfactants. When tested on other species, low HLB surfactants are the most active at low pH. These results support the concept that the physicochemical properties of the herbicide, adjuvants, and weed species should be matched for optimum activity.

Interactions of two sulfonylurea herbicides with organoclays

AbstractThe sorption of two sulfonylurea herbicides (SU), metsulfuron methyl and nicosulfuron, on pure clays and organoclays was investigated. Three clays (Arizona smectite, SAz-1, Wyoming smectite, SWy-2, and hectorite, SHCa-1), were treated with amounts of octadecylammonium (ODA) or dioctadecyldimethylammonium (DODMA) cations equal to ∼50 and 100% of the clays’ cation exchange capacity (CEC). Sorption isotherms were fitted to the Freundlich equation. While no measurable sorption was found on the pure clays (Kf = 0), organoclays prepared using both primary and quaternary amines were effective as SU sorbents. The metsulfuron methyl Kf values ranged between 196 and 1498 µmol1−1/n kg−1 L1/n, and Kf values for nicosulfuron, which were lower than those of metsulfuron methyl, ranged from 35 to 198 umol1−1/n kg−1 L1/n. As shown by sorption coefficients, Kd and KOC, SWy-2 treated with DODMA at ∼100% of the CEC was the most effective sorbent for metsulfuron, Kd = 684 L kg−1 and KOC = 2138 L kg−1. For nicosulfuron the most effective sorbent was SAz-1 with ODA at ∼ 50% of the CEC (Kd = 147 L kg−1 and KOC = 1233 L kg−1). In contrast to other weak-acid herbicides, such as phenoxy and picolinic acids, no clear relationships were found between sorption and layer charge, organic carbon content, and basal spacing of the organoclays for both sulfonylurea herbicides. Sorption of both herbicides on organoclays was assumed to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations was a very important factor.

Bentazone uptake into plant foliage as influenced by surfactants and carrier pH

The influence of surfactants and carrier pH on the foliar uptake of bentazone, a representative weak acid herbicide, was studied using bean (Vicia faba) and mustard (Sinapis alba) plants. The promoting effect of surfactants on bentazone uptake varied with surfactant hydrophobe structure, ethylene oxide (EO) content, and surfactant and bentazone concentrations. Among the 5 hydrophobe moieties tested, the efficiency order was C13 /C15 linear alcohols &gt; C10 linear alcohol &gt; C16 /C18 linear alcohols &gt; nonylphenol &gt; octylphenol. For C13 /C15 linear alcohol surfactants of differing EO content, the enhancing effect on bentazone uptake was 5 EO &gt; 10 EO &gt; 14 EO. All surfactants improved uptake more when bentazone was applied at low concentration. The effect of carrier pH (5, 7, and 9) on the uptake of bentazone, applied both as unformulated acid and as a sodium salt, was also investigated in this work. Lower carrier pH did not provide greater uptake for bentazone sodium salt and was only beneficial for the uptake of bentazone acid applied at very low concentration. The results are discussed in relation to the current knowledge on pesticide uptake as influenced by surfactants and the change in lipophilicity and solubility of weak acid chemicals at different pH.

Sorption and leaching potential of herbicides on Brazilian soils

Sorption of the herbicides alachlor, atrazine, dicamba, hexazinone, imazethapyr, metsulfuron‐methyl, nicosulfuron, simazine and sulfometuron‐methyl was characterized on six Brazilian soils, using the batch equilibration method. In general, weak acid herbicides (dicamba, imazethapyr, metsulfuron‐methyl, nicosulfuron and sulfometuron‐methyl) were the least sorbed, whereas weak bases such as triazines and nonionic herbicides (alachlor) were the most sorbed. The Kd values found showed a significant correlation with soil organic carbon content (OC) for all herbicides except imazethapyr and nicosulfuron. Koc values showed a smaller variation among soils than Kd. To estimate the leaching potential, Koc and the ground‐water ubiquity score (GUS) were used to calculate half‐lives (t1/2) that would rank these herbicides as leachers or non‐leachers. Comparison of calculated values to published values for t1/2 demonstrated that sulfonylureas and hexazinone are leachers in all soils, alachlor is transitional, and atrazine, simazine and dicamba are leachers or transitional, depending on soil type. Results discussed in this paper provide background to prioritize herbicides or chemical groups that should be evaluated in field conditions with regard to their leaching potential to ground‐water in tropical soils.

Sorption−Desorption of Imazamethabenz on Three Spanish Soils

Methyl imazamethabenz (methyl 6-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-4-yl)-p-toluate and m-toluate) is a weak acid herbicide used to control wild oats in cereal crops. This chemical has high so...

Mechanisms of Herbicide Absorption Across Plant Membranes and Accumulation in Plant Cells

In most cases, a herbicide must traverse the cell wall, the plasma membrane, and organellar membranes of a plant cell to reach its site of action where accumulation causes phytotoxicity. The physicochemical characteristics of the herbicide molecule including lipophilicity and acidity, the plant cell membranes, and the electrochemical potential in the plant cell control herbicide absorption and accumulation. Most herbicides move across plant membranes via nonfacilitated diffusion because the membrane's lipid bilayer is permeable to neutral, lipophilic xenobiotics. Passive absorption of lipophilic, ionic herbicides or weak acids can be mediated by an ion-trapping mechanism where the less lipophilic, anionic form accumulates in alkaline compartments of the plant cell. A model that includes the pH and electrical gradients across plant cell membranes better predicts accumulation concentrations in plant cells of weak acid herbicides compared to a model that uses pH only. Herbicides also may accumulate in plant cells by conversion to nonphytotoxic metabolites, binding to cellular constituents, or partitioning into lipids. Evidence exists for herbicide transport across cell membranes via carrier-mediated processes where herbicide accumulation is energy dependent; absorption is saturable and slowed by metabolic inhibitors and compounds of similar structure.

Evidence Against a Direct Membrane Effect in the Mechanism of Action of Graminicides

The aryloxyphenoxypropionate and cyclohexanedione herbicides, which inhibit acetyl-coenzyme A carboxylase (EC 6.4.1.2), have also been hypothesized to act at specific sites on the plasmalemma. An impermeant sulfhydryl binding agent was reported to block the diclofop acid-induced depolarization of the membrane potential (Em) in rigid ryegrass. A correlation between the antagonistic interaction with auxin herbicides both in the field and in the Emresponse, and the repolarization of Emin herbicide-resistant rigid ryegrass following removal of diclofop acid also provide support for this hypothesis. However, similar membrane responses in resistant grasses and broadleaf species suggest that the membrane response may not be important in the phytotoxic activity of the postemergence graminicides under field conditions. In addition, an antagonistic interaction was not observed in roots of susceptible grasses exposed to combinations of diclofop-methyl and 2,4-D. Furthermore, the repolarization of the Emin diclofop-resistant rigid ryegrass was correlated to differential acidification of the external solution and an increase in the protonated form of diclofop acid, rather than a site-specific interaction at the plasmalemma. Although the membrane response is probably not involved in herbicide phytotoxicity in agricultural systems, a higher extracellular pH in the resistant biotypes of rigid ryegrass may inhibit the movement of these weak-acid herbicides across the plasmalemma, and possibly contribute to increased herbicide tolerance.