Herbicidal pretreatments

Use of Subtoxic Herbicide Pretreatments to Improve Crop Tolerance to Herbicides... 

Research on chemical antidotes or safeners has been summarized in several reviews and published symposia (3.-9). Most of the major developments (Table I) have resulted from impirical screening programs by Industry that may have been stimulated by observations of herbicide antagonism in plants (3, 10). However, some of the research on mode of action of antidotes has been directed at finding new ways to protect crop plants from herbicides (3). The research to be discussed in this text, namely the use of subtoxic herbicide pretreatments to improve crop tolerance to selected herbicides, arises in part from research on the mode of action of R-25788 as a selective antidote for EPIC or butylate in corn. 

Increasing Crop Tolerance to Herbicides with Herbicide Pretreatments... 

At this point, evidence that similar molecules acted as effective antidotes by inducing needed metabolic pathways for herbicide detoxication was at most very speculative. However, another hypothesis emerged. Could early herbicide pretreatments increase crop tolerance to these herbicides by elevating the substrates and enzymes needed for detoxication While not a new concept in animal systems, such an idea has received little attention in plant systems and it certainly has not been exploited in any practical way. The whole idea has seemed much more credible with the study by Jacetta and Radosevich (19) of photosynthetic recovery in corn after treatment with atrazine. More specifically, they showed that inhibition of photosynthesis was reduced and the rate of recovery enhanced in corn plants treated for the second or third time with atrazine compared to "first exposed" plants (Figure 2). Furthermore, the faster recovery was related to enhanced rates of atrazine metabolism in the previously treated plants (Table III). 

Table IV. Effects of herbicide pretreatments on subsequent herbicide injury to crops.

Compounds that affect activities of hepatic microsomal enzymes can antagonize the effects of methyl parathion, presumably by decreasing metabolism of methyl parathion to methyl paraoxon or enhancing degradation to relatively nontoxic metabolites. For example, pretreatment with phenobarbital protected rats from methyl parathion s cholinergic effects (Murphy 1980) and reduced inhibition of acetylcholinesterase activity in the rat brain (Tvede et al. 1989). Phenobarbital pretreatment prevented lethality from methyl parathion in mice compared to saline-pretreated controls (Sultatos 1987). Pretreatment of rats with two other pesticides, chlordecone or mirex, also reduced inhibition of brain acetylcholinesterase activity in rats dosed with methyl parathion (2.5 mg/kg intraperitoneally), while pretreatment with the herbicide linuron decreased acetylcholine brain levels below those found with methyl parathion treatment alone (Tvede et al. 1989). 

Plant. Trifluralin was absorbed by carrot roots in greenhouse soils pretreated with the herbicide (0.75 Ib/acre). The major metabolite formed was a,a,a-trifluoro-2,6-dinitro-7V-(/i-propyl-toluene)-p-toluidine (Golab et al., 1967). Two metabolites of trifluralin that were reported in goosegrass Eleucine indica) were 3-methoxy-2,6-dinitro-A7A-dipropyl-4-(trifluoromethyl)benz-enamine and A-(2,6-dinitro-4-(trifluoromethyl)phenyl)-jV-propylpropanamide (Duke et al., 1991). 

Nilve, G. 1992. Sample pretreatment methods for determination of acidic herbicides in water, with special emphasis on supported liquid membranes. PhD dissertation, Lund University, Sweden. 

The Frank and Demint [200] method is directly applicable to water samples. After addition of sodium chloride (340g IT1) and aqueous hydrochloric acid (1 1) to bring the pH to 1, the sample was extracted with ethyl ether and the organic layer was then extracted with 0.1M sodium bicarbonate (saturated with sodium chloride and adjusted with sodium hydroxide to pH8). The aqueous solution adjusted to pHl with hydrochloric acid was extracted with ether and after evaporation of the ether to a small volume, Dalapon was esterified at room temperature by addition of diazomethane (0.5% solution in ether) and then applied to a stainless steel column (1.5m/3mm) packed with Chromosorb P (60-80 mesh) pretreated with hexamethyldisilazane and then coated with 10% FFAP. The column was operated at 140°C, with nitrogen carrier gas (30mL muT1) and electron capture detection. The recovery of Dalapon ranged from 91 to 100% the limit of detection was O.lng. Herbicides of the phenoxyacetic acid type did not interfere trichloroacetic acid could be determined simultaneously with Dalapon. 

Stimulated by the antidote research, as well as by the work of Jacetta and Radosevich (19). we decided to examine the influence of pretreatment with subtoxic rates of several herbicides on later crop tolerance to these same herbicides. For these growth room studies (25°C/18°C, 16h photoperiod, light intensity 400 uE/mZ/sec, 75% relative humidity), pretreatments at concentrations of 0.1% to 10% of the final herbicidal rate were given as a root drench or seed treatment to seeds planted in moist vermiculite in styrofoam cups. Herbicidal treatments were later applied to the roots. Plants were harvested 8-14 days after herbicide treatment (Table IV). 

Figure 3. Prevention of metribuzin (0.5 mg/L) injury to tomatoes with metribuzin pretreatments (1.0 ug/L). Left to right, control, 1.0 ug/L pretreatment followed by 0.5 ng/L herbicidal treatment 14 days later.

Figure 6. Effect of R-25788 pretreatments on the toxicity of a later herbicidal amount of R-25788 in corn seedlings.

The photo-Fenton processes are explored as photochemical pretreatment of nonbiodegradable and ubiquitous environmental pollutants and/or extremely toxic compoimds in wastewaters, such as persistent organic dyes under visible light irradiation (151,154,180,181) and under UV irradiation (139,182), azo dye factory wastewaters (140,162,183-185), herbicides (186-188), pesticides (152,153,189,190), insecticides (191), pharmaceuticals and wastewaters from medical laboratories (192-197), smdactants (198), industrial effluents with persistent toxic pollutants (199), industrial solvents and wastewaters (167,200), chlorinated solvents (201), and municipal wastewater (202). The photo-Fenton process was proposed to improve the biodegradability of especially biorecalcitrant wastewater, coming from textile industry, and the method was also suggested for water disinfection (203-205). 

The above studies show that the imprinted sorbents can be used in two modes in the reversed phase mode (sample application) and in the affinity mode (dichloromethane washing). Otherwise there are only few reports on the use of MIPs for the pretreatment of samples of environmental concern. Recently an MIP imprinted using the herbicide bentazone as template was synthesised and evaluated by... 

Nilve G, Knutsson M, and Jdnsson JA. Liquid chromatographic determination of sulfonylurea herbicides in natural waters after automated sample pretreatment using supported liquid membranes. J. Chromatogr. A 1994 668 75-82. 

Khrolenko, M., Dzygiel, P., Wieczorek, P. (2002). Combination of supported hquid membrane and solid-phase extraction for sample pretreatment of triazine herbicides in juice prior to capiUary electrophoresis detennination. J. Chromatogr. A, 975, 219-27. 

The phenomenon of cross-enhancement, or the ability of soils pretreated with specific herbicides to degrade other structurally related herbicides more rapidly than in untreated control soils, has been known for nearly 40 years. Both enhanced degradation and cross-adaptation have been observed under field conditions for phenoxyalkanoic acid herbicides (3-5). ... 

Field studies with single applications of phenoxyalkanoic acid herbicides have indicated that breakdown is rapid under temperature and moisture conditions that favour microbiological activity (5). Enhanced degradation of these herbicides, under field conditions, was first noted in the late 1940s. The use of plant bioassay procedures, led to the discovery that the persistence of 2,4-D, but not 2,4,5-T, was decreased,by pretreatment of the soil with 2,4-D (26, 27). This enhanced breakdown was later confirmed using (14C)2,4-D and radiochemical analytical techniques (29). The breakdown of the (14C)2,4-D being more rapid in soil from the treated plots, tested 8 months after the last field application, than in soil from plots treated for the first time. 

The breakdown of repeated 2,4-D and MCPA applications made to a field soil has been monitored using a white mustard bioassay procedure (21) Repeated applications of the two herbicides resulted in a reduction in degradation time from 10 weeks for 2,4-D and 20 weeks for MCPA, after one treatment, to 4 and 7 weeks, respectively, after 19 annual applications. From these pretreated soils microbial isolates were extracted and, in mineral salt medium, were found to degrade 2,4-D and MCPA more rapidly than isolates from untreated control soils (35). The microbial isolates from the 2,4-D treatments would also rapidly degrade MCPA added to... 

The presence of some metabolites in flooded rice soils (19,20) and in cranberry bogs (21) has resulted in rapid degradation of their parent compounds. To assess the role of metabolites in conditioning the soil for their respective parent compounds (in nonflooded situations), we pretreated a soil up to 4 times with a hydrolysis product and studied the fate of the subsequently applied UC-labeled parent compound for several insecticides and herbicides. 

Herbicide Family and Pretreatment Sorbent Elution Treatment LOD Recovery (%) RSD (%) Technique Year Ref. 

Chloroacetanilides and nitrogen heterocyclic herbicides Soil (5 g) PFE Pretreatment with 31.6% of water. 32 ml acetone, 1500 psi, 100°C Dried over Na2S04, filtered, evaporated to 1 ml, and adjusted to 5 ml with MTBE 93 to 103 1 to 7 GC-MS 2000 196... 

As mentioned before, analytical methods required for herbicide determination must be very sensitive, selective, and robust. Normalized methods generally use liquid and gas chromatography techniques with detectors more or less specific. Sample pretreatment such as derivatization steps or cleanup of the extracts are sometimes mandatory prior to analytical measurement. 

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