Measurement and modelling of glyphosate fate compared with that of herbicides replaced as a result of the introduction of glyphosate-resistant oilseed rape

BACKGROUND: Crops resistant to glyphosate may mitigate the increasing contamination of the environment by herbicides, since their weeding requires smaller amounts of herbicides and fewer active ingredients. However, there are few published data comparing the fate of glyphosate with that of substitute herbicides under similar soil and climatic conditions. The objectives of the work reported here were (i) to evaluate and compare the fate in soil in field conditions of glyphosate, as used on glyphosate-resistant oilseed rape, with that of two herbicides frequently used for weed control on the same crop, albeit non-resistant: trifluralin and metazachlor, and (ii) to compare field results with predictions of the pesticide root zone model (PRZM), parameterized with laboratory data. Dissipation and vertical distribution in the soil profile of glyphosate, trifluralin and metazachlor were monitored in an experimental site located in Eastern France for 1 year. RESULTS: Herbicide persistence in the field increased as follows: metazachlor < glyphosate < trifluralin, contrary to laboratory results showing glyphosate to be least persistent. The main metabolite of glyphosate-aminomethylphosphonic acid (AMPA)-was more persistent than glyphosate. AMPA and trifluralin had the largest vertical mobility, followed by metazachlor and glyphosate. PRZM underestimated the dissipation rate of glyphosate in the field and the formation of AMPA, but its predictions for trifluralin and metazachlor were correct. The simulation of herbicides and AMPA distribution in the soil profile was satisfactory, but the mobility of trifluralin and metazachlor was slightly underestimated, probably because PRZM ignores preferential flow. In general, data from the laboratory allowed an acceptable parameterization of the model, as indicated by goodness-of-fit indices. CONCLUSION: Because of the detection of AMPA in the deep soil layer, the replacement of both trifluralin and metazachlor with glyphosate might not contribute to decreasing environmental contamination by herbicides. PRZM may be used to evaluate and to compare other weed control strategies for herbicide-resistant as well as non-resistant crops.     

Removal of paraquat and atrazine from water by montmorillonite‐(Ce or Zr) phosphate cross‐linked compounds

The adsorption of paraquat (1,1′‐dimethyl‐4,4′‐bipyridilium dichloride) and atrazine (6‐chloro‐N ²‐ethyl‐N ⁴‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) from aqueous solution onto two montmorillonite‐(Ce or Zr) phosphate cross‐linked compounds at different temperatures (288 K and 308 K) has been studied using batch experiments. The adsorption isotherms obtained for paraquat on both adsorbents may be classified as H‐type of the Giles classification, which suggests that paraquat molecules are strongly adsorbed on the samples. For the adsorption of atrazine, L‐type isotherms were obtained for both montmorillonite‐(Ce or Zr) phosphate compounds, which suggests that these compounds have a medium affinity for this herbicide. The increase in temperature from 288 K to 308 K did not have any clear effect on the adsorption process of paraquat on either adsorbent whereas atrazine adsorption decreased slightly as temperature increased, possibly due to a mainly physical process. Fourier transform infrared (FTIR) spectroscopic studies revealed that at the pH generated by the adsorbents, the cationic herbicide interacted to a greater extent with the negatively charged surface of the adsorbents than did atrazine. For both herbicides, the Ce‐montmorillonite adsorbent showed a higher adsorption capacity than the Zr‐montmorillonite adsorbent. © 2000 Society of Chemical Industry     

Adsorption,desorption and dissipation of metolachlor in surface and subsurface soils

BACKGROUND: Variations in soil properties with depth influence retention and degradation of pesticides. Understanding how soil properties within a profile affect pesticide retention and degradation will result in more accurate prediction by simulation models of pesticide fate and potential groundwater contamination. Metolachlor is more persistent than other acetanilide herbicides in the soil environment and has the potential to leach into groundwater. Reasonably, information is needed about the dissipation and eventual fate of metolachlor in subsoils. The objectives were to evaluate the adsorption and desorption characteristics and to determine the dissipation rates of metolachlor in both surface and subsurface soil samples. RESULTS: Adsorption of metolachlor was greater in the high‐organic‐matter surface soil than in subsoils. Lower adsorption distribution coefficient (K_ads) values with increasing depth indicated less adsorption at lower depths and greater leaching potential of metolachlor after passage through the surface horizon. Desorption of metolachlor showed hysteresis, indicated by the higher adsorption slope (1/n_ads) compared with the desorption slope (1/n_des). Soils that adsorbed more metolachlor also desorbed less metolachlor. Metolachlor dissipation rates generally decreased with increasing soil depth. The first‐order dissipation rate was highest at the 0–50 cm depth (0.140 week^?1) and lowest at the 350–425 cm depth (0.005 week^?1). Degradation of the herbicide was significantly correlated with microbial activity in soils. CONCLUSION: Metolachlor that has escaped degradation or binding to organic matter at the soil surface might leach into the subsurface soil where it will dissipate slowly and be subject to transport to groundwater. Copyright © 2009 Society of Chemical Industry     

Mineralization of 2,4‐D,mecoprop, isoproturon and terbuthylazine in a chalk aquifer

The potential to mineralize 2,4‐dichlorophenoxyacetic acid (2,4‐D), mecoprop, isoproturon and terbuthylazine was studied in soil and aquifer chalk sampled at an agricultural field near Aalborg, Denmark. Laboratory microcosms were incubated for 258 days under aerobic conditions at 10 °C with soil and chalk from 0.15–4.45 m below the surface. The [ring‐U‐¹⁴C]‐labeled herbicides were added to obtain a concentration of 6 µg kg^?1 and mineralization was measured as evolved [¹⁴C]carbon dioxide. The herbicides were readily mineralized in soil from the plough layer, except for terbuthylazine, which was mineralized only to a limited extent. In the chalk, lag periods of at least 40 days were observed, and a maximum of 51%, 33% and 6% of the added 2,4‐D, mecoprop and isoproturon, respectively, were recovered as [¹⁴C]carbon dioxide. Large variations in both rate and extent of mineralization were observed within replicates in chalk. No mineralization of terbuthylazine in chalk was observed. As a measure of the general metabolic activity towards aromatic compounds, [ring‐U‐¹⁴C]‐benzoic acid was included. It was readily mineralized at all depths. © 2000 Society of Chemical Industry     

Evaluation of three pesticide leaching models with experimental data for alachlor, atrazine and metribuzin

The persistence and movement of residues of alachlor, alrazine and metribuzin were measured in a mini-lysimeter system in the field. This comprised a number of soil columns (11 cm diametert; 30 cm long), and permitted the vertical distribution of residues to be determined at. intervals alter application and the collection and analysis of leaehale water. Laboratory experiments were also performed to determine the degradation rates of the three herbicides and their strengths of adsorption by the test soil. The results showed an order of degradation rate of metribuzin> alachlor>atrazine and an order of adsorption of alacblor>atrazine>melribuzin. Movement of residues in the soil columns and concentrations in the leachate were inversely related to the strength of adsorption. Parameters derived from the laboratory data were used in conjunction with weather data for the period of the field experiment in three mathematical models of pesticide leaching: VARLEACH, LEACHP and PRZM2. In most instances, the models gave acceptable predictions of the distribution of residues in soil. This was particularly so for the less mobile compound alachlor. With the most mobile compound, metribuzin, residues were not well predicted at the later sampling dates. All three models gave accurate predictions of the volumes of drainage water, but none of them predicted the concentrations of herbicide in the leachate, presumably because they do not take account of preferential flow pathways of water and solute in the soil.     

Water and sediment dynamics of penoxsulam and molinate in paddy fields: field and lysimeter studies

BACKGROUND: In Chile, rice is cultivated under water‐seeded and continuously flooded conditions. Because herbicide dynamics in paddy fields and non‐flooded fields is different, 3 year experiments were performed to study the dissipation of molinate and penoxsulam in water and sediment. RESULTS: In field experiments, both herbicides dissipated by 45–55% from the initial applied amounts during the first 6 h after application in all crop seasons; in lysimeter experiments, dissipation amounts were approximately 10% for penoxsulam and 16% for molinate. Penoxsulam field water DT₅₀ values varied from 1.28 to 1.96 days during the three study seasons, and DT₉₀ values from 4.07 to 6.22 days. Molinate field water DT₅₀ values varied from 0.89 to 1.73 days, and DT₉₀ values from 2.82 to 5.48 days. Sediment residues were determined 2 days after herbicide application into the paddy water, and maximum concentrations were found 4–8 days after application. In sediment, DT₅₀ values varied from 20.20 to 27.66 days for penoxsulam and from 15.02 to 29.83 days for molinate. CONCLUSIONS: Results showed that penoxsulam and molinate losses under paddy conditions are dissipated rapidly from the water and then dissipate slowly from the sediment. Penoxsulam and molinate field water dissipation was facilitated by paddy water motion created by the wind. Sediment adsorption and degradation are considered to have a secondary effect on the dissipation of both herbicides in paddy fields. Copyright © 2011 Society of Chemical Industry     

The Ant Mosaic - Tropical Tree Crops and the Limiting of Pests and Diseases

Abstract

Field experiments were conducted to determine the effectiveness of herbicides in controlling Rottboellia cochinchinensis (Lour.) W. D. Clayton and Cyperus rotundus L. in upland rice (Oryza sativa L.) at different moisture regimes as imposed by a line source sprinkler system. Preemergence application of pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] was highly effective in controlling R. cochinchinensis irrespective of soil moisture after herbicide application. Bentazon [3‐(1‐methylethyl)‐(1H)‐2,1,3‐benzothiadiazin‐4(3H)‐one 2,2‐dioxide] and 2,4‐D [(2,4‐dichlorophenoxy)acetic acid] applied at postemergence effectively controlled C. rotundus when moisture supply was well above pan evaporation. These herbicides also had no adverse effect on rice stand and resulted in higher yield over the control. Water application rates above upland pan evaporation for a season‐long period was essential to obtain a high response to weed control either by herbicides or hand weeding. The data suggest that proper weed control by herbicides or hand weeding will not result in high upland rice grain yields if moisture level from rains fall below the critical level.     

Adsorption and bioactivity of di-allate,tri-allate and trifluralin*

Studies were conducled to cstmiate the adsorption parameter k and the bioactivity (in terms of G R₅₀) of di-allate [S-2,3-dichloroallyl N, N-di-isopropyl (thiocar bamate)], iri-allate [S-2,3,3-trichloroallyl N, N -di-isopropyl (thiocar bamate], and [trifiuralin (2,6-dinitro N, N-dlpropyl-4-trifluoromelhylaniline) in a number of Saskatchewan soil. The k values ratiged from 5 for di-allate adsorption m Asquilh loamy sand to 315 for trifluralin adsorption on Melfort loam and were closely related to the soil organic matter content. The relative degree of adsorption was irifluralin > tri-allate >di-allaie. For each herbicide, the G R₅₀ values were positively correlated wich organic matter conienl atid wilh k. It was suggested that these nonionic herbicides may be amenable to a predictive approach for field application rates in different soils. Among herbicides for any one soil, however, there was not the same relationship between G R₅₀andk. since the G R₅₀ was least for trifluralin and there was no significant difference between di-allate and tri-allate.     

The effect of initial concentration of carbofuran on the development and stability of its enhanced biodegradation in top‐soil and sub‐soil

Carbofuran was incubated in top‐soil and sub‐soil samples from a pesticide‐free site at a range of initial concentrations from 0.1 to 10 mg kg⁻¹. Amounts of the incubated soils were removed at intervals over the subsequent 12 months, and the rate of degradation of a second carbofuran dose at 10 mg kg⁻¹ was assessed. An applied concentration as low as 0.1 mg kg⁻¹ to top‐soil resulted in more rapid degradation of the fresh addition of carbofuran for at least 12 months. The degree of enhancement was generally more pronounced with the higher initial concentrations. When the same study was conducted in sub‐soil samples from the same site, an initial dose of carbofuran at 0.1 mg kg⁻¹ resulted in only small increases in rates of degradation of a second carbofuran dose. However, degradation rates in the sub‐soil samples were, in many instances, considerably greater than in the corresponding top‐soil samples, irrespective of pre‐treatment concentration or pre‐incubation period. Initial doses of 0.5 mg kg⁻¹ and higher applied to sub‐soil successfully activated the sub‐soil microflora. Application of the VARLEACH model to simulate carbofuran movement through the soil profile indicated that approximately 0.01 mg kg⁻¹ of carbofuran may reach a depth of 70 cm 400 days after a standard field application. The results therefore imply that adaptation of the sub‐soil microflora (c 1 m depth) by normal field rate applications of carbofuran is unlikely to occur. In experiments to investigate this in soils exposed to carbofuran in the field, there was no apparent relationship between top‐soil exposure and degradation rates in the corresponding sub‐soils. The results further confirmed that some sub‐soil samples have an inherent capacity for rapid biodegradation of carbofuran. The high levels of variability observed between replicates in some of the sub‐soil samples were attributed to the uneven distribution of a low population of carbofuran‐degrading micro‐organisms in sub‐surface soil. There was no apparent relationship between soil microbial biomass and degradation rates within or between top‐soil and sub‐soil samples. © 2001 Society of Chemical Industry     

DIFFERENTIAL PHYTOTOXICITY OF ATRAZINE AND AMETRYNE TO BANANAS*

Summary. In field screening trials for bananas (Musa acuminata var. Dwarf Cavendish) in Hawaii, ametryne (2-methylthio-4-ethylamino-6-isopropylamino-s-triazine) was less phytotoxic to bananas than atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine). Sand culture experiments showed that both herbicides were equally injurious to banana plants. Differential degradation of the herbicides by the plants did not account for the phytotoxicity observed. Both herbicides were partly metabolized by the plant to their common hydroxyl derivative (hydroxyatrazine) and two other unidentified metabolites after 3 and 7 days of exposure to nutrient solution containing ¹⁴C-labelled ametryne and atrazine. Phytotoxicity was directly related to leachability of the herbicides and negatively related to adsorption capacity of each soil for the herbicides. Organic matter content seemed to be correlated to the response observed. It was postulated that phytotoxicity in the field may have been attributed to differential location of the herbicide in relation to the roots.     

Residualwirkung von Chlortriazin-Herbiziden im Boden an drei rumänischen Standorten. I. Prognose der Persistenz von Simazin und Atrazin im Boden

Residual effects of chlorotriazine herbicides in soil at three Rumanian sites. I. Prediction of the persistence of simazine and atrazine Persistence of simazine and atrazine in the top 10 cm soil was measured at three sites in Rumania with variations in climate and soil conditions. Both herbicides were applied at 1 and 3 kg ai ha^?1 to uncropped plots and to plots cropped with maize (Zea mays L.). Rates of residue decline were independent of application rate and crop cover but varied between sites. The time for 50% loss of atrazine varied from 36 to 68 days and that of simazine from 48 to 70 days. Laboratory studies were made with atrazine to characterize degradation rates under standard conditions and to measure adsorption and leaching behaviour in the different soils. Weather records for the periods of the field experiments were used in conjunction with appropriate constants derived from the laboratory results, or from data in the literature, in a computer program to simulate persistence in the field. Results from the model were in reasonable agreement with the observed soil residues although there was a tendency to overestimate rates of loss on some occasions. The results suggest that the model of persistence was sufficiently accurate for practical purposes, and that its use could preclude the need for extensive analytical measurements of residues.     

The implications of spatially variable pre‐emergence herbicide efficacy for weed management

BACKGROUND

The efficacy of pre‐emergence herbicides within fields is spatially variable as a consequence of soil heterogeneity. We quantified the effect of soil organic matter on the efficacy of two pre‐emergence herbicides, flufenacet and pendimethalin, against Alopecurus myosuroides and investigated the implications of variation in organic matter for weed management using a crop–weed competition model.

RESULTS

Soil organic matter played a critical role in determining the level of control achieved. The high organic matter soil had more surviving weeds with higher biomass than the low organic matter soil. In the absence of competition, surviving plants recovered to produce the same amount of seed as if no herbicide had been applied. The competition model predicted that weeds surviving pre‐emergence herbicides could compensate for sublethal effects even when competing with the crop. The ED50 (median effective dose) was higher for weed seed production than seedling mortality or biomass. This difference was greatest on high organic matter soil.

CONCLUSION

These results show that the application rate of herbicides should be adjusted to account for within‐field variation in soil organic matter. The results from the modelling emphasised the importance of crop competition in limiting the capacity of weeds surviving pre‐emergence herbicides to compensate and replenish the seedbank. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

The herbicide flamprop-M-methyl has a new antimicrotubule mechanism of action

BACKGROUND: The herbicidal mode of action of flamprop‐M‐methyl [methyl N‐benzoyl‐N‐(3‐chloro‐4‐fluorophenyl)‐D ‐alaninate] was investigated. RESULTS: For initial characterization, a series of bioassays was used, which indicated a mode of action similar to that of mitotic disrupter herbicides. Cytochemical fluorescence studies, which included monoclonal antibodies against polymerized tubulin, were applied to elucidate effects on mitosis and microtubule assembly in maize roots. When seedlings were root treated with 50 µM of flamprop‐M‐methyl, cell division activity in meristematic root tip cells ceased within 4 h. The compound severely disturbed the orientation of spindle and phragmoblast microtubules, leading to defective spindle and phragmoblast structures. Cortical microtubules were only slightly affected. In late anaphase and early telophase cells, phragmoblast microtubules were disorganized in multiple arrays that hampered regular cell plate deposition in cytokinesis. Microtubules of the spindle apparatus were found attached to chromosomal kinetochores, but did not show regular organization associated with a zone of microtubule‐organizing centres at the opposite ends of the cell. On account of this loss of spindle organization, chromosomes remained in a condensed state of prometaphase or metaphase. Unlike known microtubule disrupter herbicides, flamprop‐M‐methyl and its biologically active metabolite flamprop did not inhibit soybean tubulin polymerization to microtubules in vitro at 50 µM . In contrast, soybean plants responded sensitively to the compounds. CONCLUSION: The results indicate that flamprop‐M‐methyl is a mitotic disrupter herbicide with a new antimicrotubule mechanism of action that affects orientation of spindle and phragmoblast microtubules, possibly by minus‐end microtubule disassembly. Copyright © 2008 Society of Chemical Industry     

Action of two herbicides on the microbial activity of soil cultivated with common bean (Phaseolus vulgaris) in conventional-till and no-till systems

The effects of application of the herbicides fluazifop‐p‐butyl and fomesafen and the commercial mixture of these herbicides on the microbial activity of a soil, cultivated with common bean under no‐till (NTS) and conventional‐till (CTS) systems, were evaluated. Microbial respiration was monitored for 63 days after application (DAA) of the herbicides, and the following evaluated at 12 and 51 DAA: microbial biomass carbon (MBC), microbial quotient (qMIC), metabolic quotient (qCO₂), percentage of bean root colonisation by mycorrhizal fungi and grain yield at the end of the cycle. A greater microbial respiratory rate was observed under NTS, with fluazifop‐p‐butyl providing the lowest respiration. At 12 DAA, MBC and qMIC were most affected negatively by fomesafen and by the commercial mixture of the two herbicides. Mycorrhizal colonisation was affected by the herbicides only at 12 DAA under CTS; however, in both periods, the highest value was found under NTS. All the herbicides caused a decrease in the MBC and qMIC values at 51 DAA; the qCO₂, which is related to the soil system stability, indicated a greater NTS balance over CTS. The herbicide fomesafen induced lower stability in the system. Lower grain yield was obtained without weed control (no herbicides) and with fomesafen‐only treatments, which may be attributed to the high weed infestation in the experimental area.     

Chemical hydrolysis of 2-chloro-4,6-bis(alkylamino)-1,3,5-triazine herbicides and their breakdown in soil under the influence of adsorption

The determination of rate constants and the calculation of the activation parameters [activation energy (E_a), free energy of activation(ΔG^≠)and entropy of activation (ΔS^≠)] demonstrated the identity of the reaction kinetics of chemical hydrolysis of the chlorinated triazine herbicides simazine, atrazine, propazine and terbuthylazine. Persistence in soil could be estimated, from the hydrolytic half-life time, only in pH regions where these compounds were also sensitive to chemical hydrolysis. In general, the rate of hydrolysis increased in the presence of soil as the result of a catalysing effect of the soil in their breakdown. When half-lives in soil of these triazine herbicides were compared with adsorption constants, a functional relationship was observed in both soil types; as adsorption increased the half-life in soil also increased.     

Absorption,translocation, and metabolism of ethalfluralin and trifluralin in Solanum spp

Seedlings of Solanum scabrum Mill. and Solanum ptycanthum Dun. were treated with [¹⁴C]ethalfluralin (N-ethyl-α,α,α-trifluoro-N-(methylallyl)-2,6-dinitro-p-toluidine) and [¹⁴C]trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) supplied in nutrient solution to determine the basis for differences in response by these two species to these two herbicides. Plants of S. scabrum absorbed more [¹⁴C]ethalfluralin and [¹⁴C]trifluralin than plants of S. ptycanthum. During the first 24 h, S. scabrum seedlings, but not S. ptycanthum seedlings absorbed more [¹⁴C]ethalfluralin than did plants treated with [¹⁴C]trifluralin. More [¹⁴C]ethalfluralin than [¹⁴C]trifluralin was found in the shoots of plants of both species. Seventy-two hours after treatment with [¹⁴C]herbicides, the conversion to water-soluble metabolites was greater for [¹⁴C]ethalfluralin than for [¹⁴C]trifluralin. In the shoots of plants from both species an average of nearly 55% of the ¹⁴C recovered was found in the water-soluble fraction following [¹⁴C]ethalfluralin treatment whereas an average of only 40% was found in the water-soluble fraction following [¹⁴C]trifluralin treatment.     

Movement of fluometuron, simazine, 36Cl− and 144Ce3+ in soil under field conditions: qualitative aspects

The distributions of two herbicides and two radioactive ions in field plots at two sites were determined at periods up to 187 days following surface applications in the spring. The results demonstrated the variability characteristic of field situations. At one site some fluometuron moved a short distance down the profile but after 187 days most remained above 6 cm whereas at the other site there was essentially no movement below 3 cm. At both sites simazine was almost entirely confined to the top 3 cm. The adsorption characteristics of the two compounds are similar so the greater mobility of fluometuron is probably a consequence of its greater solubility, Measurements of ³⁶Cl^? indicated a significant movement of water through the 30 cm depth studied. ¹⁴⁴Ce³⁺ used as a tracer of soil particles was of similar mobility to simazine. In general the movement of chloride and the two herbicides can be interpreted in terms of the concept of mobile and immobile fractions of soil water in which a proportion of the mobile water does not reach equilibrium with solutes in the bulk of the soil. Soil structural effects may therefore be more important than adsorption in controlling the movement of solutes and redistribution in association with soil particles can be significant. A parallel laboratory experiment showed that the results from a standard leaching column procedure did not necessarily indicate field performance.     

Soil persistence studies with bromoxynil, propanil and [14C]dicamba in herbicidal mixtures

The persistence of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), [¹⁴C]dicamba (3,6-dichloro-2-methoxybenzoic-7-¹⁴C acid) and propanil [N-(3,4-dichlorophenyl)propionamide] at rates equivalent to 1 kg ha^?1, were studied under laboratory conditions in a clay loam, a heavy clay and a sandy loam at 85% of field capacity and at 20±1°C, both singly and in the presence of herbicides normally applied with these chemicals as tank-mix or split-mix components. The degradation of bromoxynil was rapid with over 90% breakdown occurring within a week in the heavy clay and sandy-loam soils, while in the clay-loam approximately 80% of the bromoxynil had broken down after 7 days. In all three soils degradation was unaffected by the presence of asulam, diclofop-methyl, flamprop-methyl, MCPA, metribuzin or propanil. Propanil underwent rapid degradation in all soil treatments, with over 95% of the applied propanil being dissipated within 7 days. There were no noticeable effects on propanil degradation resulting from applications of asulam, barban, bromoxynil, dicamba, MCPA, MCPB, metribuzin or 2,4-D. The breakdown of [¹⁴C]dicamba in a particular soil was unaffected by being applied alone or in the presence of diclofop-methyl, flampropmethyl, MCPA, metribuzin, propanil or 2,4-D. The times for 50% of the applied dicamba to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50 days in the heavy clay.     

Enhanced Mineralization of [14C]Atrazine in Kochia scoparia Rhizospheric Soil from a Pesticide-Contaminated Site

Mineralization of atrazine (6-chloro-N²-ethyl-N⁴-isopropyl-1,3,5- triazine-2,4-diamine) in soil treated with a mixture of atrazine and metolachlor (2-chloro-6′-ethyl-N-(2-methoxy-1-methylethyl)acet-o-toluidide at concentrations typical of point-source contamination (50 μg g⁻¹ each) was significantly greater (P<0·001) in rhizospheric soil from Kochia scoparia (L.) Roth., a herbicide-resistant plant, than in non-vegetated and control soils. Soils were collected from an agrochemical dealership contaminated with several herbicides, including atra-zine, metolachlor, trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine and pendimethalin (N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidene), at concentrations well exceeding the field application rates. Mineralization rates of ring-labeled atrazine in both rhizospheric and non-vegetated soils were quite high (>47% of the initial ¹⁴C applied after 36 days) compared to literature values. These results suggest that plants such as Kochia might be managed at pesticide-contaminated sites to help facilitate microbial degradation of wastes such as atrazine in soil.     

Comparative effects of three isoxazole-urea herbicidal derivatives on the metabolism of enzymatically isolated soybean leaf cells*

The effects of the herbicide isouron and of its plant degradation products designated as metabolite l {N-[5-(1,1-dimethylethyl)-3-isoxazolyl]-N-methylurea} and metabolite 2 {N-[5-(1,1-dimethylethyl)-3-isoxazolyl]-urea} on the metabolism of enzymatically isolated leaf cells of soybean [Glycine max (L.) Merr., cv. Essex] were compared under laboratory conditions. Photosynthesis, protein synthesis, ribonucleic acid synthesis, and lipid synthesis were assayed by the incorporation of NaH¹⁴CO₃, [¹⁴C]-leucine, [¹⁴C]-uracil, and [¹⁴C]-acetate, respectively, into the isolated cells. Time-course and concentration studies included incubation periods of 30, 60, and 120 min and concentrations of 0.1, 1, 10 and 100 μM of the three herbicides. The urea derivative of isouron (metabolite 2) was the least active of the three compounds. The activity of the mono-methylated derivative of isouron (metabolite 1) was comparable to that of isouron and the sensitivity of the four processes to both chemicals decreased in the order: photosynthesis > ribonucleic acid synthesis > lipid synthesis > protein synthesis. The concentration of isouron that caused a 50% inhibition of photosynthesis of the isolated soybean leaf cells was calculated at 0.51 μM. The effects of isouron and metabolite 1 on photosynthesis, lipid and RNA synthesis appeared to be independent of incubation lime as maximal inhibition occurred within 30 min. Inhibition of protein synthesis by both chemicals was time-dependent, increasing in magnitude with concomitant increases in incubation time.