Determination of sugarbeet herbicides in soil samples by hplc

Determination of sugarbeet herbicides such as chloridazon, metamitron and phenmedipham in soil samples is described. After extraction with acetone, pesticides were determined by HPLC on an RP-18 column using methanol/water as mobile phase. Average recoveries were 82% for chloridazon, 93% for metamitron and 77% for phenmedipham. Quantification limits were 3·5 μg kg^?1 for chloridazon, 6·3 μg kg^?1 for metamitron and 3·6 μg kg^?1 for phenmedipham.     

Influence of different field sites on pesticide movement into subsurface drains

Pesticide movement to subsurface drains was monitored in two typical crop production areas in Germany. Field trials were conducted on two subsurfacedrained soils, a silt loam and a poorly structured sandy soil, under different climatic conditions. Over a period of one year, the drainflow was measured and the drain water was analysed for all applied herbicides. Different leaching behaviour was observed at the two field sites. Following autumn application of pendimethalin and isoproturon to the Soester Börde soil, maximum concentrations of about 62 μg litre^?1 for isoproturon and 0.7 μg litre^?1 for pendimethalin were observed in drainflow from this silt loam. The early occurrence of both herbicides in the drain water only two days after application is consistent with fast flow through macropores. In contrast, on the subsurfacedrained sandy soil in Brandenburg, isoproturon did not reach the drains until two months after autumn application and was found at maximum concentrations of only 1.4 μg litre^?1; pendimethalin was not detected in the drain water. Pesticide movement after spring application seemed to be of minor importance. At both locations, spring application led to low concentrations of pesticides in the drainflow (pendimethalin < 0.01 μ litre^?1; metolachlor ? 0.05 μ litre^?1; chloridazon ? 0.15 μ litre^?1; metamitron ? 0.02 μg litre^?1; terbuthylazine ? 1.4 μ litre^?1).     

Measurement and simulation of the movement of thiazafluron, clopyralid and metamitron in soil columns

Adsorption and leaching of the herbicides thiazafluron (1,3-dimethyl-1(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)urea), metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-tri azin-5-one) and clopyralid (3,6-dichloropicolinic acid) were studied in one sandy and two silty-clay soils. Equilibrium adsorption coefficients (K_d) were measured using a batch equilibration procedure, and mobility was studied in repacked columns of the soils under fluctuating saturated/unsaturated flow conditions. Breakthrough curves (BTCs) were consistent with an inverse relationship between leaching and adsorption with greater mobility of the weakly-adsorbed clopyralid than the more strongly adsorbed thiazafluron or metamitron. The BTC data were used to evaluate the LEACHP simulation model. Following model calibration with respect to hydrological parameters and some of the herbicide degradation rates, the best fits between predicted and observed data were with the less adsorptive and highly mobile clopyralid. In general, the model gave acceptable predictions of the timing of the concentration maxima and the shapes of the BTCs, although earlier breakthrough than that observed was predicted with the less mobile herbicides, thiazafluron and metamitron, in the silty-clay soils. For metamitron, the total amounts leached were not predicted accurately, suggesting more rapid degradation of the herbicide in the soil columns than in the kinetic studies performed in a 1:1 soil:solution ratio shaken system.     

Effects of certain herbicides on the fate of sporangiospores of Mucor piriformis and conidia of Botrytis cinerea and Penicillium expansum

The effects of several herbicides used in pome fruit orchards on the germination of spores and growth of mycelia of Botrytis cinerea, Mucor piriformis and Penicillium expansum in vitro and the survival of propagules of these fungi in soil were studied. Diuron in agar at 4–128 μg ml^?1 reduced germination of spores of B. cinerea and M. piriformis, and 2,4-D and paraquat at 32 μg ml^?1 similarly affected B. cinerea and P. expansum. Several herbicides at 128 μg ml^?1 in agar reduced growth of B. cinerea and M. piriformis but were ineffective against P. expansum. Propagule survival levels of the three fungi generally were lower in both autoclaved and non-autoclaved soil amended with herbicides than in non-amended soil. This effect was greatest in non-autoclaved soil, suggesting involvement of microbial antagonists. The most effective herbicides for reduction of fungal propagules in soil were 2,4-D, diuron, and paraquat.     

The effect of root-absorbed PSII inhibitors on Kautsky curve parameters in sugar beet

The effects of the photosystem II inhibitors metamitron and terbuthylazine on the shape of the Kautsky (chlorophyll fluorescence induction) curve were investigated in sugar beet grown in hydroponic culture. The objective of the study was to trace recovery processes following herbicide injury using Kautsky curve parameters. Metamitron is used for selective weed control in sugar beet because it is metabolized in this crop. In contrast, terbuthylazine is toxic to sugar beet. Two hours after treatment, various fluorescence induction curve parameters, such as maximum quantum efficiency (F_V/F_m), the relative changes at the J step (F_vj) and area (the area between the Kautsky curve and maximum fluorescence, F_m), were affected by metamitron at concentration ranges of 70–280 mg active ingredient (a.i.) L^?1 in plants treated at the four‐true‐leaf stage. Shortly after herbicide application, F_v/F_m was more affected by the hydrophilic metamitron [log(K_ow) = 0.83] than by the lipophilic terbuthylazine [log(K_ow) = 3.21], but these differences between compounds were alleviated as metamitron was metabolized and terbuthylazine was not. Terbuthylazine at 1 mg a.i. L^?1 affected sugar beet at the four‐ and six‐true‐leaf stages to the same extent, whereas metamitron at a dose of 140 mg a.i. L^?1 affected much more at four‐ than at the six‐true‐leaf stage. Sugar beet recovered from metamitron injury even at high doses (140 and 280 mg a.i. L^?1). Fluorescence induction curve parameters were similarly affected by terbuthylazine and, although sugar beet recovered from terbuthylazine injury at low doses (<0.2 mg a.i. L^?1), the Kautsky curve was irreversibly affected at higher doses (1–10 mg a.i. L^?1), leading finally to plant death. Older plants were affected later, and recovered sooner, from both herbicides.     

Joint action of some root-absorbed herbicides in oats (Avena sativa L.)

In growth chambers the phytotoxicity of binary mixtures of four herbicides was compared using an Additive Dose Model as reference. Of the four herbicides used, lenacil was the most potent whereas ethofumesate was the least potent herbicide; chloridazon and metamitron were equally potent. The Additive Dose Model implicitly presupposes that at any one response level the herbicides of a mixture can replace each other in proportion to their relative potency when applied separately. Metamitron and chloridazon mixtures appeared to follow the reference model. The efficacy of mixtures of lenacil and chloridazon or lenacil and metamitron, however, were increased in comparison with the herbicides applied separately. On the other hand, lenacil and ethofumesate or metamitron and ethofumesate were less phytotoxic than expected from the Additive Dose Model. The results are discussed in relation to the mode of action of the compounds. Action combinée de certains herbicides absorbés par les racines en culture d'avoine Avena sativa L. La phytotoxicité de mélanges binaires de quatre herbicides a fait l'objet d'une comparaison en phytotron, utilisant comme témoin un Modèle Dose Additive. Parmi les quatre herbicides utilisés, le lénacile s'est révélé le plus et l'éthofumésate le moins actif; le chloridazone et le métamitrone ont fait preuve d'une activitéégale. Le Modèle Dose Additive présuppose qu'à tout niveau d'activité chaque herbicide faisant partie d'un mélange puisse se substituer à l'autre en fonction de son efficacité relative en traitement simple. Les mélanges métamitrone–chloridazone semblent s'accorder au modèle de référence. Cependant, l'efficacité des mélanges chloridazone–lénacile et lénacile–métamitrone a augmenté par rapport à l'application de chaque élément séparément. D'autre part, les mélanges lénacile–éthofumesate et métamitrone–éthofumesate ont fait preuve d'une phytotoxicité moindre que celle qu'annonçait le modèle. Ces résultats sont discutés par rapport au mode d'action des composés. Kombinierte Wirkung verschiedener wurzelwirksamer Herbizide auf Avena sativa L. In Klimakammern wurde die Phytotoxizität von Zweier-Mischungen von vier Herbiziden untersucht und mit einem Modell additiver Dosierungen verglichen. Von den vier untersuchten Herbiziden war Lenacil das wirksamste und Ethofumesat das schwächste; Chloridazon und Metamitron wirkten gleich stark. Das Modell additiver Dosierungen setzt stillschweigend voraus, dass auf jedem Wirkungsniveau die Herbizide einer Mischung einander im Verhältnis ihrer relativen Wirksamkeiten, wenn allein appliziert, ersetzen können. Metamitron + Chloridazon Mischungen scheinen dem Vergleichsmodell zu folgen. Die Wirkungen der Mischungen von Lenacil + Chloridazon oder Lenacil + Metamitron, hingegen, waren im Vergleich zu den Effekten der einzeln applizierten Komponenten gesteigert. Andrerseits waren Lenacil + Ethofumesat oder Metamitron + Ethofumesat weniger phytotoxisch, als das Modell hätte erwarten lassen. Die Resultate werden mit Bezug auf die Wirkungsmechanismen der Komponenten diskutiert.     

Differential sensitivity of Atriplex patula and Chenopodium album to sugar beet herbicides: a possible cause for the upsurge of A. patula in sugar beet fields

In the last decade, the prevalence of Atriplex patula as a weed in the Belgian sugar beet area has increased. Possible reasons for its expansion in sugar beet fields, besides a poor implementation of the low‐dose phenmedipham/activator/soil‐acting herbicide (FAR) system, might be low sensitivity or evolved resistance to one or more herbicides used in sugar beet. Dose – response pot bioassays were conducted in the glasshouse to evaluate the effectiveness of five foliar‐applied sugar beet herbicides (metamitron, phenmedipham, desmedipham, ethofumesate and triallate) and three pre‐plant‐incorporated herbicides (metamitron, lenacil, dimethenamid‐P) for controlling five Belgian A. patula populations. Local metamitron‐susceptible and metamitron‐resistant populations of Chenopodium album were used as reference populations. Effective dosages and resistance indices were calculated. DNA sequence analysis of the photosystem II psbA gene was performed on putative resistant A. patula populations. Overall, A. patula exhibited large intraspecific variation in herbicide sensitivity. In general, A. patula populations were less susceptible to phenmedipham, desmedipham, ethofumesate and triallate relative to C. album populations. Two A. patula populations bear the leucine‐218 to valine mutation on the chloroplast psbA gene conferring low level to high level cross‐resistance to the photosystem II inhibitors phenmedipham, desmedipham, metamitron and lenacil. In order to avoid insufficient A. patula control and further spread, seedlings should preferentially be treated with FAR mixtures containing higher‐than‐standard doses of metamitron and phenmedipham/desmedipham and no later than the cotyledon stage.     

Residualwirkung von Chlortriazin-Herbiziden im Boden an drei rumänischen Standorten. II. Prognose möglicher Folgekulturen bei Atrazinrükständen im Boden

Residual effects of chlorotriazine herbicides in soil at three Rumanian sites. II. Prediction of the phytotoxicity of atrazine residues to following crops Total and plant-available atrazine residues in the top 10 cm soil were measured 120 days after application of 3 kg ai ha^?1 to maize (Zea mays L.) at three sites in Rumania. At one site, similar measurements were made 3?5 years after application of 100 kg ai ha^?1. Plant-available atrazine residues were estimated by extraction of soil samples with water, and by bioassay using Brassica rapa as the test plant. It was calculated that between 30 and 120μg atrazine 1^?1 was potentially available to plants in the different soils. Dose-response relationships for atrazine and the most important rotational crops with maize in Rumania—sunflower, winter wheat, soybean and flax—were determined in hydroponic culture using herbicide concentrations corresponding with the plant-available fractions measured in the different soils. ED₅₀ values were determined by probit analysis and the results showed that sunflower (ED₅₀, 22μg 1^?1) was the most sensitive crop, and soybean (ED₅₀, 78μg 1^?1) was the least. The residual phytotoxicity of atrazine to succeeding crops in the different soils was predicted using the appropriate availability and phytotoxicity data, and the results showed good agreement with those observed. The results suggest that measurements of plant-available herbicide residues afford a rapid method of assessing possible phytotoxicity to following crops.     

Orobanche ramosa L. (broomrape) control in tomato (Lycopersicon esculentum Mill.) with chlorsulfuron, glyphosate and imazaquin

Chlorsulfuron, giyphosate and imazaquin were evaluated in pot and field studies for their efficacy in controlling broomrape (Orobanche ramosa L.) in tomato (Lycopersicon esculentum Mill.) in Northern Greece. All herbicides were applied four to five weeks after tomato transplanting, when the crop was at early flowering stage and broomrape had started to develop underground attachments. The number of emerged broomrape shoots and underground attachments were less affected by herbicide treatments than the dry weight, suggesting that the herbicides suppress the growth of broomrape rather than kill its underground organs. In the pot experiments, chlorsulfuron applied at 5 g AI ha^?1 was the most effective treatment for broomrape control and the least toxic to the crop. Imazaquin and glyphosate applied at 37 and 180 g AI ha^?1, respectively, controlled broomrape but imazaquin reduced crop yield. In the field, similar rates of glyphosate and higher rates of imazaquin were not toxic to the crop but were less effective on broomrape. Chlorsulfuron applied at 10 g AI ha^?1 controlled broomrape emergence by 88%. When the herbicide was applied twice (5+10 g AI ha^?1), it gave complete control of broomrape but delayed crop maturity. The yield of tomato was not increased as a result of these treatments because of low broomrape infestation and a short competition period.     

The role of ferrous ions in the rapid degradation of oxamyl,methomyl and aldicarb in anaerobic soils

The carbamoyloxime pesticides methomyl, oxamyl and aldicarb, together with the oxidation products of aldicarb, are known to break down much more rapidly in certain anaerobic subsoils than in the aerobic topsoils from the same site. Ferrous ions have now been shown to be involved in this reaction. Oxamyl was degraded in aqueous solutions at 30°C containing 250 μg ml^?1 Fe²⁺ with a half-life of about 10 h, independent of pH in the range of 5.65–7.66; the observed products of this reaction were N,N-dimethyl-l-cyanoformamide and methanethiol. These same products, rather than the oximino hydrolysis product observed from degradation in aerobic soils, were rapidly and quantitatively formed from oxamyl in suspensions of anaerobic reduced subsoils (Fe²⁺ concentration 27–41 μg ml^?1 soil water), but oxamyl was rather stable in water-saturated Vredepeel subsoil (Fe²⁺ concentration 0.65 μg ml^?1) in which the redox potential was much higher. Methomyl behaved similarly. The rates of reaction in the suspensions of anaerobic subsoils were greater than expected from the concentrations of Fe²⁺ in the soil water, but most of the Fe²⁺ present in soil was bound to the soil particles by cation exchange and this bound Fe²⁺ may have participated. Breakdown of aldicarb was accelerated both in solutions of Fe²⁺ and in the suspensions of anaerobic reduced subsoils, though the rate enhancement was less than observed with methomyl and oxamyl; 2-methyl-2-methylthiopropionitrile and 2-methyl-2-methylthiopropionaldehyde were the observed products from aldicarb in anaerobic soil but only the former was produced in Fe²⁺ solutions; the corresponding nitriles and aldehydes were also yielded by aldicarb sulphoxide and aldicarb sulphone in the anaerobic, reduced subsoils.     

Behaviour of metamitron and hydroxy-chlorothalonil in low-humic sandy soils

The behaviour of the herbicide metamitron and of the main transformation product, hydroxy-chlorothalonil (HTI), of the fungicide chlorothalonil was studied to assess the risk of leaching from low-humic sandy soil. The adsorption of metamitron corresponded to a K_om value of about 60 dm³ kg⁻¹ (moderate adsorption). The half-life of metamitron in soil at 15 °C was only three days, presumably due to adaptation of the micro-organisms. In the autumn, the residue of metamitron in the soil profiles corresponded to less than 1% of the cumulative dosage. The half-life of chlorothalonil at 15 °C was about 12 days and about 45% of it was transformed to HTI. The adsorption of HTI to the soils corresponded to a K_om value of 260 dm³ kg⁻¹. The incubation study (15 °C) showed the transformation of HTI in the soils to be very slow. The amounts of HTI remaining in the soil profiles in the autumn corresponded to 4 and 16% of the cumulative dosage of chlorothalonil. In winter, the HTI residue decreased by 40% relative to the autumn level. Occasionally, HTI could be detected in the upper ground-water level (at a depth of about 1 m), at an average concentration of 0.1 to 0.2 µg dm⁻³. © 1999 Society of Chemical Industry     

Fate of the herbicides glyphosate, glufosinate-ammonium, phenmedipham, ethofumesate and metamitron in two Finnish arable soils

The fate of five herbicides (glyphosate, glufosinate-ammonium, phenmedipham, ethofumesate and metamitron) was studied in two Finnish sugar beet fields for 26 months. Soil types were sandy loam and clay. Two different herbicide-tolerant sugar beet cultivars and three different herbicide application schedules were used. Meteorological data were collected throughout the study and soil properties were thoroughly analysed. An extensive data set of herbicide residue concentrations in soil was collected. Five different soil depths were sampled. The study was carried out using common Finnish agricultural practices and represents typical sugar beet cultivation conditions in Finland. The overall observed order of persistence was ethofumesate > glyphosate > phenmedipham > metamitron > glufosinate-ammonium. Only ethofumesate and glyphosate persisted until the subsequent spring. Seasonal variation in herbicide dissipation was very high and dissipation ceased almost completely during winter. During the 2 year experiment no indication of potential groundwater pollution risk was obtained, but herbicides may cause surface water pollution.     

A new molecular method for the rapid detection of a metamitron‐resistant target site in Chenopodium album

BACKGROUND: Resistance to photosystem II inhibitors—triazines (atrazine) and triazinones (metamitron, metribuzin)—in Chenopodium album L. is caused by the serine 264 to glycine mutation in the D1 protein. This mutation has been detected in C. album collections from Belgium with unsatisfactory metamitron efficacy in the field and was confirmed in greenhouse resistance bioassays. Incomplete herbicide efficacy in practice can also be caused by reduced uptake due to environmental conditions. Hence, for reliable differentiation and resistance identification, a rapid method for mutation detection in the target gene psbA is required. RESULTS: Dose–response curves obtained in herbicide greenhouse assays with metamitron‐resistant and ‐susceptible reference biotypes showed that a dose of 2 L ha^?1 metamitron was suitable for discrimination. A psbA PCR‐RFLP was developed, based on the presence of a FspBI restriction enzyme recognition site, covering D1 codon 264 in susceptible genotypes. A paper‐based DNA extraction allowed direct processing of leaf samples already in the field. In order to detect the mutation even in mixed seed samples, a nested PCR‐RFLP was also developed. CONCLUSION: The method allows exhaustive surveys screening C. album leaf or seed samples for the occurrence of the D1 Ser264Gly mutation to confirm or disprove metamitron resistance in the case of unsatisfactory control. Copyright © 2010 Society of Chemical Industry     

Environmental fate of herbicides trifluralin, metazachlor, metamitron and sulcotrione compared with that of glyphosate, a substitute broad spectrum herbicide for different glyphosate-resistant crops

The introduction of crops resistant to the broad spectrum herbicide glyphosate, N-(phosphonomethyl)glycine, may constitute an answer to increased contamination of the environment by herbicides, since it should reduce the total amount of herbicide needed and the number of active ingredients. However, there are few published data comparing the fate of glyphosate in the environment, particularly in soil, with that of substitute herbicides. The objective of this study is to compare the fate of glyphosate in three soils with that of four herbicides frequently used on crops that might be glyphosate resistant: trifluralin, alpha,alpha,alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine, and metazachlor, 2-chloro-N-(pyrazol-1-ylmethyl)acet-2',6'-xylidide for oilseed rape, metamitron, 4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one for sugarbeet and sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione for maize. The distribution of herbicides between the volatilized, mineralized, extractable and non-extractable fractions was studied, along with the formation of their metabolites in laboratory experiments using 14C-labelled herbicides, over a period of 140 days. The main dissipation pathways were mineralization for glyphosate and sulcotrione, volatilization for trifluralin and non-extractable residues formation for metazachlor and metamitron. The five herbicides had low persistence. Glyphosate had the shortest half-life, which varied with soil type, whereas trifluralin had the longest. The half-lives of metazachlor and sulcotrione were comparable, whereas that of metamitron was highly variable. Glyphosate, metazachlor and sulcotrione were degraded into persistent metabolites. Low amounts of trifluralin and metamitron metabolites were observed. At 140 days after herbicide applications, the amounts of glyphosate and its metabolite residues in soils were the lowest in two soils, but not in the third soil, a loamy sand with low pH. The environmental advantage in using glyphosate due to its rapid degradation is counterbalanced by accumulation of aminomethylphosphonic acid specifically in the context of extensive use of glyphosate.     

An evaluation of the herbicidal and plant growth regulatory activity of a novel class of carbohydrate-derived 6,8-dioxabicyclo[3.2.1]octanes

Thirty-five carbohydrate-derived dioxabicyclo[3.2.l]octane derivatives have been tested for herbicidal and plant growth regulatory (PGR) activity in an Arabidopsis thaliana assay. Nine of these were herbicidal at concentrations of ? 16 μg ml^?1 in the growth medium. Three compounds, viz, 1,6-anhydro-3-deoxy-4-O-(2,6-dichlorobenzyl)-2-O-methyl-β-D-ribo-hexopyranose(I), its 4-O-benzyl analogue (II), and l,6-anhydro-2-azido-4-O-benzyl-2,3-dideoxy-β-D-ribo-hexopyranose (III) were very active herbicides, killing plants by the rosette stage at 1 μg ml^?1 or less. At lower rates, the herbicides acted as growth regulators, reducing growth rates of shoots and roots as well as flowering and seed-pod development. A further 14 compounds exerted PGR effects only, while 12 compounds were virtually inactive at 16 μg ml^?1, the highest rate tested. The ability of the active compounds to reduce seed viability on mature plants at sublethal concentrations was demonstrated in four cases.     

Inhibition of photosynthesis in intact plants of biotypes resistant or susceptible to atrazine and cross-resistance to other herbicides

A concentration of atrazine of 0?1 mm 1^?1 in the nutrient solution resulted in complete inhibition of photosynthesis in intact leaves of susceptible biotypes of Amaranthus retroflexus L., Polygonum lapathifolium L., Chenopodium album L., Solanum nigrum L., Poa annua L. and Stellaria media (L.) Vill. within a few hours of treatment, whereas the inhibition of the resistant biotypes by the same concentration varied from small to moderate. In contrast, diuron (10 or 20 μm 1^?1) produced only minor differences between resistant and susceptible biotypes. The influence of some other herbicides on photosynthesis of these resistant biotypes was also smaller than that on the susceptible biotypes. This cross-resistance was evaluated with the resistance factor for intact leaves of Brassica napus L., A. retroflexus, and S. nigrum. This factor is equal to the ratio of the herbicide concentration in leaves of the resistant biotype to that in leaves of the susceptible biotype with inhibition to half-maximum rate of photosynthesis. This concentration in the leaves was calculated from the concentration in the nutrient solution, and the total transpiration divided by leaf area from the beginning of the herbicide treatment until the moment of half-maximum of photosynthesis. The resistance factors for intact leaves of A. retroflexus, S. nigrum and B. napus were 26–30 for atraton, 3–7 for metamitron, 2–9 for bromacil, 3–5 for monolinuron, 1 for diuron and < 1 for bentazone. For isolated chloroplasts much higher values have been reported. The reason for this discrepancy is not clear. A somewhat higher resistance factor (around 50–60) was derived after infiltration of detached leaves of these species with atraton solutions.     

Use of activated carbon to protect tomato against metribuzin

Protection of tomato (Lycopersicon esculentum Mill., cv. UC 134) against metribuzin was increased by placing a small quantity of activated carbon near each seed position before pre-emergent application of the herbicide. The most protective location of the carbon was the surface of the soil, immediately above the seed. In this location, 0.10 g of activated carbon protected tomatoes from the phytotoxic effects of 0.700 kg metribuzin ha^?1 whereas, without the activated carbon, 0.175 kg metribuzin ha^?1 killed both tomato and blackberry nightshade (Solanum nigrum L.). Placing plastic discs in this location did not reduce the phytotoxicity; however, a barrier that prevented lateral invasion by the herbicide, of the zone of soil through which the plumule emerged, partially protected the plains from 0.350 kg metribuzin ha^?1, showing that metribuzin entered the plants in substantial quantities via the plumule as well as the roots. This result suggests that activated carbon maintained a lower metribuzin concentration in the plumule/root zone than the bulk concentration. It also explains way the soil surface was the most protective carbon location. These findings provide the basis for extending the use of preemergent herbicides to situations in which previously they could not be used safely or economically.     

Response of Brassica napus in tissue culture to metsulfuron-methyl and chlorsulfuron

A herbicide bioassay based on tissue cultures of Brassica napus L. was evaluated with selected sulfonylurea herbicides. Data were analysed by fitting the results to a log-logistic dose–response model. Within an experiment, the non-linear regression models were fitted simultaneously to the individual dose–response curves. The results obtained showed good response to even low concentrations of herbicide, with detection limits in the range 0.008–0.69 nmol L^?1 for chlorsulfuron and 0.02–0.13 nmol L^?1 for metsulfuron. The reproducibility of the assays, on the basis of coefficient of variation of the ED₅₀ values, was found to be 44% for chlorsulfuron and 48% for metsulfuron measurements. Assay of herbicide dissolved in aqueous soil extract showed significant interference from this matrix on the response, requiring a five times dilution of the extract to overcome this matrix effect.     

Retention and degradation of metribuzin in sandy loam and clay soils of Lebanon

The retention and degradation of metribuzin herbicide were studied under two environmental conditions. Field studies were carried out on two soils, a sandy loam soil (soil A) and a clay soil (soil B). Metribuzin was applied with a jet sprayer at 1060 g a.i. ha^?1 and 1960 g a.i. ha^?1 on soils A and B respectively. Reconstituted soil columns were used to study the herbicide movement and metabolism in the two soils. Analyses of metribuzin and its metabolites were carried out using standardized methods. The results indicated a very weak capacity of adsorption of metribuzin in the two soils, and the weak adsorbed fraction is easily desorbed. Degradation and mobility of metribuzin in the field and laboratory soil columns were very intense and rapid. Soil A favoured reductive deamination whereas soil B favoured oxidative desulphuration and the respective metabolites deaminometribuzin and diketometribuzin yield the same product deaminodiketometribuzin. Both leaching by rainfall and degradation were important in the disappearance of metribuzin from the soils.     

Some physicochemical factors influencing foliar uptake enhancement of glyphosatemono(isopropylammonium) by polyoxyethylene surfactants

Structure-concentration–foliar uptake enhancement relationships between commercial polyoxyethylene primary aliphatic alcohol (A), nonylphenol (NP), primary aliphatic amine (AM) surfactants and the herbicide glyphosatemono(isopropylammonium) were studied in experiments with wheat (Triticum aestivum L.) and field bean (Vicia faba L.) plants growing under controlled-environment conditions. Candidate surfactants had mean molar ethylene oxide (EO) contents ranging from 5 to 20 and were added at concentrations varying from 0·2 to 10 g litre^?-1 to [¹⁴C]glyphosate formulations in acetone–water. Rates and total amounts of herbicide uptake from c. 0·2–μl droplet applications of formulations to leaves were influenced by surfactant EO content, surfactant hydrophobe composition, surfactant concentration, glyphosate concentration and plant species, in a complex manner. Surfactant effects were most pronounced at 0·5 g acid equivalent (a.e.) glyphosate litre^?-1 where, for both target species, surfactants of high EO content (15–20) were most effective at enhancing herbicide uptake: surfactants of lower EO content (5–10) frequently reduced, or failed to improve, glyphosate absorption. Whereas, at optimal EO content, AM surfactants caused greatest uptake enhancement on wheat, A surfactants gave the best overall performance on field bean; NP surfactants were generally the least efficient class of adjuvants on both species. Threshold concentrations of surfactants needed to increase glyphosate uptake were much higher in field bean than wheat (c. 2 g litre^?-1 and < 1 g litre^?-1, respectively); less herbicide was taken up by both species at high AM surfactant concentrations. At 5 and 10 g a.e. glyphosate litre^?-1, there were substantial increases in herbicide absorption and surfactant addition could cause effects on uptake that were different from those observed at lower herbicide doses. In particular, the influence of EO content on glyphosate uptake was now much less marked in both species, especially with AM surfactants. The fundamental importance of glyphosate concentration for its uptake was further emphasised by experiments using formulations with constant a.i./surfactant weight ratios. Any increased foliar penetration resulting from inclusion of surfactants in 0·5 g litre^?-1 [¹⁴C]glyphosate formulations gave concomitant increases in the amounts of radiolabel that were translocated away from the site of application. At these low herbicide doses, translocation of absorbed [¹⁴C]glyphosate in wheat was c. twice that in field bean; surfactant addition to the formulation did not increase the proportion transported in wheat but substantially enhanced it in field bean.