Studies into the differential activity of the hydroxybenzonitrile herbicides: I. Photosynthetic inhibition,symptom development,and ultrastructural changes in two constrasting species

Symptom development in Matricaria inodora and Viola arvensis is compared following field rate application of ioxynil and bromoxynil salts. M. inodora is moderately susceptible to ioxynil-Na and susceptible to bromoxynil-K, whereas V. arvensis is moderately resistant to ioxynil-Na and resistant to bromoxynil-K. Levels of protein, amino acid, and reducing sugars reflected the susceptibility of M. inodora and the transient growth cessation of V. arvensis after hydroxybenzonitrile treatment. Rapid development of chlorotic and necrotic symptoms in M. inodora corresponded with complete inhibition of CO₂ fixation within 4 days of treatment. Chloroplasts were swollen, and, after 7 days, ioxynil-Na induced ultrastructural changes typical of photooxidative processes including thylakoid swelling and vesicle formation in the chloroplast. Bromoxynil-K treated mesophyll cells exhibited general cellular disruption including chloroplast swelling and plasmalemma and tonoplast rupture. In contrast, CO₂ fixation remained at 35% of control levels in bromoxynil-K-treated V. arvensis. Chloroplast grana contained twice the number of thylakoids of untreated chloroplasts and there was a small reduction in the chlorophyll a:b ratio of treated foliage. Ioxynil-Na induced a similar response with additional chloroplast swelling reflecting the greater inhibition of CO₂ fixation by this herbicide.     

In vitro activity and binding characteristics of the hydroxybenzonitriles in chloroplasts isolated from Matricaria inodora and Viola arvensis

The electron transport inhibition, uncoupling, and binding of ioxynil and bromoxynil salts is compared in chloroplast fragments isolated from two weed species with contrasting responses to the hydroxybenzonitriles. Ioxynil Na was three to four times more inhibitory than bromoxynil K towards DCPIP and SiMo reduction in both Matricaria inodora and Viola arvensis. Ioxynil Na was also a more potent uncoupler of PSI-dependent electron transport from ascorbate/DCPIP to methyl viologen. Uncoupling occurred at concentrations higher than those that inhibited electron transport. Binding studies with [¹⁴C]bromoxynil K and [¹⁴C]ioxynil Na salts revealed slightly biphasic curves with no significant difference in the amounts of the two herbicides bound at a given concentration. The ratios of inhibition constant (K_i) and binding constant (K_b) were approximately one for ioxynil Na and three for bromoxynil K. Radiolabelled herbicide displacement studies revealed that ioxynil Na could partially displace bound [¹⁴C]bromoxynil K, but bromoxynil K could not displace bound ioxynil Na at biochemically active concentrations. Ioxynil Na may be a more effective inhibitor than bromoxynil K because it binds more strongly to the thylakoid membrane.     

Comparison of the uptake, movement and metabolism of fluroxypyr in Stellaria media and Viola arvensis

The uptake, movement and metabolism of fluroxypyr* is compared in two contrasting weed species, Stellaria media (susceptible) and Viola arvensis (moderately resistant). Similar rates of uptake occurred in both species, with a rapid cuticular uptake of 50% of that applied within 4 h. Total uptake by the underlying leaf tissue reached 66.6% and 70.8% in S. media and V. arvensis after 7 days. In translocation studies, in which ¹⁴C-fluroxypyr was applied to previously sprayed plants, 5.1% of applied ¹⁴C-activity was translocated from the treated leaves of S. media after 1 day, which increased to 42.2% after 7 days, recovered mainly from the stem tissue. In V. arvensis translocation was similar after 24 h however, after 7 days over 40% of applied ¹⁴C-activity remained in the treated leaves and only 9.7% was translocated, mainly to the developing leaves and apical tissue. ¹⁴C-activity extracted from the cuticle was the methylheptyl ester of fluroxypyr in both species. In the treated leaves and apical tissue, ¹⁴C-activity was the free acid of fluroxypyr and polar conjugates with a significantly greater proportion of the acid in S. media. It is concluded that the resistance or V. arvensis is partially due to reduced translocation and greater conjugation than in the susceptible S. media.     

Absorption,translocation and metabolism of 14C-glyphosate in several weed species*

The pattern and extent of ¹⁴C-glyphosate [N-(phosphonomethyl)glycine] translocation from the treated leaf and metabolism of ¹⁴C-glyphosate were studied in field bindweed (Convolvulus arvensis L.), hedge bindweed (Convolvulus sepium L.). Canada thistle [Cirsium arvense (L.) Scop.] tall morning glory [lpomoea purpurea (L.) Roth.] and wild buckwheat (Polygonum convolvulus L.). ¹⁴C was translocated throughout the plants within 3 days with accumulation in the meristematic tips of the roots and shoots evident. Cross and longitudinal sections of stems and roots showed that the ¹⁴C was localized in the phloem. Field bindweed translocated 3–5% of the applied ¹⁴C from the treated leaf, hedge bindweed 21.6%, Canada thistle 7.8%, tall morningglory 6.5%, and wild buckwheat 5%. Field bindweed, Canada thistle, and tall morningglory metabolized the parent glyphosate to aminomethylphosphonic acid to a limited extent. This metabolite made up less than 15% of the total 14C. Of the total ¹⁴C applied to excised leaves, 50% had disappeared within 25 days.     

Uptake and translocation of benomyl and carbendazim (methyl benzimidazol-2-yl carbamate) in the symplast

Cytoplasmic uptake of carbendazim (methyl benzimidazol-2-yl carbamate; MBC) from an aqueous solution was demonstrated with isolated mesophyll cells. About 2.5% of the labelled MBC (ring-2-[¹⁴C]) in the treatment solution (1.85 μg/ml) was taken up in 44 h. When cotyledons of cucumber seedling were treated with either 347 or 36 μg [¹⁴C]-MBC/plant 1.11 and 0.13% were extracted, respectively, from the remainder of the plant, 5 days after treatment. Greatest amounts were detected in shoot apices. Likewise, when MBC and benomyl were applied at the dose of 2 μmol, 0.34 and 0.57% were detected in the untreated part of the plant with a bioassay procedure. Foliar application with 347 or 36 μg[¹⁴C]-MBC/leaf resulted in the translocation of 1.68 and 0.11% out of the treated area. By scalding the living cells of the petiole translocation was prevented suggesting that long distance movement occurred in the symplast. During a period of 14 days 1.56% of [¹⁴C]-MBC applied to cucumber leaves was metabolised and respired as CO₂. This degradation was assumed to occur enzymically within the symplast.     

Penetration of triolein and methyl oleate through isolated plant cuticles and their effect on penetration of [14C]quizalofop-ethyl and [14C]fenoxaprop-ethyl

The penetration of two model seed oil compounds, [¹⁴C]triolein (TRI) and [¹⁴C]methyl oleate (MEO) through plant cuticles and their effects on the penetration of [¹⁴C]quizalofop-ethyl and [¹⁴C]fenoxaprop-ethyl were investigated. Experiments were carried out using isolated cuticles from rubber plant (Ficus elastica Roxb.) leaves and from tomato (Lycopersicon esculentum Mill,) and pepper (Capsicum annuum L.) fruits. Chemicals were deposited in droplets on to cuticle discs maintained on agar blocks under controlled conditions. TRI and MEO were used at 1% (V/V). The transfer of radiolabel through cuticles was negligible for TRI and varied from 6 to 13% after 72 h, according to species, for MEO, The penetration results obtained for quizalofop-ethyl (0.084 mg mL^-1) and fenoxaprop-ethyl (0.189 mg mL^-1) were very similar and varied according to species. The greatest diffusion intoagar was observed for pepper (12.8% and 10.7% after 72 h, for quizalofop-ethyl and fenoxaprop-ethyl respectively), the lowest for rubber plant cuticles (1.4 and 1.3% respectively). Addition of MEO produced significant increases in the penetration of quizalofop-ethyl and fenoxaprop-ethyl through rubber plant and tomato cuticles. TRI had an enhancing effect on the two herbicides only with rubber plant cuticles. Results are discussed with particular consideration of the variations between plant species and the possible mode of action of seed oil adjuvants.     

Metribuzin metabolism in tomato: Isolation and identification of N-glucoside conjugates

Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-1,2,4-triazin-5(4H)-one] metabolism was studied in tomato (Lycopersicon esculentum Mill. “Sheyenne”). Pulse-treatment studies with seedlings and excised leaves showed that [5-¹⁴C]metribuzin was rapidly absorbed, translocated (acropetal), and metabolized to more polar products. Foliar tissues of 19-day-old seedlings metabolized 96% of the root-absorbed [¹⁴C]metribuzin in 120 hr. Excised mature leaves metabolized 85–90% of the petiole-absorbed [¹⁴C]metrubuzin in 48 hr. Polar metabolites were isolated by solvent partitioning, and purified by adsorption, thin-layer, and high-performance liquid chromatography. A minor intermediate metabolite (I) was identified as the polar β-d-(N-glucoside) conjugate of metribuzin. The biosynthesis of (I) was demonstrated with a partially purified UDP-glucose: metribuzin N-glucosyltransferase from tomato leaves. A possible correlation between foliar UDP-glucose: metribuzin N-glucosyltransferase activity levels and differences in the tolerance of selected tomato seedling cultivars to metribuzin was suggested. The major polar metabolite (II) was identified as the malonyl β-d-(N-glucoside) conjugate of metribuzin.     

Movement and metabolic fate of [14C]ethephon in flue-cured tobacco

The absorption, distribution, and metabolic fate of [¹⁴C]ethephon in flue-cured tobacco (Nicotiana tabacum L.) was studied using autoradiography, thin-layer chromatography, high-voltage paper electrophoresis, and liquid scintillation spectrometry. Labeled ethephon penetrated mature leaf tissue easily and was translocated primarily in an acropetal direction. No ¹⁴C activity was detected in any other plant part except the treated leaf. The first day after treatment, most of the translocated ¹⁴C was detected in the midrib, and after 2 days radioactivity was noticed in veinal areas distal to the point of application. Four days later, however, ¹⁴C was detected in slight amounts only in the midrib, indicating that [¹⁴C]ethephon was rapidly degraded by the leaf tissue. Depending on leaf position on the stalk, as much as 92% of the radioactivity had disappeared from the leaf tissue during the first day after treatment, and as little as 5% of the applied radioactivity was recovered 4 days later. Methanol-extracted plant residues contained insignificant amounts of ¹⁴C. All of the ¹⁴C in methanol extracts was present in the form of a labeled compound with an R_f value corresponding to that of ethephon, indicating the absence of any detectable metabolites of the parent compound. Smoke analysis of cigarettes showed that more [¹⁴C]ethylene than ¹⁴CO₂ was recovered in the main stream, whereas the trend was reversed in the case of side stream smoke.     

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.     

Weed size hierarchies in Denmark

In 962 field trials in different crops, the size of a number of weed species was investigated. The weeds were ranked according to their size on a single occasion. The ranks of the species were examined relative to two weed species, Viola arvensis and Matricaria inodora. The ranking was, however, independent of which of the two species were used as standard. The correlation between the rankings with Viola arvensis as standard, and the rankings with Matricaria inodora as standard were r= 0-93 for spring sown crops, and r= 0.84 for autumn sown crops. Sinapis arvensis was the largest weed species in spring sown crops, with a weight of 14.5 times that of Viola arvensis, followed by Brassica napus, Galeopsis spp., Thlaspi arvense and Amsinckia spp. Viola arvensis. Veronica spp. and Lamium spp. were the smallest weed species in spring sown crops. Galium aparine was the largest species in autumn sown crops, whereas spring germinating species such as Polygonum spp. were among the smallest. The ranks of Viola arvensis, Myosotis arvensis, Lapsana communis, Matricaria inodora and Lamium spp. were almost identical in spring sown crops and in autumn sown crops.     

Basis for the selective action of fluroxypyr

The dose-response, foliar uptake, translocation and metabolism of the methylheptyl ester (MHE) of fluroxypyr were examined in lambs-quarters (Chenopodium album L.), wild buckwheat (Polygonum convolvulus L.), Canada thistle (Cirsium arvense L. Scop.) and field bindweed (Convolvulus arvensis L.). Under controlled environment growth room conditions, E₅₀ values, determined from shoot dry weights of the susceptible species, wild buckwheat (16 g ha^?1) and field bindweed (40 g ha^?1), were markedly different than those of the tolerant species, lambsquarters (331 g ha^?1) and Canada thistle (800 g ha^?1). Regardless of species, more than 80% of applied [¹⁴C]fluroxypyr-MHE was absorbed by foliar surfaces 120 h after treatment. Translocation of radioactivity out of the treated leaves of susceptible species was significantly greater than that of tolerant species. For example, 120 h after treatment with [¹⁴C]-fluroxypyr-MHE, the proportion of applied radioactivity translocated in tolerant Canada thistle and lambsquarters was 15 and 10%, respectively, whereas in susceptible wild buckwheat and field bindweed it was 41 and 40% of applied radioactivity, respectively. High-performance liquid chromatography (HPLC) of plant extracts indicated four distinct chroma-tographic peaks common to all four species. More fluroxypyr was recovered in the susceptible species (70%) than in the tolerant species (30%), 120 h after application. Selectivity differences between the tolerant and susceptible species may be the result of enhanced metabolic transformation of the herbicide to more polar, non-phytotoxic compounds with limited mobility within the tolerant species. Les bases de la sélectivité du fluroxypyr La courbe dose effet, la pénétration foliaire, la migration et le métabolisme de Tester méthyle-heptyle (MHE) du fluroxypyr ont étéétudiés chez le chénopode blanc (Chenopodium album L.), la renouée faux-liseron (Polygonum con-vulvulus L.), le chardon des champs (Cirsium arvense L. Scop.) et le liseron des champs (Convolvulus arvensis L.). En conditions de crois-sance contrôlées, les valeurs ED₅₀, déterminées à partir du poids de matière sèche des parties aériennes étaient nettement différentes chez les plantes sensibles et chez les plantes résistan-tes:renouée faux-liseron, 16 g ha^?1; liseron des champs, 40 g ha^?1; chénopode blanc, 331 g ha^?1; chardon des champs, 800 g ha^?1. Quelle que soit 1'espèce, plus de 80% du [¹⁴C]fluroxypyr-MHE pénétrait dans les feuilles en 120 h. La migration de la radioactivité hors des feuilles traitérs était significativement plus importante chez les plantes sensibles que chez les plantes tolérantes. Par exemple, 120 h après le traite-ment avec du [¹⁴C]fluroxypyr-MHE, la proportion de radioactivité appliquée qui avait migré dans le chardon des champs et le chénopode, tolérants, était respectivement 15 et 10%, alors que chez la renouée faux-liseron et le liseron des champs, sensibles, elle était respectivement 41 et 40%. Des analyses par HPLC des extraits de plantes montraient quatre pics chro-matographiques distincts dans chacune des quatre espèces. Davantage de fluroxypyr était retrouvé 120 h après 1'application chez les plantes sensibles que chez les plantes résistantes (70% contre 30%). Les différences de sensibilité entre espèces pourraient être dues chez les plantes tolérantes à un métabolisme plus important de l'herbicide en composés plus polaires, non phytotoxiques et peu mobiles. Grundlagen für die selektive Wirkung von Fluroxypyr Die Dosis/Wirkungs-Beziehung, Blattaufnahme, Translokation und Metabolismus des Methylheptyl-Esters (MHE) von Fluroxypyr wurden bei Weißem Gänsefuß (Chenopodium album L.), Gemeinem Windenknöterich (Polygonum convolvulus L.), Acker-Kratzdistel (Cirsium arvense (L.) Scop.) und Gemeiner Ackerwinde (Convolvulus arvensis L.) untersucht. Unter den kontrollierten Umweltbedingungen eines Phytotrons wurden anhand des Trockengewichts die ED₅₀-Werte bestimmt, die bei den empfindlichen Arten Polygonum convolvulus mit 16 g ha^?1 und Convolvulus arvensis mit 40 g ha^?1 sich deutlich von denen der toleranten Arten Chenopodium album mit 331 g ha^?1 und Cirsium arvense mit 800 g ha^?1 unterschieden. Unabhängig von der Art waren mehr als 80 % der Aufwandmenge von [¹⁴C]-Fluroxypyr-MHE durch die Blattoberflächen 120 h nach der Behandlung aufgenommen. Die Translokation der Radioaktivität aus den behandelten Blättern war bei den empfindlichen Arten signifikant größer als bei den toleranten. Z. B. waren 120 h nach der Behandlung bei den toleranten Arten Cirsium arvense und Chenopodium album 15 bzw. 10 % der Radioaktivität transloziert, während es bei den empfindlichen Arten Polygonum convolvulus und Convolvulus arvensis 41 bzw. 40 % waren. Bei allen 4 Arten ergab eine HPLC-Untersuchung der Pflanzenextrakte 4 distinkte Peaks. Bei den empfindlichen Arten wurde 120 h nach der Anwendung mit 70 % mehr Fluroxypyr wiedergefunden als bei den toleranten (30 %). Die Selektivitätsunterschiede zwischen den toleranten und empfindlichen Arten könnten auf einen beschleunigten Metabolismus des Herbizids zu stärker polaren, nichphytotoxischen Stoffen mit eingeschränkter Mobilität bei den toleranten Arten zurückgeführt werden.     

The elimination of (N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide) by the boll weevil

Boll weevils, Anthonomus grandis Boheman, were either dipped in or injected with a solution of [¹⁴C]diflubenzuron (N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide) or fed on cotton squares that had been treated with the chemical to determine its turnover time and metabolic fate. No significant differences were observed between male and female weevils in their ability to eliminate [¹⁴C]diflubenzuron. Only minor differences were observed when immersion and injection treatments were compared. When weevils were treated with 66.3 ng of [¹⁴C]deflubenzuron per weevil by injection, the insects contained 13 to 15% of the radiolabel after 6 days and 4 to 6% after 13 days. The remainder of the radiolabel was in the frass. When weevils fed for 66 hr on cotton squares that had been treated with a wettable [¹⁴C]diflubenzuron preparation (Dimilin W-25), the insects averaged 120 ng of diflubenzuron per weevil. Forty-four hours after removing insects from the treated squares, 50% of the radiolabel had been excreted. In all cases, the radiolabel found in the frass or in the weevil was unchanged diflubenzuron. There were no data to indicate that the boll weevil could metabolize appreciable amounts of diflubenzuron.     

A study of ethylene and CO2 evolution from ethephon in tobacco

Buffers and leaf discs of mature tobacco (Nicotiana tabacum L.) were utilized to study [¹⁴C]-ethylene and ¹⁴CO₂ evolution from radiolabeled ethephon, (2-chloroethyl)phosphonic acid. Metabolic fate of [¹⁴C]ethephon in leaf discs was investigated by use of thin-layer chromatography, high-voltage paper electrophoresis, autoradiography, and liquid scintillation spectroscopy. The evolution of labeled ethylene generally increased with increasing buffer pH, buffer volume, and dosage of [¹⁴C]ethephon. [¹⁴C]Ethylene was evolved, increasingly with time, from [¹⁴C]ethephon either added to the buffer or applied to leaf discs. The rate of [¹⁴C]ethylene evolution was maximum during the first day and leveled off on the fourth day. More than 50% of the total [¹⁴C]ethylene evolution over a 96-hr period was recovered during the first 24 hr after [¹⁴C]ethephon application. No ¹⁴CO₂ was evolved when [¹⁴C]ethephon was degraded in the presence of buffer or leaf discs. Only ethephon itself, and no detectable metabolite thereof, was discovered in the methanolic extract of the leaf disc tissue. An insignificant amount of ¹⁴C activity (approximately 2% of the extracted ¹⁴C) was detected in the residue. By means of gas chromatography, it was confirmed that in buffers and tobacco leaf tissue ethephon breaks down to release ethylene but not CO₂.     

The metabolism of [14C]aldicarb in the leaves of sugar beet plants

Sugar beet plants were grown in the field, after in-furrow application of [¹⁴C]aldicarb (3 kg of aldicarb ha^?1) at planting. The ripe sugar beet plants were harvested, and the blades and petioles of the leaves were analysed separately. In the whole leaves, 15% of the ¹⁴C (all the percentages of ¹⁴C are relative to the total ¹⁴C incorporated into the whole leaves) was insoluble in ethanol+ water (1+1 by volume), 31% was organo-soluble (and thus unconjugated in the leaves), and 54% was water-soluble (mainly conjugated to plant constituents). The weights and concentrations (as aldicarb equivalents) of various identified metabolites of aldicarb, incorporated into the leaves, were determined; no aldicarb, as such, was detected.     

In vitro metabolism of diazinon and etrimfos by corn plant preparations

Homogenates prepared from excised roots or stems and leaves of corn seedlings metabolize up to 72% of [¹⁴C]pyrimidinyl-labeled diazinon (O,O-diethyl-O-[6-methyl-2-(1-methylethyl)-4-pyrimidinyl]phosphorothioate) to 6-methyl-2-(1-methylethyl)-4-hydroxypyrimidine and one unidentified metabolite. Six-day-old corn seedling homogenate had the highest degradative activity. The optimum pH for activity was 6.0 and the activity was found to reside in the cytosol. Etrimfos [O,O-dimethyl-O-(6-ethyl-4-pyrimidinyl)phosphorothioate] was not susceptible to degradation by the corn plant preparation.     

Tissue analysis and hemolymph translocation of [14C]chlorpyrifos sorbed from treated surfaces by American cockroaches

Using radiotracer methodology and dissection techniques it was demonstrated that [¹⁴]chlorpyrifos and/or its ¹⁴C-labeled metabolite(s) concentrated mainly in the gut tissues and malpighian tubules of American cockroaches, Periplaneta americana (Linnaeus), following sorption from a treated surface. Significantly lower (P ≤ 0.10) amounts of ¹⁴C were present in testes samples and no radioactive material was detected in brain tissue. After 41.5–48 hr of exposure of adult male American cockroaches to sublethal concentrations of [¹⁴C]chlorpyrifos, radioactivity was detected in the hemolymph of all cockroaches tested. The hemolymph accounted for 30.83% of the total sorbed ¹⁴C. A parabiotic experiment confirmed translocation of chlorpyrifos and/or its ¹⁴C-labeled metabolite(s) in hemolymph.     

The metabolism of 3-phenoxybenzyl alcohol,a pyrethroid metabolite,in plants

The metabolism and conjugation of 3-phenoxybenzyl alcohol, a plant metabolite of permethrin and cypermethrin, have been examined in abscised cotton leaves. Mature cotton leaves were treated by petiole uptake of an aqueous solution of [α-¹⁴C]-3-phenoxybenzyl alcohol. Initially there was rapid formation of a compound identified as the glucosyl 3-phenoxybenzyl ether. Subsequently more polar compounds were formed and these were shown to be disaccharide conjugates of the alcohol with glucose and pentose sugars. The alcohol and its mono- and disaccharide conjugates were shown to undergo interconversion in cotton leaves, and evidence was obtained from experiments with [¹⁴C]glucose for the ready exchange of the glucose units on the conjugates with free glucose in the leaves. No larger carbohydrate conjugates of 3-phenoxybenzyl alcohol were detected under the conditions used.     

Mercapturic acid formation in the metabolism of propachlor,CDAA, and fluorodifen in the rat

When [¹⁴C]F₃-fluorodifen (2,4′-dinitro-4-trifluoromethyl diphenylether), carbonyl-[¹⁴C]CDAA (N,N-diallyl-2-chloroacetamide), and carbonyl-¹⁴C-propachlor (2-chloro-N-isopropylacetanilide) were fed to rats, 57 to 86% of the ¹⁴C was excreted via the urine within 48 hr. Although very little radioactivity was excreted in the feces of CDAA-treated rats, 15–22% of the ¹⁴C was excreted in the feces of propachlor- of fluorodifentreated rats and an average of 8% of the ¹⁴C remained in these rats 48 hr after treatment. Oxidation of the ¹⁴C label to [¹⁴C]O₂ was not a major process in the metabolism of these herbicides. The only major radioactive metabolite present in the 24-h urine of fluorodifen-treated rats, 2-nitro-4-trifluoromethylphenyl mercapturic acid, accounted for 41% of the administered dose of ¹⁴C. In the metabolism of CDAA, the corresponding mercapturic acid accounted for 76% of the dose; it was the only major metabolite present in the 24-h urine. In contrast, three major metabolites were detected in the 24-h urine of propachlortreated rats, and the mercapturic acid accounted for only 20% of the dose. The mercapturic acid of each herbicide was identified by mass spectrometry.     

The importance of reduced light intensity on the growth and development of six weed species

Crops limit light for photosynthesis and growth of weeds. We studied the effect of reduced light on performance of six weed species [one invasive species (Amsinckia micrantha), three common species (Veronica persica, Capsella bursa‐pastoris, Viola arvensis) and two less common weeds (Anagallis arvensis and Scleranthus annuus)]. In two glasshouse experiments, six light levels were achieved aiming at 0%, 20%, 50%, 80%, 90% and 95% reduction of light and corresponding with daily light integrals (DLI) of 12.4, 9.63, 7.13, 2.74, 0.95 and 0.69 mol m^?2 day^?1 in experiment 1 and 21.2, 18.0, 10.7, 3.71, 1.64 and 1.20 mol m^?2 day^?1 in experiment 2. The number of leaves was strictly controlled by DLI. Chlorophyll content index, maximum photochemical efficiency of photosystem II (F_v/F_m), stomatal conductance, flowering and dry matter were strongly reduced when DLI was reduced to 0.69–3.71 mol m^?2 day^?1 for all species. Threshold DLI for flowering was ca. 3.71 mol m^?2 day^?1 for S. annuus, V. arvensis, A. arvensis and V. persica, while C. bursa‐pastoris deviated by flowering at DLI of 0.95 mol m^?2 day^?1. This may explain why C. bursa‐pastoris is common in the seedbank of Danish arable soils in spite of intensive farming with well‐fertilised and dense crops.     

Physiological basis for antagonism of clethodim by imazapic on goosegrass (Eleusine indica (L.) Gaertn.)

Greenhouse and laboratory experiments were conducted to determine the effect of imazapic on the herbicidal activity of clethodim on goosegrass. Imazapic did not affect absorption of [¹⁴C]clethodim by goosegrass. Averaged across the two treatments of clethodim alone and clethodim plus imazapic, absorption was 36 and 89% of applied [¹⁴C]clethodim at 0.5 and 96 h, respectively. The majority of [¹⁴C]clethodim (79% of applied) was absorbed by 24 h. Translocation of ¹⁴C was not affected by imazapic, and 3.6% of applied ¹⁴C had translocated into the portion of the shoot below the treated leaf at 96 h after treatment. Metabolism of clethodim was not affected by the presence of imazapic. Three major metabolites of clethodim were detected in treated tissue at all harvest intervals. The majority (58%) of [¹⁴C]clethodim was converted to a relative polar metabolite form 96 h after treatment, whether clethodim was applied alone or in the presence of imazapic. One day after treatment, the photosynthetic rate in plants treated with imazapic decreased below the rate in the non-treated check, and was less for 8 days, the duration of the study. These data suggest that the antagonism of clethodim by imazapic may be caused by imazapic reducing the photosynthetic rate of goosegrass and therefore the sensitivity of ACCase to clethodim.