The foliar uptake of isoproturon by wheat and Alopecurus myosuroides huds

The uptake and translocation of ¹⁴C-isoproturon (3-p-cumenyl-1-1,-dimethylurea) in wheat (tolerant) and backgrass (sensitive) following foliar treatment under controlled environmental conditions were examined. The amount of ¹⁴C-isoproturon translocated through the xylem was about 10 times that translocated through the phloem in both wheat and blackgrass. However, 25.5% of the applied ¹⁴C-isoproturon was translocated in the xylem in blackgrass, compared with 8.9% in wheat. ¹⁴C-isoproturon did not respond significantly to induced sink-demand in either species. Leaf-disc autoradiograms revealed the absorption of ¹⁴C-isoproturon by the minor veins and translocation into the cut vein endings. No significant differences were found in between wheat and blackgrass in this respect.     

Studies on the mechanism of selectivity of the auxin herbicide quinmerac

Investigations were conducted to elucidate the mechanism of selectivity of the auxin herbicide, quinmerac, in cleavers (Galium aparine) and the tolerant crops sugarbeet (Beta vulgaris), oilseed rape (Brassica napus) and wheat (Triticum aestivum). After root treatment with the herbicide, the selectivity has been quantified as approximately 400-fold between oilseed rape and Galium and 1000-fold between sugarbeet or wheat and the weed species. When 1 and 10 μM [¹⁴C]quinmerac were applied for 4 h, no significant differences between root absorption and translocation of ¹⁴C by Galium and the crop species were found. After 16 h, metabolism of [¹⁴C]quinmerac to the biologically inactive hydroxymethyl and dicarboxylic acid derivatives was more rapid in wheat and sugarbeet than in Galium. In oilseed rape, a lower rate of herbicide metabolism was observed. In Galium, accumulations of abscisic acid (ABA), triggered by quinmerac-stimulated ethylene biosynthesis, were found to cause the herbicidal growth inhibition which develops during 24 h of application. Within 1 h of treatment, quinmerac stimulated 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and ACC concentration specifically in Galium shoot tissue. During the next 4 h, ACC synthase activity was increased up to 50-fold, relative to the control. Within 3 h of exposure to quinmerac, increased ethylene formation followed by higher ABA levels was detected. In sugarbeet, oilseed rape and wheat, quinmerac did not stimulate ACC synthase activity and ACC and ABA levels. It is suggested that (i) the selectivity of quinmerac is primarily based upon the lower sensitivity to the herbicide of the tissue/target in the crop species, (ii) the induction process of the ACC synthase activity in the shoot tissue is the primary target of herbicidal interference. In wheat and sugarbeet, tolerance to quinmerac is additionally increased by a more rapid metabolism. © 1998 SCI.     

Survey of the presence of wild oat and blackgrass in parts of the United Kingdom

The survey was carried out over an area representing about 47% of the cereal acreage of the United Kingdom. Estimates of the acreage of fields containing wild oat (Avena fatua L. and A. ludoviciana Dur) ranged from 15% (Northern Ireland) to 74% (England, South West Region). The degree of infestation over the total acreage was 58%‘light’, 34%‘medium’, and 8%‘heavy’. Blackgrass (Alopecurus myosurodies Huds.) was found only in the central and southern regions, estimates of the infested cereal field acreage ranging from 2% (Wales) to 23% (South East England). Nine per cent of the fields were rogued for wild oat. On average, only 10% of the infested acreage received herbicide treatment for wild oat and blackgrass though, in some regions, up to 23 % was treated. The timing of treatment and the herbicide used were recorded. Information on the extent and type of herbicide used for broad-leaved weeds was also recorded. Data on sowing dates of the crops, the previous cropping, previous infestations, the soil type and the state of field drainage were obtained and were correlated with the incidence of wild oat and blackgrass where relevant.     

Investigation of an emulsifiable oil adjuvant and its components on the activity of a new grass herbicide by factorial experimentation

The herbicidal activity of a new compound [(±)-(EZ}-2-[1-allyloxy-imino)propyr]-3-hydroxy-6-(4-cyanophenyl)cyclohex-2-ene-1-one] (I) , selective to broad-leaf plants, and partially selective in some cereal crops was enhanced by an emulsifiable paraffinic oil (HVI 60E). In order to determine the amount of HVI 60E to be applied with an emulsifiable concentrate of I (IE) a factorial herbicidal assay involving six concentrations of I and five concentrations of HVI 60E (both sets including zero concentration) was conducted to obtain response surfaces of herbicidal activity. The total selectivity of IE in broad-leaf species remained for all HVI 60E concentrations. The activity against grass weeds, except Poa annua L., increased markedly with increasing HVI 60E concentration and enhancement of performance of IE by HVI 60E in broad-leaf crops could be pursued without fear of crop phytotoxicity. However, the increases in activity against grass weeds were accompanied by increases in phytotoxicity against cereal crops. The response surfaces of Echinochloa crus-galli(L.) Beauv. and Zea mays L. were coincident and this result terminated further testing in this outlet. However, the response surfaces of Avena fatua L. and Triticum aestivum L. were reasonably well separated. Further tests in this outlet and in broad-leaf crops are planned around optimum cost-performance ratios derived from combinations with cost-response surfaces. The results of a further lest showed that both components (oil and emulsifier) of HVI 60E were approximately equivalent in adjuvant enhancement activity and so, on cost grounds, a crop oil (low emulsifier content) would be preferred over a crop oil concentrate (high emulsifier content).     

Foliar absorption and translocation of nicosulfuron and rimsulfuron in five annual weed species

Ambrosia artemisüfolia L. (common ragweed) and Digitaria ischaemum Schreb. (smooth crabgrass) are not controlled by nicosulfuron and rimsulfuron at the highest recommended application rates, whereas Panicum miliaceum L. (wild proso millet), Amaranthus retroflexus L. (redroot pigweed) and Avena fatua L. (wild oat) are susceptible. The foliar absorption and translocation of ¹⁴C-nicosulfuron and ¹⁴C-rimsulf uron were studied in these weed species up to 48 h after treatment (HAT). Differences in herbicide uptake and translocation were not correlated with the species susceptibility. By 48 HAT, more than 50% of both herbicides remained on the treated leaf surface. Foliar absorption of rimsulfuron was greater than that of nicosulfuron in A. retroflexus, P. miliaceum and A. artemisüfolia. Most of the absorbed herbicide remained in the treated leaf of each weed species. Export of ¹⁴C–nicosulfuron ranged from 28 to 54% of that absorbed, in contrast to 15 to 39% for ¹⁴C–rimsulfuron. The absorption and translocation rates of both herbicides were highest within the initial 6 HAT, and decreased thereafter. Both herbicides showed approximately the same distribution pattern within each weed species.     

The origins of selectivity and performance of a new pre-emergence bleaching herbicide,WL 110547. Part II: Plant and environmental factors influencing biological activity

Pre-emergence applications of the novel tetrazole herbicide WL 110547 control a number of economically important grass and broad-leaved weed species in small grain cereals. To assess the influence of plant and environmental factors on the biological performance of WL 110547, a series of tests were carried out under controlled conditions and, where appropriate, comparisons were made with field observations. When presented with the maximum opportunity for compound uptake in the absence of soil, differences in the degree of susceptibility to WL 110547 were observed amongst both monocotyledonous and dicotyledonous species, although the latter group generally showed higher levels of phytotoxicity. This species susceptibility to WL 110547 was unaffected by temperature. Increasing the sowing depth in soil decreased the level of effect of WL 110547 on a number of monocotyledonous species, although small-seeded species (e.g. blackgrass, annual meadow grass), emerging from deep in the soil profile, subsequently developed levels of phytotoxicity comparable to, or even greater than, shallow-planted seedlings. This was attributed to less vigorous seedlings, emerging from depth, that were unable to regenerate new tissue and grow away from a treated soil layer. Reduced growth rates of wild oat, blackgrass and speedwell, induced by low temperatures, also increased the phytotoxicity of WL 110547. Furthermore, applications of WL 110547 during seedling emergence maximised herbicide effect, as did seedling emergence through moist rather than dry soil. The results are discussed in relation to the mobility of the herbicide in soil, the mode of action of WL 110547, its availability to the plant and the duration of contact between emerging shoot and treated soil layer.     

The relative tolerances of wild and cultivated oats to infection byErysiphe graminisf.sp.avenae: I. The effects of infection on vegetative growth and yield

The effects of infection by the powdery mildew fungusErysiphe graminisf.spavenaewere studied in one line of wild oat (Avena fatua), and two cultivars, Lustre and Peniarth, of cultivated oat (A. sativa) to determine if the wild oat was more tolerant of infection than the two cultivated oats. Seven weeks after inoculation, when the plants were 10-weeks-old with fully expanded flag leaves, the fungus had colonized approx. 40% of the leaf surfaces of wild oat and cv. Lustre but only about 30% of the leaf surfaces of cv. Peniarth. The lower leaves of cv. Peniarth were at least as susceptible, if not more so, than those of the other two lines but the upper leaves, including the flag leaf, were much more resistant. Although cv. Peniarth supported the production of about half the number of mildew conidia as the wild oat and cv. Lustre its total dry weight and grain yield were reduced to the greatest extent. The wild oat was clearly much more tolerant of mildew infection than cv. Peniarth and slightly more tolerant than cv. Lustre. The greater tolerances of the wild oat and cv. Lustre compared to cv. Peniarth appeared to be due to the lower sensitivities of their metabolism to the activities of the mildew fungus.     

The impact of uptake, translocation and metabolism on the differential selectivity between blackgrass and wheat for the herbicide pyroxsulam

BACKGROUND: Wheat shows selectivity to pyroxsulam, a new broad‐spectrum herbicide with high activity on blackgrass. Studies were performed to establish whether uptake, translocation or metabolism were responsible for the differential activity in wheat compared with blackgrass. In addition, the effect of the safener cloquintocet‐mexyl on metabolism was evaluated in wheat and blackgrass shoots. RESULTS: Root uptake of pyroxsulam in blackgrass was significantly higher than in wheat, suggesting a possible activity enhancement in blackgrass owing to root uptake. Translocation to foliage from root uptake as well as translocation out of treated foliage following foliar applications was low in wheat compared with blackgrass, likely owing to the rapid metabolism of pyroxsulam in wheat. Wheat metabolized pyroxsulam significantly faster than blackgrass to the less active O‐dealkylation product. Wheat shoots metabolized pyroxsulam faster when the safener cloquintocet‐mexyl was present, but cloquintocet‐mexyl did not increase the rate of metabolism in blackgrass. CONCLUSIONS: The selectivity of pyroxsulam to wheat relative to blackgrass was connected primarily with differences in the rate of metabolism and generation of an inactive metabolite. Metabolism in wheat restricted subsequent movement of radioactivity out of the treated leaf. The rapid metabolism in wheat was increased by the addition of cloquintocet‐mexyl. Copyright © 2010 Society of Chemical Industry     

Absorption, translocation and metabolism of pyridate in a tolerant crop (Zea mays) and two susceptible weeds (Polygonum lapathifolium L. and Chenopodium album L.)

Absorption, translocation and metabolism of ¹⁴C-pyridate were compared in tolerant maize. moderately susceptible Polygonum lapathifolium and susceptible Chenopodium album. Foliar absorption was limited in all species, but comparatively higher penetration levels were observed in younger leaves of dicotyledonous species. The absorbed radioactivity was not very mobile and translocation appeared mainly sym-plastic. Herbicide selectivity could not be explained on the basis of absorption and transport. Chenopodium and P. lapathifolium degraded pyridate and formed unstable water-soluble conjugates that easily released a phytotoxic metabolite. By contrast, more stable unidentified water-soluble metabolites were found in maize. That metabolic difference could explain the selectivity of pyridate.     

Non-enzymatic conjugation of fenoxaprop-ethyl with glutathione and cysteine in several grass species

Laboratory studies have shown that the amounts of glutathione (GSH) and cysteine are higher in grass species that are moderately tolerant, such as wheat (Triticum aestivum L., cv. Fredrick), and moderately susceptible, such as barley (Hor deum vitlgare L., cv. Legér) and triticale (cv. OAC Trillium), to fenoxaprop-ethyl (FE) than in species that are very susceptible to the her bicide, such as oat (Avena saliva L., cv. OAC Woodstock), wild oat (Avena fatua L.), yellow foxtail (Setaria glanca (L.) Bcauv.), large crab grass (Digitaria sanguinalis (L.) Scop.) and bar nyard grass (Echinochloa crus-galli (L.) P.B.). The safener, fenchlorazole-ethyl (FCE) was found to increase and decrease, respectively, the amounts of GSH and cysteine in the moderately tolerant and moderately susceptible species but had no effect on the susceptible species. It is sug gested that in the moderately tolerant and moderately susceptible species, especially following FCE treatment, more GSH is available to detoxify the herbicide. Glutathione-S-tranferase activity (GST) for FE was found to be very low in all of the species tested. In vitro experiments at physio-logical pH. demonstrated that FE may conjugate with GSH nonenzymatically. Therefore, it is suggested that nonenzymatic conjugation of fenoxaprop-ethyl with glutathione may be an important mechanism for tolerance of some grasses to this herbicide.     

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.     

Asulam mixtures for weed control in flax

Five field experiments were conducted from 1972 to 1975 to evaluate weed control in flax (Linum usitatissimum L.) using post-emergence treatments of asulam [methyl (4-aminobenzenesulphonyl) carbamatel alone and in combination with other herbicides. The ¹⁴C-asulam absorption by leaf segments and roots of glasshouse grown wild oats (Avena fatua L.) was also investigated. Asulam at 1.12 kg/ha gave good wild oat control and acceptable control of green foxtail (Setaria viridis (L.) Beauv.). However, wild oat control was poorer when asulam was combined with other herbicides: on a 3-year average, as compared with asulam alone at equal rates, the asulam+MCPA mixture resulted in a greater antagonism and a significant 6% reduction in flax seed yield, whereas the asulam+bromoxynil/MCPA mixture gave the least antagonistic effect, improved broadleaf weed control and increased yield by 13%. In mixtures, the potassium salt of MCPA was more compatible with asulam for weed control than the amine form. Both leaf segments and roots of wild oats absorbed and distributed less ¹⁴C-asulam from solutions containing MCPA than from those containing bromoxynil or bromoxynil/MCPA.     

Plant community diversity influences vector behaviour and Barley yellow dwarf virus population structure

The species composition of a plant community can affect the distribution and abundance of other organisms including plant pathogens. The goal of this study was to understand the role of host diversity in the transmission of two Barley yellow dwarf virus (BYDV) species that share insect vectors and hosts. Greenhouse experiments measured the transmission rate of BYDV species PAV and PAS from infected oat plants to healthy agricultural and wild grasses and from these species back to healthy oat seedlings. In the field component of the study, the rate of spread of PAV and PAS was measured in monoculture plots planted with agricultural grasses. In greenhouse experiments, the aphid vector more readily transmitted PAV from agricultural grasses and more readily inoculated PAS to the wild grass species assayed. In the field experiment, disease prevalence was greater in wheat, but there was no difference in the rate of spread of PAV and PAS. These results indicate an interaction between vector and host genotype that selects for greater PAV transmission in grain crops, contributes to differences in disease prevalence between grass types, and maintains pathogen diversity within the larger plant community (i.e. agricultural and non‐agricultural hosts).     

The role of coleoptile on barban sensitivity between wild oat and wheat

Differential sensitivity of wild oat (Avena fatua L.) and wheat (Triticum aestivum L.) to barban (4-chloro-2-butynyl-m-chlorocarbanilate) was highest when barban was applied to the coleoptile. The coleoptile acts as a physical and physiological barrier to reduce the concentration of free barban in the stem section where the sensitive meristematic sites are located. Metabolism of barban was higher ïn the coleoptile of tolerant wheat than in that of susceptible wild oat. Free barban concentration was higher in the stem of wild oat than in the stem of wheat after 24 hr, but after 48 hr, concentrations were similar. The coleoptile appears to enhance the differential sensitivity to barban between wild oat and wheat.     

Influence of water stress on absorption, translocation and phytotoxicity of fenoxaprop-ethyl and imazamethabenz-methyl in Avena fatua

The influence of water stress on the absorption and translocation of ¹⁴C-labelled fenoxapropethyl and imazamthabenz-methyl in Avena fatua L. (wild oat) was studied. The phytoioxicity to A. fatua of both herbicides with a droplet application was also examined under water stress conditions. The absorption of both fenoxaproethyl and imazamethabenz-methyl was reduced by waler stress when the plants were harvested within 24 h after herbicide application. Up to 48 h after the application, the translocation out of the treated lamina of both herbicides, based on percentage of applied ¹⁴C. was reduced under water stress conditions. When havested 96 h after herbicide application, however, water stress no longer significantly affeaed the absorption and translocation of either herbicide. When the herbicides were applied as individual droplets, water stress reduced the phytotoxicity of fenoxaprop-ethyl but not that of imazamethabenz-methyl. It is concluded that the changes in herbieide absorption and translocation may not be the major physiological processes associated with differential whole-plant response oi A faiua to fenoxaprop-ethyl and imazamefhabenz-methyl under water stress.     

Effect of chlorfenprop-methyl, flamprop-isopropyl and benzoylprop-ethyl on 14C2 fixation in Avena fatua L., Triticum aestivum L. and Hordeum vulgare L.

The effect of chlorfenprop-methyl, flampropisopropyl and benzoylprop-ethyl on ¹⁴CO₂ fixation was followed in wild oat (Avena fatua L.), barley (Hordeum vulgare L., cv. Ametyst), and wheat (Triticum aestivum L., cv. Mironovská). Experimental plants were exposed to a ¹⁴CO₂-enriched atmosphere in a special apparatus 2 h, 1, 3, and 9 days after the herbicide treatment. Chlorfenprop-methyl already inhibited ¹⁴CO₂ fixation in wild oat plants 2 h after the treatment. ¹⁴C-metabolite transport to the roots was strongly decreased. Both ¹⁴CO₂ fixation and ¹⁴C-metabolite level in the roots were significantly depressed in A. fatua when compared with untreated plants at the last sampling time. ¹⁴C incorporation into starch was inhibited from the first day after treatment, and on day 9 was lowered more than ten fold in treated plants. Flamprop-isopropyl inhibited ¹⁴CO₂ fixation in wild oat plants from day 3 after treatment, but benzoylprop-ethyl not until day 9. Both herbicides also decreased ¹⁴C incorporation into starch in A. fatua. Chlorfenprop-methyl also slightly decreased ¹⁴CO₂ fixation in barley on day 9. However, assimilate transport into the roots and ¹⁴C incorporation into starch were not affected. Flamprop-isopropyl inhibited ¹⁴CO₂ fixation in barley plants only on the first day after treatment, and assimilate transport was also reduced. By contrast, no differences from untreated plants were found at the end of the experiment. Benzoylprop-ethyl did not decrease either ¹⁴CO2 fixation or assimilate transport to the roots in wheat, but it inhibited starch synthesis. Atrazine depressed ¹⁴CO₂ fixation in wild oat plants by 91%, in wheat plants by 99% compared with untreated plants. Assimilate transport into the roots was also strongly inhibited. In contrast to atrazine, the effect of chlorfenprop-methyl, flamprop-isopropyl, and benzoylprop-ethyl on CO₂ fixation seems to be secondary.     

Hydrolytic activation versus oxidative degradation of Assert herbicide,an imidazolinone aryl-carboxylate,in susceptible wild oat versus tolerant corn and wheat

Assert herbicide, a mixture of the methyl esters of meta and para 2-imidazolinone toluate, is an imidazolinone aryl-carboxylate herbicide with selectivity against wild oat in corn and wheat. The corresponding free carboxylic acids are also herbicidal but show no selectivity. Metabolism of individual isomers of ¹⁴C-labeled Assert herbicide in these three species during the first 2 weeks after foliar treatment shows that hydrolytic activation of the esters to yield the free acids occurs only in wild oat, with the more herbicidal meta isomer producing a two- to threefold greater concentration of the free acid than the para isomer. Detoxication in corn and wheat occurs by rapid oxidation of the aryl methyl group to the corresponding alcohol followed by glucose conjugation. The imidazolinone aryl-carboxylates may owe their activity to the structural analogy of the imidazolinone substituent to branched chain amino acids such as valine, in the feedback inhibition of acetolactate synthase. The wide range of selectivity of this herbicide series is probably a function of the balance of oxidative and hydrolytic metabolism at substituents other than the imidazolinone ring.     

Could recent increases in atmospheric CO2 have acted as a selection factor in Avena fatua populations? A case study of cultivated and wild oat competition

Projected increases in atmospheric carbon dioxide concentration [CO₂] may lead to differential selection and competition between weeds and crops. Yet, the current level of atmospheric [CO₂] already reflects a rapid rise (~25%) from mid‐20th century levels. To assess whether this increase could have already resulted in differential selection between weeds and crops, two temporally distinct populations of Avena fatua (wild oat) from the same geographic location, one from the 1960s (WO_old) and one from 2014 (WO_new), were grown with and without a cultivated oat (CO) line, Clintland 64 (Avena sativa) at current and mid‐20th century [CO₂] levels (ca 320 and 402 μmol mol^?1 respectively). Monocultures of each WO population differed in their response to recent increases in atmospheric [CO₂], with WO_new showing a significantly higher response for all vegetative parameters relative to WO_old. Assessment of competitive outcomes indicated that at 315 μmol mol^?1 cultivated oat was at a competitive advantage relative to either A. fatua population for leaf area and total above‐ground biomass. However, at current levels of CO₂, an overall increase in WO_new competitive ability was observed relative to the cultivated oat line. Overall, these differences are consistent with, but not conclusive for, improved evolutionary fitness and increased early competitive ability of A. fatua relative to cultivated oat as a function of the recent increase in atmospheric [CO₂]. While additional empirical studies are needed, these preliminary results indicate that weeds may be adapting rapidly to rising levels of atmospheric carbon dioxide.     

Using a selectivity index to evaluate logarithmic spraying in grass seed crops

BACKGROUND: Grass seed crops are minor crops that cannot support the development of selective herbicides for grass weed control in grass seed crops. An option is to screen for selective herbicides with the use of logarithmic spraying technology. The aim of this paper is to assess selectivity of various herbicides in grass seed crops by using dose–response curves. RESULTS: Six grass species were subjected to logarithmic spraying with 11 herbicides and with Poa pratensis L. as a weed. The ratio between the doses that caused 10% of damage to the crop and 90% of damage to the weed was used as a selectivity index. Compounds with selectivity indices above 2 can be safely used in a crop. The two ACCase herbicides clodinafop‐propargyl and fenoxaprop‐P‐ethyl and a mixture of the two ALS herbicides mesosulfuron and iodosulfuron could be used selectively to control P. pratensis in Festuca rubra L., although the selectivity indices in no instances were greater than the desired 2.0. CONCLUSION: The logarithmic sprayer can be a rapid screening tool for identifying compounds with favourable selectivity indices. Good experimental design is needed to alleviate rates being systematically distributed and confounded with growth rate and soil fertility gradients. Copyright © 2009 Society of Chemical Industry     

Studies of the mechanism of action of asulam in plants. Part I: Antagonistic interaction of asulam and 4-aminobenzoic acid

Studies have been carried out on the herbicidal action of asulam [methyl (4-aminophenylsulphonyl)carbamate] and sulphanilamide, alone or in association either with 4-aminobenzoic acid (4ABA) or 4, 6-diamino-1-(3, 4-dichlorophenyl)-1, 2-dihydro-2, 2-dimethyl-1,3,5-triazine (DCDT). The soaking of wheat seeds (Triticum estivum L.) for 12 h at 30°C in asulam and DCDT in a 10:1 ratio doubled the inhibition of root growth produced by soaking in asulam alone; the addition of 4ABA partially reversed the activity of asulam. Foliar applications of a mixture of asulam + DCDT (1.1 + 0.55 kg ha^?1) markedly increased the activity of asulam in susceptible wheat, wild oat (Avena fatua L.), tolerant flax (Linum usitatissimum L.), and in Stellaria media L. The activity of asulam at 1.1 kg ha^?1 was reversed by 4ABA at 2.2 kg ha^?1 by about 50% in wheat and wild oat, 82% in flax and 100% in S. media. The results indicate that asulam and sulphanilamide act by similar mechanisms in apparently inhibiting the biosynthesis of folic acid.