Studies on the mode of action of asulam in bracken (Pteridium aquilinum L. Kuhn) II. Biochemical activity in the rhizome buds

The effect of asulam (methyl (4-aminobenzenesulphonyl) carbamate) on the synthesis of RNA and protein was investigated in bracken sporeling plants and excised rhizome bud tissue. Foliage application of asulam (4.4 kg/ha) reduced the RNA levels in frond buds and young fronds within 3 days, while protein levels were significantly reduced after 14 days. A significant reduction in respiratory activity of buds was observed after 2 weeks, the level of inhibition being 54% after 8 weeks. During a 3-h incubation period, O₂ uptake by excised bud issue was stimulated by 5 and 10 ppm asulam and inhibited by higher concentrations; ₃₂P uptake was inhibited at all concentrations. Asulam (5 ppm and above) inhibited bud growth and reduced RNA and protein levels in incubated buds (20 h at 30°C), and the incorporation of [₁₄C]orotic acid into RNA and [₁₄C]leucine into protein. Reduction of RNA levels and inhibition of [₁₄C]ladenine incorporation into RNA in buds occurred entirely in the ribosomal and supernatant fractions of the cellular extract. Inhibition of RNA synthesis by asulam (50 ppm) as measured by [₁₄C] orotic acid incorporation into RNA was completely antagonized by CEPA (3-chloroethylphosphonic acid) (50 ppm) and partially by 2,4-D (2,4-dichlorophenoxyacetic acid) (50 ppm) and GA (Gibberellic acid) (50 ppm). These results suggest that the interference of asulam with RNA and protein synthesis at the metabolically active sinks (rhizome buds) could be one of its major mechanisms of action in bracken.     

Studies on the mode of action of asulam,aminotriazole and glyphosate in Equisetum arvense L. (field horsetail). I: The uptake and translocation of [14c]asulam, [14C]aminotriazole and [14C]glyphosate

The uptake and translocation of [¹⁴C]asulam (methyl 4-aminophenyl-sulphonylcarbamate), [¹⁴C]aminotriazole (1-H-1,2,4-triazol-3-ylamine) and [¹⁴C]glyphosate (N-(phosphonomethyl)glycine) were assessed in Equisetum arvense L. (field horsetail), a weed of mainly horticultural situations. Under controlled-environment conditions, 21°C day/18°C night and 70% r. h., the test herbicides were applied to 2-month-old and 2-year-old plants. Seven days following the application of 0.07-0.09 °Ci (1.14mg) of the test herbicides to young E. arvense, the accumulation of ¹⁴C-label (as percentage of applied radioactivity) in the treated shoots, untreated apical and basal shoots was as follows: [¹⁴C]asulam, 13.2, 0.18 and 1.02%; [¹⁴C] aminotriazole, 67.2, 3.65 and 1-91%; [¹⁴C]glyphosate, 35.9, 0.06 and 0.11%. The equivalent mean values for the accumulation of ¹⁴C-label in 2-year-old E. arvense were [¹⁴C]asulam, 12.0, 1-15 and 1.74%; [¹⁴C]aminotriazole, 58.6, 9.44 and 4.12%; [¹⁴C]glyphosate, 33.1, 0.79 and 2.32%. In the latter experiment, test plants received 0.25-0.30 °Ci (4mg) of herbicide, they were assessed after a 14-day period and the experiment was carried out at 3-week intervals between 2 June and 25 August on outdoor-grown plants. Irrespective of test herbicide or time of application, very low levels of ¹⁴C-label accumulated in the rhizome system. Only 0.2% of the applied radioactivity was recovered in 2-year-old plants and 0.4% in 2-month-old plants. In the young plants [¹⁴C]asulam accumulated greater amounts and concentrations of ¹⁴C-label in the rhizome apices and nodes than [¹⁴C]aminotriazole or [¹⁴C]glyphosate treatments. Inadequate control of E. arvense under field conditions may be due to limited basipetal translocation and accumulation of the test herbicides in the rhizome apices and nodes.     

Effect of non-ionic nonylphenol surfactants on surface physicochemical properties, uptake and distribution of asulam and diflufenican

The effect of non-ionic nonylphenol (NP) surfactants containing 4–14 ethylene oxide (EO) molecules on the distribution of asulam and diflufenican was investigated in Pteridium aquilinum L. Kuhn and Avena fatua L. The distribution of the herbicides was dependent on the EO content and concentration of surfactant and differed between plant species and herbicide. The surface properties of contact angle, droplet diameter and surface tension were examined. For solutions of asulam, the greatest reductions in contact angle, surface tension and greatest droplet diameter were obtained with surfactants of EO 6.5–10 (at 0.001–0.1%). For solutions of diflufenican, these responses were greatest when applied with surfactant of EO 4. Surfactants of EO 6.5–10 increased the uptake and translocation of [¹⁴C]asulam in P. aquilinum, particularly at surfactant concentrations of 0.01 % and 0.1 %. All surfactants increased uptake of [¹⁴C]asulam in A. fatua with no significant effects of surfactant EO number or concentration. For both species, there was a positive correlation between the optimum surface characteristics of the herbicide droplets and the uptake of asulam. With diflufenican, greatest uptake and translocation by mature frond tissue of P. aquilinum occurred at the highest concentration of surfactant EO 4; in A. fatua, however, uptake and translocation were not significantly affected by any of the surfactants.     

Control of bracken (Pteridium esculentum G. Forst (Cockayne)) in pasture with asulam

Asulam was tested for the control of bracken in non-arable pasture on the North Coast of New South Wales between 1971 and 1976. Satisfactory results were obtained with applications between April and early July, providing treated fronds were mature. The degree of control varied between years and sites. Rates of 4–8 kg a.i./ha reduced frond numbers from 70 to 100% after 6 months but recovery commenced as early as 9–12 months after application. Whole plants and sections of rhizome were excavated 4, 6 and 9 months after treatment and a sequence of events is described for the effect of asulam on bracken. The effects of frond maturity, frond density and frost are discussed in relation to herbicide effectiveness and ecological implications of the results are discussed in relation to increasing productivity of bracken-infested pasture.     

PRELIMINARY STUDIES ON THE TRANSLOCATION OF 14C-LABELLED HERBICIDES IN BRACKEN (PTERIDIUM AQUILINUM)

Summary. The translocation of ¹⁴C-labelled herbieides in field bracken has been sludied in some preliminary experiments by means of autoradiography. A detailed account is given of the methods employed. It is shown that when 2,4-D is applied to fronds at different stages of development movement of the tracer into the rhizome is considerably greater from parts of the frond which are still immature. Trasnslocation within the frond is predominantly basipetal at all stages of development; within the rhizome the direction of movement appears to be determined primarily by the nature of the vascular connections with the treated pinna.
In a comparison between the translocation of 2,4-D, 4-CPA and dalapon no difference was apparent after 24 hours, but when the treatment period was extended to 96 hours there was evidence that 4-CPA bad been translocated in a considerably greater amount than either of the other two herbicides.
Rechrches iréliminaires sur la migration des herbieides marqués am ¹⁴ C dans la fougrère algle (Pteridium aquilinum)     

The application of picloram-14C to bracken

Experimental systems were produced from fragments of bracken rhizome and picloram-¹⁴C was applied to frond laminae, rhizome apices, frond buds and roots and translocation assessed 7 days after treatment. The isotope was readily taken up by all organs and freely translocated to associated fractions of the rhizome except in the case of laminae from which distribution was very poor. Accumulation of activity in the roots was considerable following treatment of the frond buds but was limited in the frond buds. Poor translocation of herbicide from treated frond laminae is considered a possible explanation of poor control in the field when bracken is sprayed in July. Lapplication du picioram e¹⁴C à la fougère Des systèmes expérimentaux ont étéétablis en utilisant des fragments de rhizome de fougére et du piciorame ¹⁴C a été appliqué sur les limbes des frondes, les apex des rhizomes, les bourgeons des frondes et les racines. La migration fut évaluée sept jours aprés le traitement. L'isotope a été facilement absorbé par tous les organes et librement transporté aux fractions correspondantes du rhizome, sauf dans le cas des limbes è partir desquels la distribution a été très faible. L'accumulation de ractivité dans les racines a été considérable è la suite du traitement des bourgeons de frondes mais a été Iimitée dans les bourgeons de frondes. Une faible migration de I'herbicide depuis les limbes des frondes est considérée comme une explication possible du désherbage mediocre auchamp, lorsque la fougère est traitée en juillet. Die Anwendung von PicIoram-¹⁴C zu Adlerfarn Aus Rhizomstücken von Adlerfarn wurden Versuchsp-flanzen gezogen und auf die Wedelspreiten, Rhizomapices, Wedelknospen und Wurzein- Picloram-¹⁴C appliziert. Die Transiokation des Herbizids wurde eine Woche nach der Behandlung gemessen. Das lsotop wurde von alien Pflan-zenorganen schnell aufgenommen und in die dem Rhizom benachbarten Pflanzenteile transloziert; bei Behandlung der Wedelspreiten war jedoch nur eine geringe Verteilung fest-zustellen. Nach Behandlung der Wedelknospen war in den Wurzein eine beträchtliche, aber in den Wedelknospen nur eine geringe Aktivitätsanreicherung feststellbar. Für den schwachen Bekämpfungserfolg des Adierfarns im Freiland bei Spritzungen im Juli, wird die geringe Transiokation des Herbizids aus den Wedelspreiten als mögliche Erklärung angesehen.     

Studies in the growth of Pteridium aquilinum (L.) Kuhn. (bracken) 1. Regeneration of rhizome segments

The patterns of regeneration of Pteridium aquilinum (L.) Kuhn. (bracken) rhizome segments grown in pot culture are described. The overall capacity for regeneration was unaffected by the rhizome type planted, i.e., whether it consisted of only a length of frond-bearing ‘short shoote’ or whether this was attached to part of the main storage and exploratory ‘long shoot’. In all cases rhizomes extended, produced new lateral buds and developed fronds during the first summer. Regenerative capacity was also similar for segments with or without apical buds. Different patterns of growth were observed in plants grown from different types of segment: in particular, more new lateral buds were produced on rhizome segments originally lacking an apex. As rhizomes extended, the distance between successive lateral buds increased. The results are discussed in relation to the possible roles of correlative inhibition and patterns of translocation between fronds and rhizomes and to information on field populations.     

Selectivity factors relative to asulam for Senecio jacobaea L. control in Medicago sativa L. II. Metabolism*

Metabolism of ¹⁴C asulam applied with surfactant was studied in Senecio Jacobaea L. and Medicago saliva L. Plants were harvested 48, 96 and 144 h after treatment and extracted with acetone. The aqueous residue of the acetone extract was partitioned with ethyl acetate and the ¹⁴C activity in the ethyl-acetate phase, the aqueous phase and the plant residue was determined. A significant amount of ¹⁴C activity was not extracted by acetone from either species. This amount increased with time in M. sativa but remained relatively constant in S. jacobaea. More ¹⁴C activity was found in the aqueous phase than in the ethyl-acetate phase in M. sativa while the reverse was true in S. jacobaea. Significantly lower amounts of free asulam were identified in M. sativa than in S. jacnbaea. Still, results of these and previous studies on retention, uptake and translocation do not completely account for differences in sensitivity found in greenhouse and field applications. Other possible explanations for selectivity are discussed.     

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.     

Studies on the mode of action of asulam in bracken (Pteridium aquilinum L. Kuhn). III. Longterm control of field bracken.

The longterm control of bracken (Pteridium aquilinum) following applications of asulam was studied for 5 years al a site in the West of Scotland; there was no post-spray management. The time of treatment was found to be of considerable importance, satisfactory long-term control being obtained only when 4.4–8.8 kg/ha were applied around the stage of full frond expansion (26 July–16 August). Application of 4.4 kg/ha asulam on 26 July decreased frond density by 99, 95, 85, and 73% in the 4 years following treatment; about 50% recovery was observed in the fifth year. The latter may be partially due to the encroachment of rhizomes from the adjacent untreated plots and the lack of post-spray management. Examination of rhizomes dug from 1 m² areas showed that asulam killed more than 90% of the active frond buds and apices in the first year, but left undamaged 48% of the dormant buds on the deeper rhizome branches; the latter appeared to be the main source of reinfestation of treated plots in subseqquent years. The total number of living buds increased from 17% of the untreated in the first year to 38% in the fifth year; the majority of these buds were dormant. Formulation of asulam (4.4 kg/ha) with 1% ethylan CP only improved the reduction in frond density over a 4-year period by 10%. Recherches sur le mode d'action de l'asulame sur la fougère (Pteridium aquilinum L. Kuhn). III, Elimination durable de la fougère. Lélimination durable de la fougére (Pteridium aquilinum) a la suite d'applications d'asulame a éte etudiee pendant 5 annees dans une station de I'ouest de I'Ecosse; il n'y a pas eu d'autre intervention apres le trailement. W a ete constate que Vepoque du traitement avait une importance considerable, une elimination durable n'ayant pu etre obtenue qu'en appliquant 4,4 a 8,8 kg/ha d'asulame au stade de plein developpement des frondes (26 juillet-16 aout), L'application de 4,4 kg d'asulame le 26 juillet diminua la densite des frondes de 99, 95, 88, et 73% au cours des 4 annees suivant le traitement; une repousse d'environ 50% fut observee la cinquieme annee, Ce resultat final peut etre du en partie a I'empietement de rhizomes provenant des parcelles adjacentes non traitees et a I'absence d'intervention apres le traitement. L'examen des rhizomes extraits sur des surfaces de 1 m² montra que I'asulame avait tue plus de 90% des bourgeons et des apex des frondes en activite au cours de la premiere annee, mais n'avait pas endommage 48% des bourgeons dormants situes sur les ramifications les plus profondes des rhizomes; ce fait est apparu comme la cause principale de la reinfestation des parcelles traitees au cours des annees suivantes. Le nombre total des bourgeons vivants est passe de 17% des temoins la premiere annee a 38% la cinquieme annee; la majorite de ces bourgeons etaient dormants. La formulation de I'asulame avec 1% seulement d'etylan CP” a augmente de 10% la reduction de la densite des frondes au cours de la periode de 4 ans. Untersuchungen zur Wirkuttgsweise von Asulam bei Adlerfarn (Pteridium aquilinum L. Kuhn). III. Langfristige Bekdmpfung In einem Versuch in Westschottland wurde die langfristige Bekampfung von Adlerfarn (Pteridium aquilinum) nach Asulam-Behandlungen uber 5 Jahre untersucht. In diesen Versuchen wurden nach der Herbizidbehandlung keine weiteren Bekampfungsmassnahmen durchgefiihrt. Es konnte festgestellt werden, dass der Bekampfungszeitpunkt von erheblicher Bedeutung ist. Eine befriedigende langfristige Bekampfung wurde nur erzielt, wenn 4.4–8.8 kg/ha zum Zeitpunkt der vollen Wedelentwicklung (26, Juli-16. August) angewendet wurden. Eine Behandlung mit 4,4 kg/ha Asulam am 26, Juli fuhrte in den 4 Jahren nach der Applikation zu einem Ruckgang der Wedeldichte um 99, 95, 85 und 73%, und im fiinften Jahr betrug die Wedeldichte wieder etwa 50% der ursprunglichen. Das letztere lasst sich teilweise mit dem Eindringen von Rhizomen aus benachbarten unbehandelten Parzellen und dem Fehlen weiterer Massnahmen mach der Herbizidbehandlung erklaren. Die Untersuchung von I m² grossen Flachen zeigte, dass durch Asulam im ersten Jahr iiber 90% der aktiven Wedelknospcn und -apices getotet wurden, aber 48% der dormanten Wedelknospen und -apices der tieferliegenden Rhizomstijcke unbeeinfiusst blieben. Diese Organe scheinen die Hauptursache der Wiederverseuchung in den folgenden Jahren gewesen zu sein. Die Gesamtzahl an lebenden Knospen nahm, bezogen auf Unbehandelt, von 17% im ersten Jahr auf 38% im fiinften Jahr zu, die meisten Knospen waren aber dormant. Zusatz von 1% Ethylan CP zu Asulam (4,4 kg/ha) verbesserte im Beobachtungszeitraum von 4 Jahren die Wirkung (Verminderung der Wedeldichle) nur um 10%.     

Selectivity factors relative to asulam for Senecio jacobaea L. control in Medicago sativa L. I. Retention,uptake and translocation*

Senecio jacobaea L. and Medicago sativa L. plants grown in a glasshouse were treated with foliar applications of aqueous solutions of asulam. Retention on foliage, uptake and translocation were measured in both species. Retention was greater in S. jacobaea than in M. sativa when no surfactant was added and similar when surfactant was added. Addition of surfactant modified spray distribution and increased asulam uptake in M. saliva but did not in S. jacobaea. S. jacobaea translocated over twice as much asulam from the treated area as M. sativa. These data suggest that surfactant should not be added for maximum selectivity. Differences in species response to asulam treatments are partially, but not entirely, explained by differences in retention, uptake and translocation.     

Studies on the mode of action of asulam,aminotriazole and glyphosate in Equisetum arvense L. (field horsetail). II: The metabolism of [14C]asulam, [14C]aminotriazole and [14C]glyphosate

The metabolism of [¹⁴C]asulam (methyl 4-aminophenylsulphonylcarbamate), [¹⁴C] aminotriazole (1H-1,2,4-triazol-3-ylamine) and [¹⁴C]glyphosate (N-(phosphonomethyl)glycine) were assessed in Equisetum arvense L. (field horsetail). Following application of the test herbicides (4mg?0.3 °Ci herbicide/shoot) to the shoots of 2-year-old pot-grown plants, the total recovery of ¹⁴C-label after 1 week and 8 weeks was high for all three herbicides (>80-0% of applied radioactivity). Asulam was persistent (>69-7% of recovered radioactivity) in both shoots and rhizomes. Sulphanilamide, a hydrolysis product of asulam, accounted for the remainder of the recovered radioactivity. Aminotriazole showed evidence of conjugation in shoots and rhizomes. The principal ¹⁴C-labelled component in shoots was composed of high proportions of aminotriazole (>76-3%) together with the metabolites: X (ninhydrin positive), β-(3-amino-1,2,4-triazolyl-1-)α-alanine, Y (diazotization positive) and various unidentified compounds. Rhizomes generally contained lower proportions of intact aminotriazole (>59.4%) together with the metabolites X,Y and unidentified compounds. The proportion of aminotriazole did not decrease with time in shoots or rhizomes; however, the ratio of metabolite X: Y moved in favour of Y as the interval after treatment increased. Glyphosate was extensively metabolised in shoots and rhizomes to yield aminomethylphosphonic acid (AMPA) and various unidentified compounds. Differential metabolism appears to be one of the factors which may govern the persistence and toxicity of the test herbicides in E. arvense.     

Application timing of asulam for torpedograss (Panicum repens L.) control in sugarcane in Okinawa island

Field and glasshouse experiments were conducted from 1995 through 1996 to evaluate application timing of asulam (methyl sulfanilylcarbamate) for torpedograss (Panicum repens L.) control in relation to plant age in sugarcane. Above‐ground shoots of torpedograss were completely controlled with asulam at 2–4 kg active ingredient (a.i.) ha^?1 applied 60 or 80 days after planting (DAP) in artificially infested pots. But some newly developed rhizome buds survived after asulam application resulting in 1–25 and 76–100% or more regrowth in 60 and 80 DAP‐applied pots, respectively. Whereas the herbicide at 2–4 kg a.i. ha^?1 applied within 60 DAP completely controlled above‐ground shoots, applied 80 DAP at 2 kg a.i. ha^?1 it did not completely control the weed in the artificially infested field. Regrowth levels were 1–25 and 76–100% or more in 60 and 80 DAP‐applied plots, respectively. Asulam at 2–3 kg a.i. ha^?1 applied 20, 40, 60 or 80 DAP in a naturally infested field completely controlled above‐ground shoots and regrowth levels were 76–100 or more, 51–75, 1–25 and 26–50% in these same DAP applied plots, respectively. The herbicide applied at 4 kg a.i. ha^?1 caused chlorosis on younger sugarcane leaves (one‐leaf stage), but when applied at 2–3 kg a.i. ha^?1, no injury symptoms were shown. The herbicide at 2–4 kg a.i. ha^?1 applied within 60 DAP resulted in remarkably higher yield and shoot biomass of sugarcane than that applied 80 DAP. This study suggested that asulam at 2–3 kg a.i. ha^?1 should be applied 60 days after planting for the maximum control of torpedograss regrowth and better yield of sugarcane. This study also indicated that torpedograss cannot be completely controlled with a single application of asulam in a naturally infested field because of rhizome fragmentation by cross plowing and distribution of rhizomes into different soil layers that require different times to emerge. The shoots emerging after asulam application could not be controlled. Another study is required to determine the interval between sequential applications of asulam for better control of torpedograss in a naturally infested field.     

Uptake and translocation of [14C]asulam and [14C]bromacil by roots of maize and bean plants

The uptake and translocation of ¹⁴C-ring-labeled asulam (methylsulfanilcarbamate) and bromacil (5-bromo-3-sec-butyl-6-methyluracil), were compared after root application to maize (Zea mays L.) and bean (Phaseolus vulgaris L.). Autoradiographs showed the distribution of bromacil throughout these and other plant species, and the retention of asulam in the roots. The recovery of both compounds in quantitative radioassays was between 90 and 100%. The absorption of bromacil and asulam was rather similar. Absorption of bromacil increased up to 20% of the applied dose in bean plants after 2 days of exposure, and up to 11% in maize plants after 4 days. Absorption of asulam in bean plants was 22% of the applied dose after 2 days, and 8% in maize plants after 4 days. The pattern of distribution of bromacil and asulam was completely different. After 4 h of exposure of the roots about half of the absorbed bromacil had accumulated in the shoots, while two-thirds or more was translocated to the shoots after exposure periods of 1 to 4 days. Not more than one-eighth of the absorbed asulam was found in the shoots. In consequence, the bromacil content in the transpiration stream relative to that in the ambient solution was much higher than that of asulam. The leakage of asulam from bean and maize roots into herbicide-free nutrient solution was lower than that of bromacil. The reasons for these differences are not yet clear. There was only some metabolism of asulam in maize, but not in bean plants. No metabolites of bromacil were detected in the two plant species.     

The effectiveness of old and new strategies for the long‐term control of Pteridium aquilinum,an 8‐year test

There is a need for management strategies to control dominant perennial weeds and restore seminatural communities. We compared the effects of five weed control treatments on dense Pteridium aquilinum relative to an untreated experimental control over an 8‐year period with the aim of restoring acid grassland. The weed control treatments tested were as follows: cutting and bruising, both twice and thrice annually, and herbicide treatment (asulam in year 1 followed by annual spot retreatment of all emergent fronds). Pteridium aquilinum performance and plant species composition were monitored. Data were analysed using Bayesian mixed‐effect models and multivariate techniques. Cutting twice and thrice yearly and the asulam treatment all reduced frond density to zero; both bruising treatments were ineffective. The plant communities in the cut and asulam‐treated plots showed differences from the untreated and bruised plots; the asulam‐treated plots contained more ruderal species and the cut plots were more typical of acid grassland. Acid grassland recovery was fastest in the asulam‐treated plots, but the cut plots caught up after approximately 5 years. There were two important conclusions. First, an intractable weed like P. aquilinum can be eradicated and a vegetation more suited for grazing can be achieved by the continuous application of some treatments over many years. Here, success was achieved by cutting twice/thrice annually, or by a single asulam application followed by annual spot spraying of all emergent fronds for 8 years. Second, bruising, a treatment favoured by some conservation organisations, did not work and cannot be recommended. The use of long‐term, continuously applied treatments might be considered for all perennial weeds with large underground root/rhizome systems.     

Soil persistence studies with [14C]MCPA in combination with other herbicides and pesticides

The persistence of [¹⁴C]MCPA at a rate equivalent to 1 kg ha^?1 was studied under laboratory conditions in a clay loam, heavy clay and sandy loam at 85% of field capacity moisture and 20±1°C both alone and in the presence of tri-allate, trifluralin, tri-allate and trifluralin, malathion, Vitaflow DB, malathion and Vitaflow DB, bromoxynil, bromoxynil and asulam, bromoxynil and difenzoquat, dicamba, dicamba and mecoprop, linuron, MCPB, metribuzin, propanil, TCA, benzoylprop-ethyl, diclofop-methyl, and flamprop-methyl. Except in the soils treated with asulam, the half-lives of [¹⁴C]MCPA in all three soil types were similar, being approximately 13±1 days, thus indicating that none of the other chemicals studied adversely affected the soil degradation of MCPA. In the asulam treated soils, the half-lives of the MCPA were about 3 days longer than in non-asulam treated soils; the effect was most marked in the clay loam.     

Mechanism of resistance to fenarimol in Aspergillus nidulans

Mycelial uptake of [¹⁴C]fenarimol (10 μg/ml) by 20 fenarimol-resistant mutants of Aspergillus nidulans was compared with uptake by wild-type strain 003. Uptake of the fungicide during the initial 10 min of incubation was significantly lower in all mutant strains than in the wild-type strain indicating that resistance is related with reduced uptake. Upon prolonged incubation a gradual decrease of accumulated radioactivity in the wild-type strain was observed. A few mutants displayed resistance to unrelated chemicals such as p-fluorophenylalanine or d-serine; this phenomenon appeared not to be due to a decreased uptake of the corresponding natural amino acids. Incorporation of [³H]adenine and [¹⁴C]leucine by mycelium of mutant M193 was hardly inhibited after 5 hr of incubation with the fungicide, whereas a distinct effect was found with the wild-type strain. At this time also fungitoxicity to the wild-type strain became apparent. Probably, this effect is indirectly caused by inhibition of ergosterol biosynthesis. Mycelium of mutant M193 incorporated [¹⁴C]acetate slightly less effectively than the wild-type strain. After 2 hr of incubation with this radiochemical leakage of [¹⁴C]acetate metabolites from mycelium of the mutant strain was observed. This indicates that resistance might be correlated with increased excretion of fungal metabolites, which in turn may be related with reduced fitness of fenarimol-resistant mutants.     

Studies of the mechanism of action of asulam in plants. Part II: Effect of asulam on the biosynthesis of folic acid

The possible relationship between folate levels in plants and their tolerance to asulam has been examined. Plants with a high content of folates were relatively less susceptible to asulam. There was evidence of a depletion of the folates in the shoots of plants treated with asulam, either alone at 1. 1 kg ha^?1 or in combination with 4, 6-diamino-1-(3, 4-dichlorophenyl)-1, 2-dihydro-2, 2-dimethyl-1, 3, 5-triazine (DCDT) at 1.1 kg ha^?1. In wheat shoot tips (Triricum estivum L.), asulam at 1.1 kg ha^?1 reduced the levels of the N⁵-methyl, N¹⁰-formyl and N⁵-formyl derivatives of 5, 6, 7, 8-tetrahydrofolates, particularly when applied in combination with DCDT. Studies with cell-free extracts of wheat seedlings revealed that asulam inhibited the enzymic synthesis of 7, 8-dihydro-pteroateina manner similar to sulphanilamide inhibition. The biochemical site of action of asulam in plants is discussed.     

Studies into the differential activity of the hydroxybenzonitrile herbicides: II. Uptake,movement and metabolism in two contrasting species

Uptake, movement, and metabolism of unformulated ioxynil and bromoxynil salts were investigated in Matricaria inodora and Viola arvensis. The morphology of these two species did not give rise to different spray retention and contact angles. After 7 days, uptake of [¹⁴C]ioxynil-Na reached 8.26% of applied ¹⁴C activity in M. inodora and 16.77% of that in V. arvensis compared with 1.54 and 3.83%, respectively, for [¹⁴C]bromoxynil-K. Over 98% of the ¹⁴C activity detected in the plant after 7 days remained in the treated leaves of V. arvensis following [¹⁴C]ioxynil-Na treatment. However, 8.7% of the ¹⁴C activity detected in [¹⁴C]ioxynil-Na-treated M. inodora was recovered from the apex and developing leaves reflecting a greater translocation. [¹⁴C]Bromoxynil-K was more mobile in both species and after 7 days 87.5 and 91.39% were detected in the treated leaves of M. inodora and V. arvensis, respectively. In both species the majority of translocated ¹⁴C activity was recovered from the apex and developing leaves. Up to 20% of the applied [¹⁴C]ioxynil-Na and [¹⁴C]bromoxynil-K was not detected within the treated plant. Extraction of treated plants revealed no detectable metabolic breakdown of ioxynil-Na to halogenated derivatives in either species. However, metabolic breakdown of bromoxynil-K was apparent in V. arvensis. No significant root exudation was detected when [¹⁴C]ioxynil-Na and [¹⁴C]bromoxynil-K were applied to hydroponically grown S. media and V. arvensis. Losses of ¹⁴C activity were due to herbicide volatility or degradation to volatile products on the leaf surface.     

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.