Physiological effects of methyl 2-[4(2,4-dichlorophenoxy)phenoxy] propanoate on oat,wild oat,and wheat

Methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoate (dichlofop-methyl) is a selective herbicide for wild oat (Avena fatua L.) control in wheat (Triticum aestivum L.). Dichlofop-methyl inhibited IAA-stimulated elongation of oat and wheat coleoptile segments by 51 and 13%, respectively, at 10 μM concentrations. Dichlofop-methyl alone had no auxin activity at concentrations of 0.1, 1.0, and 10 μM. The inhibitory effect of dichlofop-methyl was overcome partially by increasing the IAA concentration or by application of 3,6-dichloro-o-anisic acid (dicamba), a herbicide with weak auxin activity. The de-esterified free acid metabolite, 2-[4-(2,4-dichlorophenoxy)phenoxy]-propionic acid (dichlofop), at 10 μM inhibited auxin-stimulated oat coleoptile elongation by 23%, but it did not affect wheat coleoptile elongation at the same concentration. Both dichlofop-methyl and dichlofop inhibited root growth in excised shoots and seedlings of wild oat but had no effect on wheat. Dichlofop was a more effective inhibitor of root growth than dichlofop-methyl. The results suggest that dichlofop-methyl functions as a strong auxin antagonist, while the metabolite, dichlofop, inhibits root growth and development by another mechanism. The herbicidal effect of dichlofop-methyl may be the net effect of two biologically active forms of the compound each with a different mode of action acting at different sites within a susceptible plant.     

Differential effect of diclofop-methyl on fatty acid biosynthesis in leaves of sensitive and tolerant plant species

The herbicide diclofop-methyl caused an early and pronounced inhibition of the incorporation of [¹⁴C]acetate into leaf lipids of the sensitive plant species maize (Zea may L.), wild oat (Avena fatua L.), and barnyardgrass (Echinochloa crus-galli L.). With an EC₅₀ value of approximately 10^?7M inhibition was already apparent 0.5–4 hr after herbicide application. The fatty acid biosynthesis of tolerant bean (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.), and soybean (Glycine max L.) was not affected, with one exception [wheat (Triticum aestivum L.) belongs to the more tolerant species]; the inhibition of fatty acid biosynthesis, however, was in the same order of magnitude as in sensitive plants. More detailed studies showed that in wheat a recovery from inhibition of fatty acid biosynthesis occurred. Four days after herbicide application (0.18 kg diclofop-methyl/ha) in wheat normal fatty acid biosynthesis was restored, whereas in sensitive maize a 60% inhibition was maintained over the whole experimental period (8 days). The results support the view that tolerance of wheat to diclofop-methyl is based on its inactivation in leaves, whereas the tolerance of dicotyledonous species may probably lie at the level of the site of action of diclofop-methyl. In experiments with intact leaves, the inhibition of fatty acid biosynthesis resulted in an enhanced flow of [¹⁴C]acetate into organic acids and amino acids. This effect, however, was not always reproducible in experiments with leaf pieces or isolated root tips.     

The response of resistant and susceptible plants to diclofop-methyl

Wild oat (Avena fatua L.) plants sprayed at the 2-or 3-leaf stages of growth with diclotop-methyl developed chlorosis over the entire leaf blade of all leaves. The leaves became necfrotic ⁷days after spraying Shool growth was inhibited. In wheat (Triticum aesicum L cv.Waldron) discrete chlorotic areas developed only where the herbicide convicted the 2nd or 3rd leaf with no visible injury so new growth uf'ter treutment. Growth inhibition of susceptible oat (Avena sativa L. cv. Garry) was sensitive to placement of diclutop-methyl near the upica and meristematic sites of the plant. Chlorosis and necrosis were independent of herbicide placement. Selective herbicide placement induced chlorosis only or both chlorosis and growth inhibition Root growth in wild oat and barley (Hordeum rulgare L. cv. Dickson) was strongly inhibited by 1–0 μM diclofop-methyl. Wild oat shoots were killed when seedlings were root-treated with 10 μM diclofop-melhyl. The 100 μM rool treatment killed barley shoots but only stunted the growth of wheat shoots by approximately 50%. In root-ireated wheat plants the shoots were turgid and developed a light purple colour, whereas in foliar-treated plants the shoots developed discrete chlorotic areas.     

Potassium uptake in atrazine-treated roots of sensitive and resistan plants

The action of atrazine and its biodegradation products on the membrane transport of potassium in roots was evaluated in both sensitive and resistant plants. Excised roots of maize and oat showed inhibition of potassium uptake efficiency in the presence of 1.4 × 10^?4M atrazine and 1.4 × 10^?4M deethylated atrazine. Other biodegradation products such as 2-chloro-4-amino-6-ethylamino-1,3,5-triazine,2-chloro-4,6-,bisamino-1,3,5-triazine, and 2-chloro-4-amino-1,3,5-triazine showed no inhibitory effect on the K⁺ uptake capacity. Two maize hybrids showing different uptake efficiency were inhibited differently by atrazine. We suggest that atrazine and deethylated atrazine inhibited the K⁺ transport interacting directly with the plant cell membranes without discerning between resistant and sensitive plants.     

Effects of herbicidal carbamates on mitochondria and chloroplasts

At concentrations near 2 × 10^?4M, barban, chlorpropham, and phenmedipham are inhibitors of the electron transfer in potato and mung bean mitochondria. The inhibition seems to be localized in the flavoprotein region. It affects preferentially the exogenous NADH dehydrogenation, in potato mitochondria (I₅₀, 10^?4M). Succinate dehydrogenation is less inhibited. At noninhibiting concentrations, the studied carbamates cannot uncouple the oxidative phosphorylations. Photosynthesis is completely inhibited by 2.10^?7M phenmedipham, 5 × 10^?5M barban, and 2 × 10^?4M chlorpropham. The inhibition takes place at the PS II level. Moreover, barban and chlorpropham are uncouplers of the photophosphorylations for concentrations between 5 × 10^?5 and 5 × 10^?4M. The effects observed on mitochondrial respiration can also be found on respiration of Acer cultured cells. The effects on isolated chloroplast photosynthesis are also observed for slightly higher concentrations on cultured Chlorella and on pea and oat leaf fragments.     

Control of Avena fatua L. and Cirsium arvense (L.) Scop. with mixtures of 3,6-dichloropicolinic acid and four herbicides for control of A. fatua

Control of Avena fatua (L.) (wild oat) with diclofop methyl applied at 0·7 kg ha^?1 at the two-leaf stage and difenzoquat at 0·84 kg ha^?1 at the four-leaf stage in wheat (Triticum aestivum L.) under field conditions was good and not affected when either of these herbicides was mixed with 3,6-dichloropicolinic acid as the monoethanolamine salt at 0·14, 0·20 or 0·30 kg ha^?1. In the glasshouse, mixtures containing 3,6-dichloropicolinic acid at rates as high as 0·6 kg ha^?1 also did not affect control of A. fatua. When barban at 0·35 kg ha^?1, or flamprop methyl at 0·56 kg ha^?1 was mixed with similar rates of 3,6-dichloropicolinic acid and applied at the two-leaf and four-leaf stage of A. fatua respectively, a reduction in control of A. fatua (antagonism) occurred under both field and glasshouse conditions. The herbicides for control of A. fatua did not influence the fresh weight suppression of C. arvense shoots obtained in the glasshouse with 3,6-dichloropico-colinic acid at 0·3 kg ha^?1. Early tolerance of wheat (cv. Neepawa) was acceptable with all mixtures. Wheat yields with diclofop methyl or difenzoquat alone or in mixture with 3,6-dichloropicolinic acid were increased over the yields from the A. fatua-infested control.     

Selective control of graminaceous weeds in spring wheat

Infestation of wheat fields in Israel by graminaceous weeds is increasing due to effective control of broad-leaf weeds and lack of crop rotation. The main graminaceous weeds are wild oat (Avena sterilis L.) and wild canarygrass (Phalaris paradoxa L. andPh. brachystachys L.). In eight experiments carried out in wheat fields during 1975–1977, wild oat was effectively controlled by post-emergence applications of barban, isoproturon and difenzoquat, while wild canarygrass was controlled by post-emergence applications of barban, isoproturon and metha-benzthiazuron.     

Metabolism of substituted diphenylether herbicides in plants. I. Enzymatic cleavage of fluorodifen in peas (Pisum sativum L.)

A “soluble” glutathione S-transferase that catalyzes the cleavage of the herbicide, 2,4′-dinitro-4-trifluoromethyl diphenylether (fluorodifen), was isolated and partially characterized from epicotyl tissues of pea seedlings. A 32-fold purification of the enzyme was achieved by differential centrifugation, ammonium sulfate precipitation, Sephadex gel filtration, and DEAE-cellulose ion exchange chromatography. The enzyme had a pH optimum of 9.3–9.5 and was specific for reduced glutathione, with an estimated apparent K_m value of 7.4 × 10^?4M. Limited specificity studies with four substituted ¹⁴C-labeled diphenylether compounds indicated that fluorodifen was the only effective substrate, with an estimated apparent K_m value of 1.2 × 10^?5M. Differences and similarities between the pea epicotyl enzyme and other plant and animal glutathione S-transferases were discussed from the standpoint of substrate specificity, pH optima, distribution, stability, and inhibitor studies.     

Herbicide-resistant crops: yield penalties and weed thresholds for oilseed rape (Brassica napus L.)

An expérimental procedure was designed to provide a simple model for types of analyses necessary to determine weed density thresholds for advantageous use of crop plants engineered for herbicide resistance. Oilseed rape (Brassica napus L., cv. Tower) biotypes resistant (RES) and susceptible (SUS) to atrazine were used as model crop plants, and wild oat (Avena fatua L.) was used as the model weed. Along a wild oat density gradient equivalent to 0–128 plants m^?2, RES plants consistently experienced biomass and yield reductions of approximately 10–20% compared to SUS plants. When atrazine was applied at 1.5 kg ha^?1 to control wild oats competing with RES plants, RES biomasses and yields were stabilized at the same level as that where 25–30 wild oats m^?2 reduce yields of SUS plants. This implies that with wild oat densities of 25–30 plants m^?2, it becomes agronomically advantageous to crop with RES plants plus atrazine rather than to crop with higher-yielding SUS plants.     

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.     

Effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on photosynthesis and respiration of Nitella dactyl cells

Long-term experiments with dactyl cells of Nitella flexilis showed that the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) at a concentration of 1 × 10^?5M affected not only O₂ evolution in the light but also O₂ uptake in the dark. The inhibition of O₂ production was transitory, but dark respiration did not recover. DCMU induced the formation of giant mitochondria which disappeared before cell death. It was concluded that the algicidic effect of 1 × 10^?5M DCMU on N. flexilis, but not necessarily the elongation of mitochondria, was due to the inhibition of mitochondrial respiration and not of photosynthesis.     

Binding of mercury compounds to the chloroplasts in relation to the inhibition of the Hill reaction

The binding behavior of mercuric chloride (HgCl₂), phenylmercuric acetate (PMA), and ethylmercuric chloride (EMC) to the spinach chloroplasts in relation to the inhibition of the Hill reaction was studied at pH 6.8 and 7.8 using ²⁰³Hg labeled compounds. The pH of the reaction medium did not influence the amount of mercury binding of the chloroplast at various mercurial concentrations, but it altered the inhibition curve of the Hill reaction. Between 0–1 × 10^?5M the binding of Hg²⁺ and EMC were similar and increased linearly with the concentration, while the binding of PMA was similar to the binding of Hg²⁺ only at a concentration below 4 × 10^?6M and was less when the concentration was above 4 × 10^?6M. However, the inhibition of the Hill reaction by these mercury compounds was quite different; at pH 7.8, the I₅₀ values for Hg²⁺, PMA, and EMC were 5 × 10^?6, 2.5 × 10^?6, and 2.5 × 10^?6M, respectively, while at pH 6.8, these values were 4 × 10^?6, 4 × 10^?5, and 2 × 10^?4M, respectively. The differential block of electron flow by the mercury compounds at pH 6.8 and 7.8 was further confirmed by electron spin resonance study.     

Effect of alachlor on Avena seedlings: Inhibition of growth and interaction with gibberellic acid and indoleacetic acid

The growth of Avena seedlings grown in sand was found to be inhibited by alachlor with the time of onset of inhibition after treatment being a function of herbicide concentration. There was a 12 hr lag period following a subirrigation with 2.5 × 10^?4M alachlor before growth inhibition could be detected. This lag period may be due to uptake and translocation of alachlor from the roots to the site of inhibition or to the exhaustion of certain growth-limiting substance(s) whose biosynthesis is inhibited by alachlor. Additions of gibberellic acid by subirrigation simultaneously with alachlor or after alachlor treatment did not prevent growth inhibition. However, treatment with 10^?3M gibberellic acid 24 hr prior to alachlor treatment overcame the alachlor inhibition. On the other hand, in contrast to gibberellic acid, indoleacetic acid did not prevent inhibition by alachlor.     

Gibberellin influence on membrane disruption by thiocarbamate herbicides

Vernolate (0, 8, 16, 31, 62, 125.0, or 250.0 ppbw) incorporated into sand inhibited the growth of wheat (Triticum aestivum L. cv Holley) at 125.0 ppbw. These growth inhibition and morphological responses were virtually identical to wheat response to EPTC at 125 ppbw. ¹⁴C from vernolate (carbonyl labeled) (125.0 ppbw) was absorbed into wheat seedlings at approximately 1.8 μM on the presumption that the ¹⁴C present was [¹⁴C]vernolate. Since the response of wheat to the thiocarbamate herbicides resembles a gibberellic acid (GA) deficiency and cell enlargement requires the presence of functional plasmalemmas and tonoplasts, the question of membrane disruption by excessive concentrations of thiocarbamate herbicides and potential reversal thereof by GA₃ was studied by measuring the efflux of K⁺, Na⁺, and Mg²⁺. GA₃ (0.003 μM) stimulated lettuce leaf disc growth in diameter and fresh weight. This GA-stimulated increase in size and weight was reversed by 1 mM EPTC. Betacyanin efflux from beet leaf tonoplasts was increased by 1 mM EPTC and this efflux was not reversed by exogenous GA₃ (0.3 μM). This influence by supraoptimal EPTC concentrations was shown to be via membrane disruption, which obviated any possible GA influence by eliminating the functionality of the membranes requisite to the development of a GA response. It is concluded that viable mode-of-action studies must measure physiological responses consistent with the symptomology of herbicide responses normally observed with each herbicide at field concentrations.     

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.     

Acetylcholinesterase of Aphis citricola: Properties and significance in determining toxicity of systemic organophosphorus and carbamate compounds

In apterous adults of the spirea aphid, Aphis citricola van der Goot, the optimum conditions for determining acetylcholinesterase (AChE) activity consist of reaction mixture of 0.1 M phosphate buffer (pH 7.5), 10^?3M acetylthiocholine (ASCh), and enzyme extract equivalent to 80 ± 3 μg protein incubated for 15 min at 30°C. The K_m value for ASCh (6.7 × 10^?5M) was much lower than that of butyrylthiocholine (BuSCh) (1.25 × 10^?2M). The enzyme activity was almost completely inhibited by 10^?6M paraoxon or 10^?5M eserine and was 84% inhibited by 10^?5M BW284C51 (a specific AChE inhibitor). DTNB was found to inhibit the enzyme activity and was therefore added at the end of the reaction. AChE activity of A. citricola was inhibited in vitro and in vivo by dimethoxon > dimethoate, and aldicarb sulfoxide > aldicarb > aldicarb sulfone. The in vivo effect correlates well with the toxicity level of the various toxicants. A neurotoxicity index which combines both mortality and in vivo inhibition of the aphid AChE activity is suggested as a measure for determining the toxicity of organophosphorus and carbamate compounds toward aphids.     

Aryl N-methoxy-N-methylcarbamates: Potent competitive reversible inhibitors of cholinesterases

A series of 27 substituted aryl N-methoxy-N-methylcarbamates were synthesized and their ability to reversibly inhibit house fly-head and bovine-erythrocyte acetylcholinesterase and horse-serum cholinesterase was determined. These compounds were all competitive, reversible inhibitors of bovine erythrocyte acetylcholinesterase but some of them showed mixed competitive inhibition against the house fly-head and horse-serum enzymes. Dissociation constants (K_i) as small as 9.9 × 10^?9M and as large as 1.4 × 10^?4M were observed. A highly satisfactory correlation between log K_i for the inhibition of fly-head acetylcholinesterase by the N-methoxy-N-methylcarbamates and ?log I₅₀ for the inhibition of the same enzyme by the corresponding methylcarbamates was noted. Analysis of the anticholinesterase data by multiple regression showed -log K_i to be related to Hansch's π constant and ring position terms. The results indicate that reversible binding of these compounds to acetylcholinesterase occurs by hydrophobic bonding.     

Differential effects of isoproturon on the photosynthetic apparatus and yield of two varieties of wheat and L. rigidum

The effect of isoproturon on the ultrastructure of the photosynthetic apparatus, ribulose bisphosphate carboxylase activity, protein and chlorophyll content, and the grain yield was investigated in two wheat cultivars (Triticum sativum L. cvs Castan and Esquilache) and a weed (Lolium rigidum Gaud.). Field experiments used applications of 1–65 and 2–5 kg a.i. ha^?1 isoproturon post-emergence, and growth chamber experiments used nutrient solution with the addition of isoproturon (1·7 × 10^?4 M). The ultrastructure of the photosynthetic apparatus of the cv. Esquilache was much affected by the herbicide. In the case of cv. Castan, slight disorganization of the grana and intergrana was observed. Isoproturon decreased the activity of ribulose bisphosphate carboxylase. A decrease in protein and chlorophyll content was also observed in the cv. Esquilache and in L. rigidum. These alterations were much less evident in the cv. Castan, where, moreover, no loss of protein occurred. The yield of the treated cv. Castan plants was slightly greater than that of the control plants in two consecutive years. However, the yields of the cv. Esquilache were significantly less when the herbicide was applied in the first year at commencement of tillering in a dry season but not when applied at an advanced stage as in the second year in a wet season.     

The action of insecticides on neurosecretory neurons in the stick insect, Carausius morosus

The action of insecticides on the spontaneous electrical activity of neurohemal tissue in the stick insect, Carausius morosus, has been studied using extracellular electrodes. The pyrethroid, permethrin, causes a massive increase in the frequency of the spontaneously generated action potentials at concentrations between 5 × 10^?5 and 5 × 10^?8M. Concentrations as low as 5 × 10^?11M are still effective in producing bursting activity.DDT, at concentrations between 5 × 10^?5M and 5 × 10^?6M, produces an overall increase in activity although the bursting activity is less violent than that shown with permethrin. DDT, 5 × 10^?7M, is ineffective at altering the resting pattern.Carbaryl and coroxon cause a transitory increase in electrical activity at 1 × 10^?4M, but are ineffective at 1 × 10^?5M.It is concluded that insecticides could have a direct effect upon the neurohormonal balance in insects.     

Response of wheat and oat seedlings to root-applied diclofop-methyl and 2,4-dichlorophenoxyacetic acid

Roots of wheat and oat seedlings were treated with diclofop-methyl (methyl 2-[4-(2′,4′-dichlorophenoxy)phenoxy]propanoate) in a specially designed Plexiglas treatment apparatus. Diclofopmethyl severely inhibited the root growth of susceptible oat seedlings but roots of resistant wheat seedlings were unaffected. Diclofop-methyl at 0.3 μM reduced the growth of oat roots to 50% of the control. Direct contact between diclofop-methyl and the inhibited root zone was necessary for growth inhibition since other parts of the seedling (roots and shoots) isolated from contact with diclofop-methyl solution by a physical barrier were unaffected. Diclofop (2-[4-(2′,4′-dichlorophenoxy)phenoxy]propionic acid), the free acid metabolite of diclofop-methyl, was somewhat more phytotoxic than the parent compound. The herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), which engenders auxin responses, slightly enhanced the inhibition of oat root growth by diclofop-methyl. The primary wheat metabolite, ring-hydroxylated diclofop, was nonphytotoxic to oat root growth, whereas the acetylated derivative of the primary water-soluble oat metabolite (neutral glucose ester of diclofop) inhibited oat root growth to the same extent as diclofop-methyl. These results support the hypothesis that the basis for selectivity between resistant wheat and susceptible oat is the metabolism of diclofop-methyl by aryl hydroxylation and conjugation but not glucose ester conjugation. Translocation is also not an important factor in the phytotoxic activity of diclofop-methyl.