Photosynthesis Involvement in the Mechanism of Action of Diphenyl Ether Herbicides

Abstract

ABSTRACIPhotosynthesis is not required for the toxicity of diphenyl ether herbicides, nor are chloroplast thylakoids the primary site of diphenyl ether herbicide activity. Isolated spinach (Spinacia oleracea L.) chloroplast fragments produced malonyl dialdehyde, indicating lipid peroxidation, when paraquat (1,1'-dimethyl4,4'-bipyridinium ion) or diuron 13-(3,4-dichlorophenyl)-I,I-dimethylureaI were added to the medium, but no malonyl dialdehyde was produced when chloroplast fragments were treated with the methyl ester of acifluorfen (methyl 5-12-chloro-4-(trifluoromethyl)phenoxy-2-nitrobenzoic acid), oxyfluorfen 12-chloro-l-(3-ethoxy4-nitrophenoxy)4-(trifluoromethyl)benzenel, or MC15608(methyl 5-12-chloro44trifluoromethyl)phenoxyj2-chlorobenzoate). In most cases the toxicity of acifluorfen-methyl, oxyfluorfen, or MC15608 to the unicellular green alga Chiamydomonas eugametos (Moewus) did not decrease after simultaneous treatment with diuron. However, diuron significantly reduced cell death after paraquat treatment at all but the highest paraquat concentration tested (0.1 millimolar). These data indicate electron transport of photosynthesis is not serving the same function for diphenyl ether herbicides as for paraquat. Additional evidence for differential action of paraquat was obtained from the superoxide scavenger copper penicillamine (copper complex of 2-amino-3-mercapto-3-methylbutanoic acid). Copper penicillamine eliminated paraquat toxicity in cucumber (Cucumis sativus L.) cotyledons but did not reduce diphenyl ether herbicide toxicity. DPE3 herbicides initiate lipid peroxidation and eventually disrupt cell membranes causing cell death (7,13,18 initiating lipid peroxidation of the polyunsaturated fatty acids in the membrane of Scenedesmus acutus.The purpose of this study was to determine if photosynthesis is involved in DPE herbicide membrane disruption and cell death as it is for cell death by paraquat and diuron (5). This research used isolated spinach chloroplast fragments, the unicellular green alga Chlamydomonas, and excised cucumber cotyledons to evaluate the requirement of photosynthesis for DPE caused cell death. MATERIALS AND METHODSIsolated Chloroplast Fragments. Spinach chloroplast fragments were prepared by the method ofTakahama (24) with some modification. Large veins were removed from 25 to 30 g of commercially grown spinach leaves, and then the leaves were cut into 1 cm2 pieces. Leaf pieces were blended 30 s in a Waring Blendor with 125 ml of 17 mM Tris-HCI buffer (pH 7.4). The debris was filtered through eight layers of cheesecloth and centrifuged at 2000g for 1 min in a Sorvall HB-4 rotor. The supernatant was centrifuged 10,000g for 10 min, and the pellet was resuspended in 17 mm Tris -HCI buffer (pH 7.4) which was again centrifuged at 10,000g for 10 min. The chloroplast fragments in the pellet were resuspended in 17 mM Tris-HCI buffer, pH 7.4. All isolations were conducted at 4C, and Chl concentrations were determined using the absorption coefficients for Chl a and b as given by MacKinney (...

Influence of Chloroplast Development on the Activation of the Diphenyl Ether Herbicide Acifluorfen-Methyl

“…The results of our studies on factors influencing the activity ofAFM support the conclusions ofearlier investigations that photosynthetic electron flow is not required to initiate DPE mediated lipid peroxidation and that the pigments in situ, in etiolated tissue, probably the carotenes, are sufficient to activate these herbicides and bring about significant membrane disruption (4,7,15,20). However, our experiments also indicate a previously unappreciated role for biosynthetic processes in the full optimization of the herbicidal activity.…”

“…One ofthe more frustrating anomalies of investigations of the DPE herbicides has been the failure of these compounds to readily stimulate appreciable lipid peroxidation in in vitro chloroplast systems despite the utilization of thylakoids isolated from fully greened leaves, which contain all the pigments heretofore considered necessary for photoactivation (7). Furthermore, we had observed that green oat protoplasts, which were bleached by treatments with paraquat, did not seem to be susceptible to AFM (data not shown).…”

“…In isolated chloroplasts, the propensity of 4'-nitro DPEs for stimulating a Mehler reaction was found to differ significantly from the responses to paraquat (25). Furthermore, 4'-C1 analogs which do not appear capable of reduction by biologically generated potentials, are also effective membrane disruptors (21,22,25 (4,7,15,20). However, our experiments also indicate a previously unappreciated role for biosynthetic processes in the full optimization of the herbicidal activity.…”

Mode of Action Studies on Nitrodiphenyl Ether Herbicides

“…These results with Scenedesmus contrast with the effects of DCMU (4,8,9) or the absence of PSI or PSII (4) on DPE toxicity in other systems. In higher plants, these measurements and studies on photosynthetically incompetent cucumber seedlings (8) indicate that photosynthetic electron transport is not required to elicit DPE toxicity.…”

“…This observation was taken as evidence supporting the hypothesis that the initial event in DPE toxicity was its reduction to a radical by PSI. However, while Matringe et al (20) reported that DCMU protected greened cucumber seedlings from LS 82-556, a compound with apparently the same mode of action as DPEs, other workers have reported little or no protection against DPEs by DCMU in photosynthetically competent tissue (4,8,9), when protection against paraquat is observed (4, 9). In addition, the fact that certain compounds have very similar toxic effects to DPEs on plants, but possess redox properties which would preclude their reduction by PSI (9,10) suggests that other explanations must be sought to account for the involvement of photosynthetic electron transport in the mode of action of DPEs.…”