The Generation of Singlet Oxygen (¹O₂) by the Nitrodiphenyl Ether Herbicide Oxyfluorfen Is Independent of Photosynthesis

Abstract

The mechanism of action of the p-nitrodiphenyl ether herbicides has remained ambiguous because of conflicting reports in the literature. The diphenyl ether herbicide oxyfluorfen causes a light induced consumption of oxygen which resembles the electron acceptor reaction of paraquat. However, this reaction is not linked to the transport of electrons through photosystem I. This conclusion is based on the observation that the rate of oxygen consumption, in the presence of oxyfluorfen, does not demonstrate a first order rate dependence on light intensity. Using the bleaching of N,N-dimethyl p-nitrosoaniline as a specific detector of singlet oxygen, we demonstrate that oxyfluorfen is a potent generator of this toxic radical. The production of singlet oxygen occurs in the presence of inhibitors of photosynthetic electron transport (oxyfluorfen at 10-4 molar and paraquat) and also under temperature conditions (3'C) which prevent electron transport. This light induced reaction results in oxygen consumption and is the primary cause of lethality for oxyfluorfen. The production of singlet oxygen occurs rapidly and at low herbicide concentrations (10-' molar). The reaction occurs without photosynthetic electron transport but does require an intact thylakoid membrane.

Protoporphyrin IX Content Correlates with Activity of Photobleaching Herbicides

“…The absorption spectrum of PPIX, with peaks in the blue, green, and red spectral regions, fits the published action spectra for these herbicides (11,12,31,32). Duke and Kenyon (7) argued that the toxic oxygen species in NDPE action is singlet oxygen and Haworth and Hess (15) recently demonstrated that singlet oxygen is the active toxic oxygen species in NDPE action. After dark incubation with the herbicide, the temperature independence of the light-induced photodestruction indicates that a photodynamic dye is responsible for the activity (17).…”

“…Photobleaching NDPE, cyclic imide, and oxadiazole herbicides inhibit Chl synthesis (13,34,35). Furthermore, there is good evidence that photosynthetic light reactions are not required for activity of these herbicides (6,9,12,15) or accumulation of PPIX in herbicide-treated tissues (25). Photobleaching DPE herbicides were recently found to strongly inhibit protoporphyrinogen oxidase (26).…”

Reactive oxygen species generation and signaling in plants

“…[50][51][52] Oxyfluorfen added to isolated thylakoid membranes, in vitro, generates 1 O 2 during illumination. 42 Treatment with the protox inhibitor acifluorfen-sodium (AF-Na) in the light induced the overaccumulation of protoporphyrinogen IX that migrates out of the chloroplast to the cytoplasm where it is oxidized to Proto IX by plasma membrane bound AF-Na-insensitive protoporphyrinogen oxidase. 47,53 A part of Proto IX so generated in the plasma membrane migrates back to the chloroplast and partitions between the cytoplasm and the chloroplast.…”

The Diphenylether Herbicide Lactofen Induces Cell Death and Expression of Defense-Related Genes in Soybean

“…therein). In the presence of light, the accumulated protoporphyrin IX, which is now improperly compartmentalized, is unable to be utilized for further synthesis and thus reacts with oxygen to give rise to singlet oxygen (Haworth and Hess, 1988). It is thought that singlet oxygen, a form of reactive oxygen species (ROS), in turn causes lipid peroxidation, especially on membranes (Orr and Hess, 1982), damage of other cellular constituents, and eventually cell death and, thus, the herbicidal action of these chemicals (Hess, 2000).…”