Pyrazine herbicides

The most successful pyrazine derivative was diquat-dibromide (see Fig. 1, the structure I). This non-selective, contact herbicide has been used to control many submerged and floating aquatic macrophytes which interferes with the photosynthetic process, releasing strong oxidizers that rapidly disrupt and inactivate cells and cellular functions (at present banned in many EU countries). Severe oral diquat intoxication has been associated with cerebral haemorrhages and severe acute renal failure (Peiro et al., 2007). Also quinoxaline herbicides (containing the pyrazine fragment) are very useful herbicides. Among them propaquizafop (Fig. 1, II) and quizalofop-ethyl (Fig. 1, III) are the most important derivatives (Frater et al., 1987; Sakata et al., 1983).

Outlawed Herbicides

Fig. 1. Structures of diquat-dibromide (I), propaquizafop (II) and quizalofop-ethyl (III). 2.1 Diquat

Diquat-dibromide (6,7-dihydrodipyrido[1,2-a:2',1'-c]pyrazinediium-dibromide; for the structure see Fig. 1, I) is a quaternary ammonium salt used as a non-selective contact herbicide and desiccant, absorbed by the foliage with some translocation in the xylem. It is used for preharvest desiccation of many crops, as a defoliant on hops, for general weed control on non crop land etc. (Ritter et al., 2000; Ivany, 2005). It is applied as an aquatic

Fig. 1. Structures of diquat-dibromide (I), propaquizafop (II) and quizalofop-ethyl (III). 2.1 Diquat

Diquat-dibromide (6,7-dihydrodipyrido[1,2-a:2',1'-c]pyrazinediium-dibromide; for the structure see Fig. 1, I) is a quaternary ammonium salt used as a non-selective contact herbicide and desiccant, absorbed by the foliage with some translocation in the xylem. It is used for preharvest desiccation of many crops, as a defoliant on hops, for general weed control on non crop land etc. (Ritter et al., 2000; Ivany, 2005). It is applied as an aquatic herbicide in many countries since the late 1950s for control of emergent and submerged aquatic weeds (Ritter et al., 2000). According to Massachusetts Department of Agricultural Resources (2010) following weeds are controlled by diquat: i) submersed aquatics: Ultricularia, Ceratophyllum demersum, Elodea spp., Najas spp., Myriophyllum spp., Hydrilla verticillata, Potamogeton spp.; ii) floating aquatics: Salvinia spp., Eichhornia crassipes, Pistia Stratiotes, Lemna spp., Hydrocotyle spp.; iii) marginal weeds: Typha spp. ; iv) algae: Pithophora spp. , Spyrogyra spp. (filamentous algae). Diquat is stable in neutral and acidic solutions but unstable in alkaline medium. It breaks down by the UV radiation and the degradation increases with pH > 9 (Diaz et al., 2002). It is also biodegraded in water by microorganisms that uses this herbicide as a source of carbon or nitrogen (Petit et al., 1995). Trade names for diquat-dibromide formulations included Desiquat®, Midstream®, Reglone®, and Reglex®. Mixtures of diquat with another quaternary herbicide paraquat (1,1'-dimethyl-4,4'-bipyridinium-dichloride) were sold under trade names including Actor®, Dukatalon®, Opal®, Pathclear® (also includes simazine and aminotriazole), Preeglox®, Preglone®, Seccatutto®, Spray Seed®, and Weedol® (Lock & Wilks, 2001).

Nitrogenase Photosystem

Fig. 2. Scheme of the photosynthetic electron transport in photosystem I (PS I). (Figure taken from http://www.bio.ic.ac.uk/research/barber/psIIimages/PSI.jpg with permission of Prof. Barber, Imperial College London).

The first paper dealing with the mode of action of diquat was published in 1960 by Mees who indicated that oxygen and light were essential for its herbicidal effect. Later Zweig et al. (1965) found that diquat caused a deviation of electron flow from photosystem (PS) I what resulted in an inhibition of NADP+ reduction and the production of a reduced diquat radical. In Fig. 2 is shown scheme of the photosynthetic electron transport (PET) in PS I. In plants, the PS I complex catalyzes the oxidation of plastocyanin and the reduction of ferredoxin (Fd). From the primary donor, P700, electrons are transferred to the primary acceptor, Ao and then to phylloquinone (Ai) operating as a single electron acceptor. From Ai electrons are transferred to a 4Fe-4S cluster (FX) and subsequently to two 4Fe-4S clusters, FA and Fb, located on the stromal side of the reaction center close to FX. PS I produces a strong reductant that transfers electrons to Fd. Ferredoxin, one of the strongest soluble reductants found in cells, operates in the stromal aqueous phase of the chloroplast, transferring electrons from PS I to ferredoxin-NADP+ oxidoreductase. The final electron acceptor in the photosynthetic electron transport chain is NADP+, which is fully reduced by two electrons (and one proton) to form NADPH, a strong reductant which serves as a mobile electron carrier in the stromal aqueous phase of the chloroplast (Whitmarsch, 1998). Due to deviation of electron flow from Fd, an inhibition of NADP+ reduction occurs and a reduced diquat radical is formed. Davenport (1963) found that in the presence of oxygen the reduced diquat free radical was reoxidized with the production of hydrogen peroxide. Thus, an one-electron reduction of diquat results in a cation free radical that reacts rapidly with molecular oxygen and generates reactive oxygen species such as the superoxide anion radical (Mason, 1990). Reactive oxygen species cause oxidative stress in the cell with consecutive damage of biological membranes. In herbicide classification diquat, similarly to paraquat, is classified as HRAC Group D herbicide causing PS I electron diversion (HRAC 2005). Injury to diquat-treated crop plants occurs in the form of spots of dead leaf tissue wherever spray droplets contact the leaves indicating that this herbicide belongs to membrane disruptors. The use of diquat for the control of aquatic weeds is widespread in the US (US Environmental Protection Agency, 1995) whereas it is forbidden in the EU (European Commission, 2001, 2002).

As mentioned above, diquat toxicity to both aquatic plants and animals originates from the formation of reactive oxygen species in both chloroplasts and mitochondria (Cedergreen et al., 2006; Sanchez et al., 2006). The field effects of diquat to natural strands of aquatic vegetation were studied by Peterson et al. (1997) and Campbell at al. (2000). The filamentous cyanobacteria were slightly less tolerant than the unicellular cyanobacteria and the most sensitive was genus Anabena (Peterson et al., 1997). Gorzerino et al. (2009) showed that diquat, used as the commercial preparation Reglone 2®, inhibited the growth of Lemna minor in indoor microcosms. According to findings of Campbell et al. (2000) diquat has a minimal ecological impact to benthic invertebrates and fish; on the other hand, aquatic plants in the vicinity of application to surface waters appear to be at risk (nevertheless this is expected, as diquat-dibromide kills aquatic plants). Howewer, Koschnick et al. (2006) observed that the accession of Landoltia from Lake County (Florida) had developed resistance to diquat and the resistance mechanism was independent of photosynthetic electron transport.

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