Paraquat (Methylviologen): Its Interference with Primary Photochemical Reactions

Paraquat (methylviologen: l,l'-dimethyl-4,4'bipyridinium) has been used widely as a herbicide for some years. Methylviologen (MV) has also been known as an extremely electronegative artifi­ cial electron acceptor, and used in laboratories for the studies of biological electron transfer systems. Its midpoint redox potential, -446 mV, is as low as that of ferredoxin, the lowest redox potential among physiological electron carriers. In plant photosynthesis, electrons from photosystem I are highly negative enough to reduce MV. In its herbicidal action, paraquat (MV) that has been sprayed on leaves and penetrated into chloroplasts is pho­ toreduced as above in place of ferredoxin, the nat­ ural electron acceptor. Reduced MV, a free radi­ cal, in turn reduces oxygen and generates a super­ oxide anion, Oj, a highly toxic radical that attacks numerous substances in the plant cells, thus fatally wounding the whole plant. This nonselective her­ bicide is useful and convenient in the field because it is quickly detoxicated by its strong adsorption to soil particles once it is in contact with the soil. This enables sowing and planting crops immediately after herbicide spraying. The immediate electron donor to MV is often regarded to be iron-sulfur clusters, FeSA/FeSB, mainly because the reoxida­ tion of P430, allegedly FeSA/FeSB (formerly Center A/Center B, respectively), is accelerated by MV [1, 2], Another iron-sulfur cluster, FeSx (formerly