2-Methylcinnolinium Herbicides: Effect of 2-Methylcinnolinium-4-(O-methylphosphonate) on Photosynthetic Electron Transport

This is the first paper on the mode of action of a new class of broadly active foliar herbicides, 2-methylcinnoliniums. One of the most active compounds of this class is 2-methylcinnolinium-4-(O-methyl phosphonate) (MCP). The symptoms of phytotoxicity include desiccation, chlorosis, and necrosis, and are very similar to those caused by methylviologen. Both MCP and methylviologen have reversible oxidation-reduction waves as measured by cyclic voltammetry, and both molecules have reduction potentials that are sufficiently positive to compete for electrons with the endogenous electron acceptors of photosystem I (PS I). MCP is a highly active inhibitor of whole-chain electron transport, similar to methylviologen, with an I₅₀ of about 25 μ. This compound is not an inhibitor of photosystem II (PS II). However, when PS II is inhibited by diuron, partial-chain electron transport from the artificial electron donor DPIPH2 to NADP is still strongly inhibited (I₅₀ = 40 μ) by this molecule. We conclude that MCP acts in the same fashion as methylviologen (Bowyer, J. R.; Camilleri, P. In Herbicides·, Hutson, D. H., Roberts, T. R., Eds.; Wiley: New York, 1987; pp 105-145), by competing with endogenous acceptors for electrons that would normally flow through the reducing side of PSI to NADP.