Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: November 23, 1998
Publication Date: N/A
Interpretive Summary: This chapter is an overview of the herbicides that inhibit a key enzyme in porphyrin synthesis. The topics covered in this chapter include a description of the various chemical classes known to inhibit this site, how these herbicides are used, their mode of action, mechanisms of plant resistance, and toxicological issues associated with these compounds.
Technical Abstract: Inhibitors of protoporphyrinogen oxidase (Protox) comprise a diverse spectrum of chemical classes, including diphenyl ethers, various heterocyclic phenyl ethers, oxadiazoles, phenyl imides,triazolinones, and pyrazoles. The common structural attribute of these compounds is that they are all at least bicyclic and have similar electrical charge distribution. Most of the commercial Protox inhibitors were developed for post emergent weed management in soybeans, although they are used in rice, cotton, and other crops. Some of the newer compounds are effective in small grains and maize as soil- incorporated herbicides. These herbicides have a complex mechanism of action, in which the catalytic product of Protox accumulates and causes rapid photooxidative damage. No evolved resistance to this herbicide class has been reported in weed populations. Natural resistance of compatible crops appears to be due to rapid metabolic degradation of the inhibitor. Crops with resistance to these herbicides have been created through the introduction of transgenes encoding inhibitor-resistant, bacterial Protox. While the half life of these compounds in soil varies from a few days to 9 months, this category of herbicides has not been significantly associated with ground water contamination or any other ecotoxicological effects. No significant toxicological effects on mammals have been associated with concentrations of these compounds to which humans might be exposed. At high concentrations, these compounds have profound but reversible effects on porphyrin metabolism of mammals and there are species-specific differences in sensitivity.