Safeners are chemical agents that reduce the phytotoxicity of herbicides to crop plants by a physiological or molecular mechanism, without compromising weed control efficacy. Commercialized safeners are used for the protection of large-seeded grass crops, such as corn, grain sorghum, and wet-sown rice, against preplant-incorporated or preemergence-applied herbicides of the thiocarbamate and chloroacetanilide families. Safeners also have been developed to protect winter cereal crops such as wheat against postemergence applications of aryloxyphenoxypropionate and sulfonylurea herbicides. The use of safeners for the protection of corn and rice against sulfonylurea, imidazolinone, cyclohexanedione, isoxazole, and triketone herbicides also is well established. A safener-induced enhancement of herbicide detoxification in safened plants is widely accepted as the major mechanism involved in safener action. Safeners induce cofactors such as glutathione and herbicide-detoxifying enzymes such as glutathione S-transferases, cytochrome P450 monooxygenases, and glucosyl transferases. In addition, safeners enhance the vacuolar transport of glutathione or glucose conjugates of selected herbicides. The safener-mediated induction of herbicide-detoxifying enzymes appears to be part of a general stress response.

Herbicide resistance is the heritable ability of a weed biotype or population to survive a herbicide application that would effectively kill a susceptible population of the weed. In the U.K. the most widespread and financially important herbicide-resistant weed is blackgrass. Investigations to elucidate the molecular mechanisms conferring herbicide resistance to blackgrass populations have been ongoing for two decades. Although the identification of target site–resistant populations has proved to be relatively straightforward (using, for example, target site assays in vitro), the study and understanding of resistance mechanisms involved in enhanced metabolism has proven to be more problematic. Research has focused on the cytochrome P450 monooxygenase and glutathione S-transferase (GST) enzyme families, both of which have been shown to be important in herbicide metabolism in many weed and crop species. GST activity and abundance are greater in a selection of herbicide-resistant blackgrass biotypes, and herbicide treatment of field populations of blackgrass results in the survival of the proportion of population possessing the greatest GST activity and abundance. In addition, GST activity in the field increases between winter and spring, and this coincides with reduced efficacy of important blackgrass herbicides. GST activities within field populations of blackgrass are highly varied, and this plasticity is discussed in relation to the development of resistant populations in field situations. This article describes research results in blackgrass and compares them with GST studies in other weed species as well as with other mechanisms for enhanced metabolism-based resistance.

Propanil is an acylanilide herbicide introduced in the early 1960s to control dicotyledonous weeds and grasses, including Echinochloa species in cultivated rice. Since then, propanil has been used extensively in rice production in the United States and in several other countries. Propanil is an inhibitor of photosystem II, but rice is tolerant to propanil because of the presence of a high level of aryl acylamidase that catalytically degrades the compound to nonphytotoxic products, i.e., 3,4-dichloroaniline and propionic acid. About 10 yr ago, biotypes of barnyardgrass and junglerice were discovered to be resistant to propanil. The resistance mechanism of these two biotypes has been shown to be elevated levels of aryl acylamidase activity. Various strategies to combat propanil resistance and to more fully understand the biochemistry involved in this resistance have been investigated. These include studies on the interactions of herbicides and other chemicals with propanil, rotation of rice with other crops (consequently the use of other herbicide modes of action), and use of alternative herbicides in rice. Certain compounds, including some organophosphate insecticides, are potent inhibitors of aryl acylamidase, which can act as synergists with propanil to increase phytotoxicity. Another compound that lacks insecticidal or herbicidal activity, PPG-124, has been commercialized as a herbicide synergist for propanil. These chemical and biochemical interactions and other factors involved in propanil-resistant Echinochloa weeds are presented and discussed.