Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2004
Publication Date: 5/7/2004
Citation: Li, J., Smeda, R.J., Nelson, K.A., Dayan, F.E. Physiological basis for resistance to diphenyl ether herbicides in common waterhemp (amaranthus rudis). Weed Science. 2004. V. 52. P. 333-338. Interpretive Summary: Common waterhemp (Amaranthus rudis) is an undesirable weed in soybean fields. This plant is normally readily controlled by applying herbicides such as acifluorfen or lactofen. New biotypes no longer controlled by recommended rates of these herbicides have appeared in fields of Kansas and Missouri. We report that two of these biotypes from Meadville and Bethel (MO) were up to 44 times more resistant to the herbicides than the wildtype plants. Our study shows that the resistant plants no longer accumulate large quantities of a photodynamic and harmful pigment that is normally the cause of death in treated plants.
Technical Abstract: Seeds of common waterhemp (Amaranthus rudis) biotypes no longer controlled by labeled application rates of acifluorfen were collected from two Missouri soybean fields. Plants grown from these seeds were tested for resistance to two diphenyl ether herbicides. Resistant factors, calculated as the ratio of the dose to inhibit dry weight accumulation by 50% (GR50) for resistant compared to sensitive waterhemp, was 9.5 and 11 for the Meadville, and 28 and 44 for the Bethel biotypes exposed to acifluorfen and lactofen, respectively. Electrolyte leakage assays determined that light-induced lipid peroxidation by acifluorfen was greatest on a control population (Bradford), intermediate for the Meadville biotype and lowest for the Bethel biotype. Levels of the photodynamic pigment protoporphyrin IX accumulating in leaf discs exposed to the diphenyl ether herbicide acifluorfen were much lower in the resistant biotypes than in the susceptible wild-type, and there was a strong correlation between the protoporphyrin IX levels and the degree of disruption of membrane integrity. Furthermore, the binding of acifluorfen to its target site in chloroplasts appeared to have been altered in the resistant biotypes. While the molecular and biochemical factors involved in mechanism of resistance remain to be fully characterized, this study establishes that the physiological basis for the evolved resistance to diphenyl ethers in common waterhemp rests on the reduction of protoporphyrin IX accumulation.