Researchers, product registration personnel, and growers desire the ability to chemically detect residual amounts of herbicides in soil at concentrations below those necessary to cause phytotoxicity to sensitive nontarget or rotational crop plants. Alfalfa, cotton, soybean, and sunflower, crops sensitive to low concentrations of aminocyclopyrachlor in soil, were planted at field test sites approximately 1 yr after aminocyclopyrachlor methyl was applied. Soil samples were collected when rotational crops were planted and were analyzed for aminocyclopyrachlor by a method based on high performance liquid chromatography tandem mass spectrometry (HPLC/MS/MS), with a limit of detection (LOD) of 0.1 part per billion (ppb) (soil oven-dry weight basis). Loglogistic dose–response analysis correlated visual phytotoxic plant responses to residual concentrations of aminocyclopyrachlor in the soil. Concentrations of aminocyclopyrachlor estimated to cause 25% phytotoxicity to alfalfa, cotton, soybean, and sunflower were 5.4, 3.2, 2.0, and 6.2 ppb, respectively, 20 to 60 times greater than the LOD of the analytical method available for soil analysis. Results from these studies suggest this HPLC/MS/MS method of analysis can be used to indicate potential risk and severity of plant response for alfalfa, cotton, soybean, and sunflower, and for other plant species once dose–response curves for these additional species are established. This chemical assay may be particularly important if researchers desire to study the concentration, movement, and dissipation of aminocyclopyrachlor in soil or as part of a forensic investigation to better understand the cause of an unanticipated or undesirable plant response.

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Studies were conducted to determine how a serine to threonine mutation at position 264 on the Qb binding niche of the D1 protein [urea-resistant/triazine resistant (UR/TR biotype)] in common purslane (Portulaca oleracea) impacted carotenoid and chlorophyll pigment pools and measurements of photochemical and non-photochemical quenching (NPQ) following applications of various inhibitors of carotenoid biosynthesis (CBI) and the Photosystem II (PSII) inhibitor diuron when applied alone, or in mixtures, as compared to wildtype (WT) purslane. Non-photochemical quenching decreased 138 to 531% in comparison to the untreated checks following any herbicide application. Most CBI herbicides and diuron did not change chl a and chl b in the UR/TR biotype, while these same herbicide treatments tended to sharply decrease chlorophyll pigments in the WT population. Zeaxanthin levels were sharply elevated when CBI herbicides were applied alone to both purslane biotypes. β-carotene reduced in both biotypes following herbicide applications in comparison to the untreated check. Neoxanthin, antheraxanthin, and lutein were generally increased or remained similar to the untreated controls in the herbicide treated UR/TR biotype, while levels of these carotenoids tended to decrease in the herbicide treated WT population. Diuron alone increased neoxanthin, antheraxanthin, lutein, and zeaxanthin by 4 to 200% in the UR/TR biotype, but decreased these same carotenoids 25 to 62% in the WT population. The applications of CBI and PSII herbicides demonstrate that redox signaling in response to this mutation in the D1 protein may impact the retention of plant pigment concentrations in the light harvesting complexes of PSII, which would be vital for stress tolerance in this biotype.