Allelopathy has been hypothesized to promote the success of invasive plants. Support for the role of allelopathy in invasions has emerged from research on the candidate allelochemical ())-catechin, which is secreted by spotted knapweed. Here we describe new methods to quantify catechin in liquid and soil. With a new technique, we assayed catechin production by individual plants in liquid media and found levels up to two orders of magnitude less than previously reported. An acetone/water solution provided consistent recovery of catechin from soil, with percent recovery depending upon soil type. We evaluated soils from two spotted knapweed sites in Montana, USA, but found no measurable catechin. Idaho fescue, a native species reportedly sensitive to catechin, only exhibited slightly reduced growth at concentrations 10 times higher than previously reported to cause 100% mortality. Our results emphasize that more research is required to clarify the role of catechin in the invasion of spotted knapweed.

Allelopathy is a notoriously difficult mechanism to demonstrate. There has been a recent resurgence of interest in allelopathy because of the work done on the invasive weed spotted knapweed and its putative allelochemical, (+/-)-catechin. In this study we collected and analyzed soil samples taken from three, long-term knapweed infested sites in Montana, USA during the summer and fall of 2005. We only detected catechin in all the soil cores at one time point (August, 2005) at two of the sites. Field levels from these two sites were nearly three orders of magnitude lower than what has previously been reported to cause reduced growth in a sensitive native species. Fourteen percent of the remaining soil cores contained low but detectable levels (<0.11 ppm) of (+/-)-catechin. Additional experiments indicated that soil moisture appears to play a significant role in whether or not catechin degrades rapidly or remains in the soil. Adding to previous work, this paper sheds doubt on the importance of this chemical in spotted knapweed invasion success.

Laboratory studies were conducted using 14 C-aminocyclopyrachlor (DPX-MAT28) and its 14 C-methyl ester formulation (DPX-KJM44) to (1) determine the adjuvants' effects on absorption, (2) compare the absorption and translocation of aminocyclopyrachlor free acid with the methyl ester, and (3) determine the rate at which aminocyclopyrachlor-methyl ester is metabolized to the free acid in Canada thistle. Canada thistle plants were grown from root cuttings and treated in the rosette growth stage. The effect of different adjuvants on absorption was determined by treating individual leaves with formulated herbicide plus 14 C-herbicide alone or with methylated seed oil (MSO), crop oil concentrate, or nonionic surfactant with and without urea ammonium nitrate and ammonium sulfate. Plants were harvested 96 h after treatment (HAT). For absorption and translocation experiments, plants were oversprayed with aminocyclopyrachlor or its methyl ester at a rate of 0.14 kg ae ha 21 in combination with 1% MSO. Formulated herbicide plus 14 C-herbicide was then applied to a protected leaf, and plants were harvested 24 to 192 HAT. Plants were harvested and radioactivity was determined in the treated leaf and in aboveground and belowground tissues. Metabolism of aminocyclopyrachlor-methyl ester to the free acid was determined 2, 6, and 24 HAT. All aboveground biomass was analyzed by high-performance liquid chromatography to establish the ratio of methyl ester to free acid. MSO applied with either herbicide formulation resulted in the highest absorption compared with no surfactant. Significantly greater aminocyclopyrachlor-methyl ester was absorbed, compared with the free acid, which was reflected in the greater aboveground translocation for the methyl ester. Both formulations had similar amounts of translocation to the roots, with 8.6% (SE 6 3.3) for the methyl ester compared with 6.2% (SE 6 2.5) for the free acid. Approximately 80% of the methyl ester was converted to the free acid at 6 HAT. Based on this conversion rate, aminocyclopyrachlor translocated as the free acid in Canada thistle. Nomenclature: Aminocyclopyrachlor

Aminocyclopyrachlor, a newly discovered synthetic auxin herbicide, and its methyl ester, appear to control a number of perennial broadleaf weeds. The potential volatility of this new herbicide and its methyl ester were determined under laboratory conditions and were also compared to dicamba and aminopyralid with the use of enclosed chamber and open-air plant bioassays. Bioassays consisting of visual estimates of epinastic responses and kidney bean and soybean leaf-width measurements were developed to measure vapor release from glass and plastic. Vapor release of aminocyclopyrachlor from glass surfaces was undetectable under laboratory conditions, and no phytotoxic responses were observed when plants were exposed to vapors emanating from various surfaces. Results were similar to those of aminopyralid, indicating the risk of plant injury from vapor movement of aminocyclopyrachlor and aminopyralid was very low. When combined with 1% methylated seed oil, vapor release of aminocyclopyrachlor-methyl ester reached 86% 192 h after application to glass surfaces. Phytotoxic responses of plants exposed to vapors emanating from various surfaces treated with aminocyclopyrachlor-methyl ester were similar to responses to dicamba under enclosed incubation conditions, but were less in outdoor, open-air environments. Studies are needed to understand better the risk of injury to nontarget plants due to vapor movement of aminocyclopyrachlor-methyl ester under field applications.

Aminopyralid is a new auxinic herbicide that provides Canada thistle control at lower use rates than clopyralid. Studies were conducted to determine if differences in absorption, translocation, or metabolism account for aminopyralid's greater biological activity. Radiolabeled aminopyralid and clopyralid were applied to individual leaves of rosette-stage Canada thistle plants. Nonionic surfactant was used for the absorption studies because it provided higher aminopyralid absorption than methylated seed oil or crop oil concentrate. Clopyralid was absorbed very rapidly, reaching 72% 24 h after treatment (HAT) and remaining near or above 80% during a 192-h time course. During the same time period, aminopyralid absorption increased from 34 to 60%. Clopyralid translocation out of the treated leaf was significantly higher than aminopyralid, 39% compared with 17%, respectively, 192 HAT. More of applied clopyralid translocated to aboveground tissue 192 HAT (27%) than to roots (12%), whereas aminopyralid translocation was similar in aboveground tissue (10%) and roots (7%) 192 HAT. Neither aminopyralid nor clopyralid was metabolized 192 HAT. Although aminopyralid is effective at lower use rates than clopyralid, clopyralid absorption and translocation were higher in Canada thistle. These results suggest that aminopyralid's chemical structure may provide for greater biological activity at the target site than clopyralid.