: Post-emergence application of carfentrazone-ethyl at rates as low as 2É2 g ha~1 caused greater leaf injury and growth reduction in ivyleaf morningglory (Ipomoea hederacea) and velvetleaf (Abutilon theophrasti) than in soybean (Glycine max). The herbicide was more rapidly metabolized in the crop than in the weed species, with 26É7, 54É3 and 60É6% of the parent compound remaining in soybean, ivyleaf morningglory and velvetleaf, respectively, 24 h after exposure. The free acid metabolite, carfentrazone, was present in all species and accounted for 21É2È27É4% of the total radioactivity. Unknown metabolites 0 and 0É22) were four to Ðve times more abundant in soybean than in the (R f weed species. Carfentrazone-ethyl induced more leakage from leaf discs from the weeds than those from soybean and the degree of injury correlated with the amount of protoporphyrin IX (Proto IX) present in the treated tissues. Both carfentrazone-ethyl and carfentrazone were potent inhibitors of protoporphyrinogen oxidase (Protox). Therefore, the selectivity of this herbicide may, at least in part, be attributed to the lower accumulation of Proto IX in soybean than in the weeds, probably because of the ability of soybean to metabolize more carfentrazone into unknown metabolites than the weeds.
Consistent with field observations, sicklepod exhibited considerable tolerance to sulfentrazone, and coffee senna showed relatively high sensitivity to this herbicide in greenhouse tests. Germination was not inhibited in either species at up to 12.9 μM of the herbicide. However, the chlorophyll content of herbicide-treated coffee senna cotyledonary leaves was greatly reduced, and seedlings died within 10 d after treatment, while sicklepod seedlings were not visibly affected. Shoot height of coffee senna was inhibited 90% by sulfentrazone at 0.5 kg ai ha−1, while the growth of sicklepod was not affected up to 2.0 kg ai ha−1. Root uptake of radiolabeled sulfentrazone was 74% greater in coffee senna than sicklepod, but the amount of radioactivity recovered from the shoots of both species after 12 h was not different. Eighty-three percent of the parent compound remained in coffee senna leaf tissue after 9 h root exposure to the herbicide. In contrast, sicklepod took up relatively less sulfentrazone through the root and metabolized sulfentrazone in the foliage more rapidly than coffee senna, with 91.6% of the herbicide being metabolized during the first 9 h of exposure. These results suggest that the tolerance of sicklepod to sulfentrazone is primarily due to a relatively high rate of metabolism of the herbicide compared to coffee senna.
Several grass and broadleaf weed species around the world have evolved multiple-herbicide resistance at alarmingly increasing rates. Research on the biochemical and molecular resistance mechanisms of multiple-resistant weed populations indicate a prevalence of herbicide metabolism catalyzed by enzyme systems such as cytochrome P450 monooxygenases and glutathioneS-transferases and, to a lesser extent, by glucosyl transferases. A symposium was conducted to gain an understanding of the current state of research on metabolic resistance mechanisms in weed species that pose major management problems around the world. These topics, as well as future directions of investigations that were identified in the symposium, are summarized herein. In addition, the latest information on selected topics such as the role of safeners in inducing crop tolerance to herbicides, selectivity to clomazone, glyphosate metabolism in crops and weeds, and bioactivation of natural molecules is reviewed.
Sulfentrazone was foliar applied at 34 and 56 g ai ha−1alone or in combination with surfactants to soybean cultivars Hutcheson and Centennial and to sicklepod, coffee senna, smallflower morningglory, velvetleaf, and yellow nutsedge. The most sensitive weeds, including coffee senna, smallflower morningglory, and velvetleaf, were severely injured by the lowest rate when sulfentrazone was applied with surfactants. Sulfentrazone provided the highest control of yellow nutsedge with X-77. Soybeans were not severely injured by sulfentrazone applied alone, but 55% foliar injury occurred when the herbicide was applied with X-77. However, the seedlings were not killed. Sicklepod was the most tolerant of the weeds tested. In the absence of surfactants, the order of radiolabeled sulfentrazone absorption by the foliage was Centennial (5.8%) = Hutcheson (8.5%) = coffee senna (10.4%) < yellow nutsedge (17.0%) < velvetleaf (22.3%) = smallflower morningglory (24%). Sicklepod leaves did not retain droplets containing sulfentrazone when no surfactant was used. Species with the highest foliar absorption also showed the greatest phytotoxic response to the herbicide. Addition of surfactants to the spray mixture enhanced the foliar absorption and overall phytotoxicity of sulfentrazone in the weeds. An inverse relationship was detected between the foliar absorption of sulfentrazone without surfactants and the amount of cuticular wax present on the leaves. No such correlation was observed when surfactants were used. Thus, surfactants overcame the barrier to absorption imposed by the cuticular wax and, under these conditions, selectivity apparently became dependent upon species-specific cellular tolerance to sulfentrazone.
Evolution of resistance to pesticides is a problem challenging the sustainability of global food production. Resistance to herbicides is driven by the intense selection pressure imparted by synthetic herbicides on which we rely to manage weeds. Target-site resistance (TSR) mechanisms involve changes to the herbicide target protein and provide resistance only to herbicides within a single mechanism of action. Non-target site resistance (NTSR) mechanisms reduce the quantity of herbicide reaching the target site and/or modify the herbicide. NTSR mechanisms include reduced absorption and/or translocation, increased sequestration, and enhanced metabolic degradation. Of these diverse mechanisms contributing to NTSR, metabolism-based herbicide resistance represents a major threat because it can impart resistance to herbicides from varied chemical classes across any number of mechanisms of action.
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