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 glutathione S-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.

In laboratory experiments, the safener CGA-92194 {α-[(1,3-dioxolan-2-yl-methoxy)imino] benzeneacetonitrile} increased the in vitro chemical reactivity of the herbicide metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] for glutathione (GSH) when tested at 1, 50, and 100 μM. Pretreatment of sorghum [Sorghum bicolor (L.) Moench. cv. ‘G623’] seeds with the safener CGA-92194 (1.25 g ai/kg of seeds) enhanced significantly the amount of 14C-metolachlor absorbed by the roots and translocated to the shoots and leaves of grain sorghum seedlings grown hydroponically in a nutrient solution. Thin-layer chromatographic analysis of methanol-soluble extracts from roots or shoots and leaves of sorghum seedlings treated with 14C-metolachlor revealed the detection of 8 to 10 metabolites of this herbicide. One of the major metabolites detected was tentatively identified as the conjugate of metolachlor with GSH. A safener-induced stimulation of metolachlor metabolism via conjugation to GSH was observed in sorghum seedlings grown from seeds pretreated with CGA-92194. Addition of tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane], a potent inhibitor of plant GSH-S-transferase enzymes, to the nutrient solution of hydroponically grown sorghum seedlings reduced the formation of GS-metolachlor in both control and safener-treated seedlings and minimized the stimulatory effect of the safener CGA-92194 on the conjugation of metolachlor to GSH. It is suggested that the safener CGA-92194 may protect grain sorghum against injury from the herbicide metolachlor by stimulating the spontaneous and enzymatic conjugation of this herbicide with GSH.

A herbicide safener, CGA-43089 [α-(cyanomethoximino)-benzacetonitrile], has been discovered which will allow the safe use of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethylacetamide] in grain sorghum [Sorghum bicolor (L.) Moench]. Experiments revealed that sorghum is protected from injury by metolachlor when it is applied at rates as high as 4 kg/ha. Better protection was achieved when sorghum seed were treated directly with the safener than when it was mixed with the herbicide. A CGA-43089 rate of 1.25 to 1.5 g per kg of sorghum seed was adequate for protection. Sorghum tolerance to metolachlor was maintained with CGA-43089 when metolachlor was combined with several s-triazine herbicides. Metolachlor effectively controlled several annual grasses with an increase in sorghum yield.

In 1987-89, field experiments were conducted to evaluate rates of activated carbon in transplant water to reduce injury to tobacco from imazaquin. Rates of activated carbon were 0, 2.2, 4.5, and 9 kg ha−1 in 1987 and 0, 9, 18, and 36 kg ha−1 in 1988-89. Imazaquin rates were 0, 0.3, and 0.6 kg ha−1 applied over-the-top immediately after transplanting. Results indicated that 9 kg ha−1 of activated carbon was the minimum amount to reduce injury from imazaquin. No additional protection was achieved with rates above 9 kg ha−1, whereas lower rates appeared to offer less protection based on visible stunting ratings.

Five field experiments were conducted during 1987–89 to determine the effectiveness of using activated carbon in transplant water to reduce injury to tobacco from herbicides incorporated before transplanting. Herbicides used were dicamba, chlorimuron, atrazine, lactofen, flumetralin, pendimethalin, pebulate, and imazaquin. Activated carbon was applied in the transplant water at a rate of 9 kg ha–1 in 1987 and 18 kg ha–1 in 1988 and 1989. Measurements taken were visual stunting ratings, yield, grade index, fresh and dry weights of roots and shoots, and percent sugars and total alkaloids in cured leaves. Activated carbon consistently reduced phytotoxicity of imazaquin and chlorimuron. As much as four- and two-fold yield increases were obtained when imazaquin and chlorimuron, respectively, were applied in conjunction with activated carbon. Less consistent results were obtained in reducing the phytotoxic effects of dicamba, atrazine, and flumetralin. Phytotoxicity without activated carbon was not consistently obtained with lactofen, pendimethalin, or pebulate.

Experiments were conducted in 1993 and 1994 to determine if seed treated with disulfoton at 3, 6, or 12 g ai kg−1 reduced bleaching of rice foliage caused by clomazone applied delayed preemergence at 0.56, 1.1, 1.7, or 2.2 kg ai ha−1 on silt loam and silty clay soils. Clomazone injured rice more on silt loam soil than on silty clay soil. Disulfoton reduced bleaching caused by clomazone. In additional field experiments conducted during 1995 and 1996, clomazone at rates as high as 2.2 kg ha−1 did not affect grain yield in seven of 10 experiments. Clomazone at rates below 1.7 kg ha−1 did not reduce grain yield in any of the experiments.

Foramsulfuron (AE F130360) is a sulfonylurea herbicide for postemergence control of grasses and some broadleaf weeds in corn. Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of corn hybrids to foramsulfuron. Differences in corn tolerance were quantified by determining the herbicide rate required to cause injury and reduce corn height by 15% (GR15). Seven hybrids were screened in the greenhouse where GR15 values indicated that the some of the corn hybrids were sensitive to foramsulfuron. All but one of these hybrids exhibited an increase in tolerance when the safener, isoxadifen-ethyl, was applied with foramsulfuron. Experiments using 14C-foramsulfuron were conducted to determine whether isoxadifen-ethyl affected foramsulfuron absorption, translocation, and metabolism in two of the corn hybrids. There was no difference in absorption between a sensitive hybrid Novartis 58D1 and a more tolerant hybrid Novartis 59Q9, but the addition of isoxadifen-ethyl increased foramsulfuron absorption in both hybrids 24 h after treatment. Less than 1 and 3% of the 14C-foramsulfuron was translocated to plant portions above and below the treated leaf, respectively. Differences in hybrid tolerance were primarily due to differential herbicide metabolism. Foramsulfuron metabolism, with and without isoxadifen-ethyl, was similar at 4 h after treatment for both the sensitive and tolerant hybrids. However, by 24 h after treatment the more tolerant hybrid metabolized foramsulfuron to more polar compounds to a greater degree than the sensitive hybrid.

Genome sequencing of many model systems, including the human, yeast, Arabidopsis, and rice genomes, has been completed. As a complement to genomic techniques, proteomics has emerged to study the composition, structure, function, and interaction of the expressed proteins or proteome in a given cell, tissue, or organism. During the past several years, tremendous progress has been made with proteomic techniques in human and yeast, but progress has been slower in plants due to several factors. Proteomic techniques have been used in plants, especially in agriculturally important crops and weeds, to understand mechanisms of herbicide tolerance and weed resistance. Proteomic techniques, starting from protein extraction to protein identification, have been developed and advanced for large-scale proteomics studies, but limitations and problems for these techniques still exist for plant proteomics. Further technological improvements are needed to enhance quantitative and comparative large-scale proteomics studies. Applications of these techniques may help weed scientists to understand stress tolerance in crops and investigate weediness traits further. Proteomic techniques have unique strengths but also offer several challenges. Together with transcriptomics and metabolomics, these techniques for analyzing global patterns of gene expression offer new and novel techniques for better understanding biological questions of interest to weed scientists in the future.

Saflufenacil is a PRE herbicide for the control of broadleaf weeds. Field and growth room studies were conducted to explore the tolerance of corn to POST treatments of saflufenacil and BAS 781. Additionally, the potential use of sodium as a safener for saflufenacil was evaluated. Crop injury caused by saflufenacil or BAS 781 was 8 and 38%, respectively, when applied at twice the recommended dose at the spike to two-leaf stage of crop growth. This injury increased to 28 and 65%, respectively, when applied at the three- to four-leaf stage. This level of crop injury resulted in yield loss, particularly when applied at the three- to four-leaf stage. The addition of Na-bentazon to saflufenacil reduced this injury and increased crop dry weight under both field and laboratory conditions. In the field, Na-bentazon also increased corn collar height and yield compared with saflufenacil applied alone. Na-bentazon reduced injury through a reduction in foliar uptake of saflufenacil. Sodium derived from baking soda also provided a safening effect, but only at the lowest dose of saflufenacil tested.