Prior research indicated that analysis of nuclear DNA content by flow cytometry could be used to distinguish smooth pigweed × tall waterhemp hybrids when the parent lines are known. Flow cytometry was performed on nuclei isolated from several Illinois populations of smooth pigweed and tall waterhemp. The smooth pigweed and tall waterhemp analyzed had nonoverlapping 2C nuclear DNA content values, with mean values of 1.04 and 1.34 pg, respectively. The consistent difference in DNA content observed between the two species indicates that DNA content analysis can be used to distinguish their hybrid progeny in natural populations.

The biochemical and molecular basis of resistance to acetolactate synthase (ALS)–inhibiting herbicides was investigated in eight resistant (R) and three susceptible (S) wild radish populations. In vitro enzyme assays revealed an ALS herbicide–resistant ALS enzyme in all R populations. ALS enzyme extracted from the shoots of all eight R populations was highly resistant to the ALS-inhibiting sulfonylurea herbicide chlorsulfuron (20- to 160-fold) and the triazolopyrimidine herbicide metosulam (10- to 46-fold) and moderately resistant to metsulfuron (three to eightfold). There was little or no cross-resistance to the imidazolinone herbicides imazapyr and imazethapyr. The ALS gene fragment covering potential mutation sites in these populations was amplified, sequenced, and compared. All eight R populations had point mutations in the codon for the proline residue in Domain A. However, the point mutations varied and encoded four different amino acid substitutions: histidine, threonine, alanine, and serine. No nucleotide difference in the DNA sequence of Domains C and D resulting in amino acid substitutions was observed between the R and S populations examined. In addition, a three- to fivefold higher ALS-specific activity was consistently observed in all R populations compared with S populations, whereas Northern blot analysis detected a similar level of ALS mRNA, suggesting a possible translational–posttranslational regulation of the enzyme. It is concluded that selection pressure from chlorsulfuron on eight separate wild radish populations has resulted in target site mutation at the same proline residue in the ALS gene. Higher ALS activity also may play a role in the resistance level.

The weedy Setaria species (giant, green, yellow, knotroot, and bristly foxtail) compose one of the worst weed groups interfering with world agriculture and in other disturbed and managed habitats. These species, together with their crop counterparts (foxtail millet, korali), form the foxtail species-group (spp.-gp). Five successive waves of Setaria spp. invasion from preagricultural times to the present have resulted in widespread infestation of the disturbed, arable, temperate regions of the earth. These invasions have resulted in considerable economic and environmental costs. The success of the Setaria spp.-gp is because of their intimate evolutionary relationship with humans, disturbance, agriculture, and land management. The ability to adapt rapidly to local conditions is the hallmark of this weedy group. Genotypic and phenotypic biodiversity provides this spp.-gp with traits that allow it to invade, colonize, adapt to, and endure in a wide range of habitats around the world. The phenotypic life-history traits important to the success of weedy Setaria spp. begin with the induction of dormancy in seed during embryogenesis. The formation of long-lived, heterogeneous seed pools in the soil is the inevitable consequence of the dormant seed rain. In soil seed pools, after-ripening, the occurrence and timing of seedling emergence, and the induction of secondary (summer) dormancy are regulated by seasonally and diurnally varying soil oxygen, water, and temperature signals. Precise and variable timing of seedling emergence ensures Setaria a dominant place in disturbed and managed communities during the growth and reproductive phases that follow. Once established in a community, phenotypic plasticity inherent in an individual weedy Setaria sp. allows it to maximize its growth, form, and reproduction to the specific local conditions it encounters, including competitive interactions with neighbors. Traits controlling the plastic development of plant architecture include the ability to form one or more tillering shoots whose stature and number are precisely sized to local conditions. A complex pattern of branching, from plant to spikelet, provides diverse microenvironments within which different levels of dormancy are induced in individual seeds on a panicle and among panicles on a common plant. Traits for adaptation to stress in weedy Setaria spp. include tolerance to many inhibitory chemicals (e.g., herbicides, salt), mechanical damage, and drought. Genetic traits such as self-pollination and small genome size contribute to a highly diverse collection of locally adapted genotypes and phenotypes ready to exploit any opportunities provided by a cropping system. Self-pollinating Setaria spp. exist in wild, weed, and crop variants, an ideal genetic condition ensuring both long-term stability and novelty. Weedy Setaria spp. populations have low to exceedingly low amounts of total genetic variation, unusually low intrapopulation genetic diversity, and unusually high genetic diversity between populations compared with an average plant species. These traits result spatially in local populations that are unusually homogeneous, typically consisting of a single multilocus genotype. Either a generally or a specifically adapted genotype of an individual species might predominate in that local population. Across the landscape, different single-genotype populations dominate particular local sites, providing novel genetics to the region by dispersal and gene flow when conditions change. Across North America, populations of green foxtail and knotroot foxtail are genetically differentiated along a north–south gradient. The history of invasion and colonization, the successful life histories of locally adapted weedy Setaria spp., and the evolutionary potential of this weed group emphasize the need for accurate prediction of its behavior. Weedy Setaria spp. management is the management of local selection pressure and consequent adaptation. Farmers, land managers, policy makers and regulators, homeowners, and consumers need accurate information about weedy Setaria spp. to predict and guide management decisions based on economics, risk, and environmental sustainability.

Sunflower broomrape and Egyptian broomrape are two holoparasites of sunflower that cause severe yield loss. The aim of the current study was to clarify the relationship between temperature and sunflower broomrape and Egyptian broomrape parasitism of the resistant sunflower variety ‘Ambar’ (R) and the susceptible variety ‘Adi’ (S). Experiments were conducted in pots under controlled conditions in a multiclimate greenhouse at five temperature (day–night) regimes: 17:9 C, 20:12 C, 23:15 C, 26:18 C, and 29:21 C. The resistant confectionery sunflower variety (R) was susceptible to sunflower broomrape only at low temperature regimes of 17:9 C, whereas the S variety was highly susceptible from 29 to 9 C. Temperature correlated negatively with the number of sunflower broomrape and Egyptian broomrape shoots that emerged above the soil in the R variety, and positively in the S variety. The number of degenerated tubercles found on the roots of the resistant variety correlated positively with temperature in both sunflower broomrape and Egyptian broomrape. No such correlation could be found in the susceptible sunflower variety grown in the presence of these two parasites. The resistance response of the R variety to sunflower broomrape and Egyptian broomrape was similar regardless of temperature regime. Parasite seeds were successfully germinated and attached to the sunflower roots but the parasite at the infection site degenerated and died. On the other hand, the susceptible variety (S) was equally susceptible to Egyptian broomrape and sunflower broomrape in all the temperature regimes tested, and susceptibility increased as temperature increased.

Biotypes of eastern black nightshade were found in Illinois and Indiana that were not controlled by postemergence applications of imazethapyr. Greenhouse studies were conducted to determine whole-plant responses to the imidazolinone herbicides imazethapyr and imazamox and to the sulfonylurea herbicide primisulfuron-methyl. The Illinois and Indiana resistant biotypes were highly resistant to imazamox and imazethapyr but were not cross-resistant to primisulfuron-methyl. DNA sequencing of acetolactate synthase (ALS) genes showed that the molecular basis for resistance in both resistant biotypes was a single base-pair mutation within Domain C that changed an alanine to a threonine in their encoded enzymes. In vitro enzyme assays showed that the Illinois resistant biotype's ALS enzyme was 789- and 881-fold less sensitive than that of the susceptible biotype to imazamox and imazethapyr, respectively. The Indiana resistant biotype's ALS enzyme was 110- and 158-fold less sensitive than that of the susceptible biotype to imazamox and imazethapyr, respectively. These results are consistent with the amino acid substitution found in the ALS enzyme of both resistant biotypes of eastern black nightshade.

Understanding pesticide metabolism in plants and microorganisms is necessary for pesticide development, for safe and efficient use, as well as for developing pesticide bioremediation strategies for contaminated soil and water. Pesticide biotransformation may occur via multistep processes known as metabolism or cometabolism. Cometabolism is the biotransformation of an organic compound that is not used as an energy source or as a constitutive element of the organism. Individual reactions of degradation–detoxification pathways include oxidation, reduction, hydrolysis, and conjugation. Metabolic pathway diversity depends on the chemical structure of the xenobiotic compound, the organism, environmental conditions, metabolic factors, and the regulating expression of these biochemical pathways. Knowledge of these enzymatic processes, especially concepts related to pesticide mechanism of action, resistance, selectivity, tolerance, and environmental fate, has advanced our understanding of pesticide science, and of plant and microbial biochemistry and physiology. There are some fundamental similarities and differences between plant and microbial pesticide metabolism. In this review, directed to researchers in weed science, we present concepts that were discussed at a symposium of the American Chemical Society (ACS) in 1999 and in the subsequent book Pesticide Biotransformation in Plants and Microorganism: Similarities and Divergences, edited by J. C. Hall, R. E. Hoagland, and R. M. Zablotowicz, and published by Oxford University Press, 2001.

Auxin-mediated responses of kochia biotypes resistant to dicamba (HRd) or resistant to dicamba and fluroxypyr (HRdf) were compared with those of two susceptible biotypes. Rates of shoot and root gravitropic response and patterns of apical dominance, as determined by lateral bud sprouting after decapitation, were determined in the absence of herbicide treatment. Shoots of susceptible plants reoriented toward vertical at a rate of 23.4° h−1, whereas the rates of HRd and HRdf shoot reorientation were significantly slower at 7.2° and 14.4° h−1, respectively. Root gravitropic responses were not different between resistant and susceptible biotypes. In contrast to susceptible biotypes, both apical and basal lateral buds on HRd plants elongated after decapitation, although differences between HRd and susceptible biotypes became smaller during succeeding weeks. The elongation pattern of HRdf lateral buds was intermediate to that of susceptible and HRd plants. Inhibition assays of root growth by natural and synthetic auxins showed that HRd root growth was less sensitive to dicamba, 2,4-D, naphthalene-1-acetic acid, and indole-3-acetic acid than was root growth of HRdf or the susceptible biotypes. Collectively, results support the hypothesis that auxin binding or signal transduction pathways are impaired in resistant biotypes and that HRd may contain different lesions than does HRdf.

The inheritance and linkage of enhanced metabolism-based herbicide cross-resistance was examined in a multiple resistant population of rigid ryegrass. F1 hybrids between resistant and susceptible populations showed an intermediate response to all the four herbicides tested, with no indication of maternal inheritance. Segregation of F2 families fitted a single-gene model for resistance to simazine, chlorotoluron, and chlorsulfuron. But there was more than the expected mortality from the low dose of tralkoxydim. Segregating F2 populations were selected by high rates of each of the four herbicides to create selected F2 families. Analysis of the response of these families demonstrated that simazine resistance is linked to chlorotoluron resistance. No other herbicide resistances were linked. Therefore, in this population of rigid ryegrass, at least three separate genes are responsible for metabolism-based cross-resistance. This study shows that multiple herbicide resistance in rigid ryegrass is the result of accumulation of a number of different resistance genes.

Resistance to protoporphyrinogen oxidase (protox)-inhibiting herbicides was identified in a population of common waterhemp that had been treated with acifluorfen for several years. The protox-resistant biotype of common waterhemp was approximately 34, 82, 8, and 4 times more resistant than a susceptible common waterhemp biotype to acifluorfen, lactofen, fomesafen, and sulfentrazone, respectively. The resistant biotype also showed a high level of resistance to acetolactate synthase–inhibiting herbicides thifensulfuron and imazethapyr but not to glyphosate or paraquat. An organophosphate insecticide was applied with acifluorfen or lactofen to determine if metabolism could be the mechanism of resistance. No differences were observed between resistant plants treated with an organophosphate plus a protox-inhibiting herbicide and plants treated with a protox-inhibiting herbicide alone.

Laboratory and field experiments were conducted to assess the allelopathic potential of buckwheat. In the field, buckwheat demonstrated strong inhibitory activity by suppressing weeds. In laboratory studies, aqueous and organic solvent extracts of the aerial parts of common buckwheat inhibited the root and shoot growth of lettuce seedlings. The chloroform and ethyl acetate extracts showed maximum activity, and plants grown in the presence of the ethyl acetate extract showed severe root browning. The allelopathic constituents of the ethyl acetate phase were isolated and identified as gallic acid and (+)-catechin by nuclear magnetic resonance spectroscopy. Gallic acid and (+)-catechin were present in the upper part of buckwheat at concentrations of 0.02 and 0.01%, of fresh weight, respectively. Gallic acid was found to be selectively and strongly inhibitory to root and shoot growth of tested plants at 100 and 10 μg ml−1. (+)-Catechin, however, inhibited plant growth to a lesser extent. These results suggest that buckwheat may have allelopathic potential and that when used as a ground cover crop or green manure may produce inhibitors, which could suppress weeds.

Two DNA molecular marker techniques were used to evaluate genetic diversity in 58 accessions of jointed goatgrass and 6 accessions of the related wild species barb goatgrass. Random amplified polymorphic DNA (RAPD) assays were performed on 8 U.S. and 50 Eurasian jointed goatgrass accessions using 30 random decamer primers. The frequency of scorable polymorphic bands within jointed goatgrass was 6 out of 90 (6.7%). Cluster analysis of RAPD data showed small genetic distances (values of 0.005 or less) among jointed goatgrass accessions. To validate the effectiveness of RAPD techniques to detect genetic diversity in tetraploid Aegilops species, six accessions of barb goatgrass were assayed using a subset of 20 decamer primers (from the original 30). RAPD data for barb goatgrass were pooled with jointed goatgrass data from the same primers. A total of 63 scorable bands were generated, of which 27 (43%) were polymorphic between two or more accessions. RAPD analysis readily distinguished between the two species and detected much greater levels of genetic diversity within barb goatgrass than between the jointed goatgrass accessions. Amplified fragment length polymorphism (AFLP) assays were performed on a subset of the 58 jointed goatgrass accessions, 3 U.S. and 13 Eurasian. These accessions were selected to represent a range in geographic diversity within our collection. Ten primer combinations generated 560 scorable bands of which 28 (5%) were polymorphic. Cluster analysis of AFLP data showed a slightly smaller range in genetic distance (0.0002 to 0.0022) among accessions compared with RAPD results; however, AFLPs distinguished among all but 2 of the 16 accessions surveyed. Although AFLP produced more scorable bands than RAPD did, both methods revealed limited genetic diversity in jointed goatgrass.

The effects of three postemergence herbicides that inhibit carotenoid biosynthesis were tested on field dodder. Flurochloridone, sulcotrione, and mesotrione treatments led to bleaching symptoms in field dodder stems. The effect of flurochloridone was rapid; 2 d after treatment (DAT) the stem was bleached and contained only 2% β-carotene, with a massive accumulation of phytoene in comparison with the control. However, flurochloridone treatment did not inhibit stem elongation, and full recovery of pigment composition at newly elongated stems was recorded 6 DAT. The effects of sulcotrione and mesotrione were similar, but the recovery was slower than with flurochloridone. The developing stems were fully bleached 6 DAT, with no detectable β-carotene, and subsequently no recovery was observed. All three herbicides led to mass destruction of the plastids in the parenchyma cells of the cortex and pith tissue. This disruption of the plastids was associated with depletion in starch content. Sulcotrione and mesotrione treatments reduced field dodder biomass accumulation, whereas flurochloridone delayed biomass accumulation by 6 d only. The differences in phytotoxicity between flurochloridone, sulcotrione, and mesotrione might be due to their different translocation patterns. Flurochloridone is xylem-mobile and is therefore localized at the applied area because of the low transpiration rate of field dodder. Sulcotrione and mesotrione are phloem-mobile and tend to accumulate in sinks such as the primordial tissue. The massive destruction of the amyloplasts and the rapid decline in starch content in response to all the herbicide treatments may indicate the possible involvement of carotenoids in stabilizing the amyloplast outer membrane in field dodder.

Studies were conducted to evaluate the absorption, translocation, and metabolism of 14C–CGA-362622 when foliar-applied to purple and yellow nutsedge. Less than 53% of the herbicide was absorbed after 96 h. Both nutsedge species translocated appreciable amounts of herbicide (30%) out of treated leaves. Translocation was both acropetal and basipetal, with at least 25% transported basipetally. Neither nutsedge species translocated more than 4% of applied radioactivity to the tubers and roots. Most of the metabolites formed by the nutsedge species were more polar than 14C–CGA-362622 and averaged 69 and 61% of the radioactivity in purple and yellow nutsedge, respectively. The half-life of CGA-362622 was estimated at 4 h in both purple and yellow nutsedge.

A common sunflower population from Howard, SD (HSD) was previously determined to be cross-resistant to imazethapyr and chlorimuron-ethyl, both acetolactate synthase–inhibiting (ALS) herbicides. Experiments were conducted to determine if target-site polymorphisms could act as a mechanism of ALS-inhibitor herbicide resistance in the HSD common sunflower. Approximately 1,600 nucleotides were amplified by polymerase chain reaction and sequenced from putative ALS gene(s) in common sunflower and Jerusalem artichoke. In sunflower, two different amplification products were detected that differed by a nine-basepair deletion. This suggested the presence of at least two ALS genes in common sunflower that could contribute to the herbicide resistance phenotype. In addition, an Ala205 to Val205 substitution was observed in several clones from resistant common sunflower (amino acid position is relative to the full-length mouse-ear cress ALS protein). Previously documented mutations at this position in other species indicated that it might play a vital role in conferring resistance to one or more ALS-inhibitor herbicides.

Greenhouse studies were conducted to determine the response of common lambsquarters and velvetleaf to glyphosate applied alone or with 20 g L−1 of ammonium sulfate (AMS). Minimal response of common lambsquarters to glyphosate plus AMS was observed. The GR50 values for velvetleaf decreased dramatically from 451 to 92 g ha−1 for glyphosate applied alone and glyphosate plus AMS, respectively. The addition of AMS did not affect foliar absorption of 14C-glyphosate in common lambsquarters but increased absorption in velvetleaf. A twofold increase in translocation, as a percentage of total 14C-glyphosate absorbed, occurred in velvetleaf with the addition of AMS. Increased control of velvetleaf with glyphosate plus AMS may be partially explained by greater glyphosate absorption and translocation. Increased translocation of glyphosate applied with AMS in velvetleaf was an indirect effect of greater foliar uptake as well as greater partitioning of glyphosate out of the treated leaf.

Application of glyphosate in combination with planting soybeans in narrow rows is an effective practice for management of weeds in glyphosate-resistant soybean. Farmers in Michigan reported higher levels of Sclerotinia stem rot (caused by Sclerotinia sclerotiorum) in fields of glyphosate-resistant soybean. Studies were conducted to determine if glyphosate or shading reduced the defense response of glyphosate-resistant soybean to S. sclerotiorum. Glyphosate caused shikimate accumulation in glyphosate-susceptible cultivar GL2415 but not in glyphosate-resistant cultivar GL2600RR. Ethylacetate extracts containing the plant defense compound glyceollin inhibited S. sclerotiorum hyphae in a rate-dependent manner. Glyphosate had no effect on either baseline or induced levels of glyceollin in glyphosate-resistant soybean, indicating that glyphosate did not impair plant defense responses to S. sclerotiorum. Shade levels of 60 and 90% in the greenhouse did not inhibit the induction of glyceollin synthesis. Glyphosate herbicide and shading did not affect the glyphosate-resistant soybean defense response to S. sclerotiorum.

A field study was conducted during 2000 and 2001 at Stoneville, MS, to determine the effects of isopropylamine, trimethylsulfonium (Tms), diammonium, and aminomethanamide dihydrogen tetraoxosulfate (Adt) salt formulations of glyphosate on weed control, growth, chlorophyll content, nodulation, nitrogen content, and grain yield in glyphosate-resistant soybean and to assess potential glyphosate accumulation in soybean nodules. Glyphosate-Tms and glyphosate-Adt injured soybean, and visible injury ranged from 29 to 38% 2 d after late postemergence (LPOST) application; however, soybean recovered by 14 d. Glyphosate formulations had no effect on chlorophyll content, root and shoot dry weight, or nodule number but reduced nodule biomass by 21 to 28% 14 d LPOST. Glyphosate levels in nodules from treated plants ranged from 39 to 147 ng g−1 (dry weight), and leghemoglobin content was reduced by as much as 10%. Control of five predominant weed species 14 d after LPOST was > 83% with one application and > 96% with two applications regardless of the glyphosate salts used. Soybean yields were generally higher with two applications than with one application regardless of glyphosate formulation. These results indicate that soybean injury and inhibition of nodule development with certain glyphosate formulations can occur, but soybean has the potential to recover from glyphosate stress.

Canada thistle and dandelion are troublesome weeds found throughout the northern United States. Carbohydrate concentrations in roots of these plants change in response to freezing temperatures as plants prepare to overwinter. Herbicides applied in the fall provide more effective control of these weeds than does treatment applied in early spring. Experiments were conducted near Scottsbluff, NE, from 1999 to 2001 to examine changes in glucose, fructose, sucrose, and fructans in roots of Canada thistle and dandelion in response to the fall-applied herbicide. Dicamba applied 10 d after the first fall frost reduced the quantities of low degree-of-polymerization (DP) fructans and provided better control of Canada thistle and dandelion than did dicamba applied 11 d before the first frost. Dicamba and dicamba plus SAN 836H fall-applied were more effective in controlling Canada thistle and dandelion than was 2,4-D fall-applied. As the rate of dicamba and dicamba plus SAN 836H increased, the quantities of low-DP fructans in plant roots declined, and plant control increased. Activity of fructan 1-exohydrolase in roots of Canada thistle was increased by dicamba fall-applied and was closely associated with decline in the quantities of low-DP fructans.

Absorption, translocation, and metabolism of the herbicide CGA 362622 were studied in cotton, spurred anoda, and smooth pigweed. 14C-labeled CGA 362622 was foliarly applied to cotton at the cotyledon to the one-leaf growth stage or at the two- to three-leaf growth stage and to spurred anoda and smooth pigweed at the four- to six-leaf growth stage. Differential absorption, translocation, and metabolism contributed to the differential tolerance of cotton, spurred anoda, and smooth pigweed to CGA 362622. Rapid translocation and a slow rate of metabolism appear to explain the susceptibility of smooth pigweed. Reduced absorption and translocation as well as rapid metabolism contribute to the CGA 362622 tolerance of cotton at the two growth stages. Limited translocation may explain the intermediate tolerance of spurred anoda to CGA 362622.