Saflufenacil (Kixor™) is a new herbicide of the pyrimidinedione chemical class for preplant burndown and selective preemergence dicot weed control in multiple crops, including corn. In this study, the mode of action of saflufenacil was investigated. For initial characterization, a series of biotests was used in a physionomics approach for comprehensive physiological profiling of saflufenacil effects. With the use of treated duckweed plants, metabolite profiling was performed based on quantification of metabolite changes, relative to untreated controls. Physiological and metabolite profiling suggested a mode of action similar to inhibitors of protoporphyrinogen IX oxidase (PPO) in tetrapyrrole biosynthetic pathway. Saflufenacil inhibited PPO enzyme activity in vitro with 50% inhibition of 0.4 nM for the enzymes isolated from black nightshade, velvetleaf, and corn. PPO inhibition by saflufenacil caused accumulations of protoporphyrin IX (Proto) and hydrogen peroxide (H₂O₂) in leaf tissue of black nightshade and velvetleaf. In corn, only slight increases in Proto and H₂O₂ were found, which reflects in planta tolerance of this crop. The results show that saflufenacil is a new PPO-inhibiting, peroxidizing herbicide.

Nomenclature: Bifenox; diuron; saflufenacil; black nightshade, Solanum nigrum L. SOLNI; duckweed, Lemna paucicostata (L.) Hegelm. LEMPA; velvetleaf, Abutilon theophrasti Medik. ABUTH; corn, Zea mays L. ZEAMX.

Multiyear field data from Spain were used to model seedling emergence for three bedstraw species (Galium) that can coexist in winter cereal fields. The relationships between cumulative emergence and both growing degree days (GDD) and hydrothermal time (HTT) in soil were analyzed as sigmoid growth functions (Weibull). Iterations of base temperature and base water potential were used to optimize the HTT scale. All species were well described with Weibull functions. Both GDD and HTT models provided good descriptions of catchweed bedstraw emergence, as its seedlings have less dependence on soil water potential than false cleavers and threehorn bedstraw, which were described best with HTT. The HTT model for catchweed bedstraw was validated successfully with independent data from the United Kingdom. The models may be useful for predicting bedstraw emergence in semiarid Mediterranean regions and elsewhere.

Nomenclature: Catchweed bedstraw, Galium aparine L.; false cleavers, G. spurium L.; threehorn bedstraw, G. tricornutum Dandy.

Experiments were conducted to investigate the effects of shading on and location of the mother plant on germination and hormone content of Palmer amaranth seed. Increasing the shading from 0 to 87% decreased germination of fresh, viable seeds in dark from 25 to 12%. Abscisic acid (ABA) content of seeds from plants in 0% shade increased from 13.3 ng g⁻¹ dry seed to 19.1 ng g⁻¹ dry seed with 87% shade. Shading of the mother plant did not influence the 1,000-seed weight of Palmer amaranth. Seeds that matured in the top and middle third of the mother plant had 67 to 78% greater germination than those that matured in the bottom third of the mother plant. Endogenous gibberellic acid (GA) content of seeds did not differ between locations on the mother plant; however, the ABA content of seeds produced on the bottom third of the plant was 46 and 59% higher than the ABA content of seeds produced at the middle and top third of the plant, respectively. Endogenous ABA or GA content of seeds and 1,000-seed weight had no relationship with seed germination over and above the treatment effects. This research suggests that shading and plant location can influence germination of fresh, viable seeds of Palmer amaranth, which will be a dormancy strategy for seed dispersal over time.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

Basic factors contributing to the rapid evolution and broad distribution of acetolactate synthase (ALS)-inhibiting herbicide resistance in smallflower umbrella sedge L. have not yet been investigated. The objectives of this study were to examine patterns of cross-resistance to ALS herbicides and genetic diversity within and among smallflower umbrella sedge populations in California rice fields to provide insight into the processes contributing to resistance spread. Twelve different patterns of herbicide cross-resistance were found across the 56 populations sampled. The frequency of populations with at least one resistant individual in the North, Central and South Sacramento Valley, and the San Joaquin Valley were 76, 86, 67, and 50%, respectively. Analysis of the genetic diversity of 29 populations using 73 sequence-related amplified polymorphism molecular markers revealed little genetic diversity within populations, with estimates of Nei's gene diversity index, h, ranging from 0 to 0.049, and Shannon's information index (I) ranging from 0 to 0.079. Hierarchical analyses of molecular variance indicated that the majority of genetic variation was partitioned among populations, rather than within populations or among regional groups. No isolation by distance was evident. Unweighted pair group method with arithmetic averages analysis indicated that population clustering was not region specific. The results suggest that resistance to ALS-inhibiting herbicides in smallflower umbrella sedge populations from California rice fields appears to have evolved independently multiple times rather than spread from a single population where resistance originated. Consequently, prevention and management of smallflower umbrella sedge in California rice fields should emphasize in-field strategies that focus on decreasing the selection pressure caused by ALS-inhibiting herbicides.

Nomenclature: Bensulfuron-methyl; bispyribac-sodium; halosulfuron-methyl; imazethapyr; penoxsulam; propoxycarbazone-sodium; smallflower umbrella sedge, Cyperus difformis L.; rice, Oryza sativa L.

Horseweed has rapidly become a major weed in soybean and cotton production fields of the United States, and Indiana farmers ranked horseweed as one of the five worst weeds in their fields during a mail survey in 2003. Glyphosate resistance in horseweed is conferred by a single, incompletely dominant gene. Horseweed populations possess a high level of variability in their response to glyphosate. Horseweed has also evolved resistance to acetolactate synthase (ALS) inhibitors, and biotypes resistant to ALS-inhibiting herbicides and glyphosate are in many of the same areas. An experiment was designed to determine whether glyphosate resistance can be transferred by pollen. We found glyphosate-resistant plants in 1.1 to 3.8% of the progeny. Segregation ratios fit the expected 3  :  1 resistant  : sensitive ratios confirming that glyphosate resistance in horseweed can transfer to closely located glyphosate-susceptible biotypes under open-pollinated conditions at low frequencies. The hypothesis of a follow-up experiment was that first-generation progeny of parent plants, selected on a continuum of low to high phenotypic response to glyphosate, will inherit respective low to high phenotypic responses to glyphosate. The variability in field-collected populations (low-level to high-level glyphosate resistance) ranged from 2 to 14 times the commonly recommended field use rate of glyphosate. However, low- and high-levels of glyphosate resistance were not observed in first-generation progeny. We conclude that differential glyphosate responses observed among parental populations was due to different frequencies of the resistance allele within the populations, rather than the presence of different resistance alleles.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

The intensification of crop management in the U.K. over the past 60 years has resulted in the decline of the populations of a number of annual plant species adapted to arable habitats. In contrast, other species continue to be common as arable weeds. A community assembly approach was taken to explain these recent changes in the weed flora using databases of plant functional traits, a pot experiment, and weed surveys of the Broadbalk long-term experiment. The hypothesis was tested that species that have been selected against by increased fertilizer inputs and herbicide use share an adverse combination of traits. An analysis comparing the combination of maximum height, seed weight, and time of first flowering of 29 common and 32 rare or threatened U.K. autumn weeds established that rare or threatened species occupied an area of trait space that was distinct from the common species. A rare weed trait syndrome of short stature, large seed, and late flowering was identified. The theory that species with a trait syndrome that is currently unfavorable are better adapted for less fertile environments was supported by the pot experiment. Species with a combination of short stature and large seed had a relatively greater competitive ability in low compared to high fertility treatments. Analysis of survey data from the Broadbalk long-term experiment confirmed that, as N inputs increased, the abundance of the two functional groups that contained only common species remained stable or increased; whereas, the groups dominated by rare or threatened species declined as fertility increased. An understanding of the response traits of arable plants to management filters, including fertilizer inputs and herbicide, is valuable for designing conservation strategies for rare species or predicting future shifts in the functional diversity of weed communities including the potential for invasive species to establish.

We explored the allelopathic potential of kudzu as a function of its phenolics. Aqueous and methanol extracts of different kudzu organs (leaf, stem, root, and seed) were assayed for allelopathy with the use of lettuce and radish seeds. Both leaf and root extracts significantly inhibited all of the measured germination indices (total germination, speed of germination, and coefficient of the rate of germination) (all P < 0.01). When treated with leaf extract, the total germination of both species was ∼ 20% less than the control. Furthermore, the leaf extract significantly reduced the speed of germination to 38 and 53% that of the lettuce and radish controls, respectively. Lettuce and radish seeds soaked in leaf and root extracts for 24 h imbibed less water (∼ 30% for both species) than those soaked in distilled water (control), suggesting that a reduction of water imbibition might be one of the mechanisms of germination retardation. Stem and seed extracts affected neither the water uptake nor the germination indices of radish and lettuce seeds. Kudzu leaves and roots contain higher amounts of total phenolics (P  =  0.001) and soluble phenolics (P  =  0.005) than stems and seeds, consistent with the results of the germination bioassays. In agar plate bioassays, both litter and rhizosphere soil had phytotoxic effects on the radicle growth of radish (P  =  0.003) and perennial ryegrass (P  =  0.001) seedlings. Perennial ryegrass and cobbler's pegs seedlings grown on leaf and root leachate-amended soil gave ∼ 40% shorter roots and shoots and ∼ 50% less dry weight than those grown in leachate-free soil. Kudzu litter was incorporated and incubated in soil for 6 wk, at which point the soluble phenolics in the soil solution were at a level (> 150 ppm) considered to be allelopathic.

Nomenclature: Cobbler's pegs, Bidens pilosa L.; kudzu, Pueraria montana (Lour.); perennial ryegrass, Lolium perenne L.; lettuce, Lactuca sativa L.; radish, Raphanaus sativus L.

Research was conducted to establish a method to investigate the resistance level of flixweed to tribenuron-methyl and the evolved biochemical resistance mechanism. Four resistant biotypes were collected from wheat fields in Mazhuangcun, Jiacun, Dishangcun, and Bafangcun in the Hebei province of China where tribenuron-methyl had been continuously used for more than 10 yr. Two susceptible biotypes were collected from wheat fields where tribenuron-methyl was never applied. Different biotypes were assessed by petri-dish bioassay, whole-plant bioassay, and acetolactate synthase (ALS) assay. Comparisons of data indicated a similarity between methods and that experiments demonstrated that petri-dish bioassay was a feasible method to identify flixweed resistant to tribenuron-methyl. Data indicated differences among the flixweed biotypes when assessed by the petri-dish bioassay, whole-plant bioassay, or ALS enzyme assay, and a close association was obtained for the three bioassay methods. ALS resistance varied by biotypes with Mazhuangcun > Jiacun > Dishangcun > Bafangcun. Target-site enzyme assay data indicated that the resistant biotype's enhanced ALS activity was the biochemical mechanism that induced flixweed's evolved resistance to tribenuron-methyl. The concentrations of tribenuron-methyl causing 50% inhibition of ALS activity of the four resistant biotypes were 1,359, 513, 184, and 164 nM; in the susceptible biotypes these concentrations were 64 and 65 nM. Resistance indexes were 21, 8, 3, and 3 for Mazhuangcun, Jiacun, Dishangcun, and Bafangcun biotypes, respectively.

Nomenclature: Tribenuron-methyl; flixweed, Descurainia sophia (L.) Webb ex Prantl DESSO; wheat, Triticum aestivum L.

Biennial wormwood and lanceleaf sage have become serious weeds of several crops within the northern Great Plains of the United States and Canada. Both species are prolific seed producers but little is known about their potential for developing persistent seedbanks. Field studies were conducted to determine the influence of duration (7, 8, 11, 19, 20, and 23 mo) and depth of burial (0, 2.5, and 10 cm) on biennial wormwood and lanceleaf sage seed viability and decay. Biennial wormwood and lanceleaf sage seeds were buried in September 2003 (burial 1) and September 2004 (burial 2). In burial 1, biennial wormwood and lanceleaf sage seed viability was 65 and 66%, respectively, after 23 mo of burial. In burial 2, biennial wormwood and lanceleaf sage seed viability was 8 and 3%, respectively, after 23 mo of burial. The difference was likely because of higher soil moisture during burial 2, which promoted seed decay. Controlled-environment studies sought to determine the influence of stratification environments (freezing, chilling, and freeze–thaw) followed by exposure to diurnally fluctuating temperatures on germination of biennial wormwood and lanceleaf sage seeds. Stratified biennial wormwood seed germination was 95% or greater when incubated in fluctuating day/night temperatures of 37/20 or 37/25 C. Stratified lanceleaf sage seeds from freezing and chilling environments did not differ in germination following incubation in fluctuating temperatures and averaged 56 and 55%, respectively. Germination of stratified lanceleaf sage seeds from the freezing and thawing environment was higher than 50% during the thawing cycle, suggesting the possibility of early season emergence of this species. Our study indicates that biennial wormwood and lanceleaf sage have the potential to develop a seedbank that can persist for more than 2 yr. High moisture levels in the soil seedbank can lead to reduced seed survival.

Nomenclature: Biennial wormwood, Artemisia biennis Willd. ARTBI; lanceleaf sage, Salvia reflexa Hornem. SALRE.

Much research has been conducted on mesotrione activity on crops and weeds, but information is lacking in regards to the relative contribution of soil and foliar absorption of mesotrione. Three experiments conducted at Virginia Tech's Glade Road Research Facility in Blacksburg, VA, evaluated the effects of 50 and 90% relative humidity (RH) on the activity of mesotrione applied to foliage, soil, and soil plus foliage. Tall fescue injury ranged from 0 to 21% and was significant in 6 of 20 comparisons. Three of these injury events were caused by soil plus foliar applications, which were always more injurious than foliar only applications, which were more injurious than soil-only applications. Both application placement and RH significantly influenced smooth crabgrass responses to mesotrione. Smooth crabgrass phytotoxicity was lowest when mesotrione was applied only to foliage and highest when mesotrione was applied to soil and foliage. Increasing RH from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity when mesotrione was applied only to foliage. By dissecting the plant canopy, it was noted at 14 d after treatment, when averaged over RH, that white leaves comprise 16% of leaves when only foliage was treated and 55 and 62% when applied to soil plus foliage and soil only, respectively. Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil or both soil and foliage, but in older leaves when applied only to foliage. Data indicate mesotrione entering plants through soil travels quickly to growing points and has an equal or greater effect on plant phytotoxicity than foliar-absorbed mesotrione. In addition, foliar-absorbed mesotrione appears to increase in plants significantly with increasing RH, but does not move rapidly to growing points.

Nomenclature: Mesotrione; smooth crabgrass, Digitaria ischaemum Schreb. ex Muhl. DIGIS; tall fescue, Schedonorus phoenix (Scop.) Holub FESAR, also Lolium arundinaceum (Schreb.) S.J. Darbyshire

Introduction of glyphosate resistance into crops through genetic modification has revolutionized crop protection. Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species. Factors that have contributed to this shift in weed species composition in Georgia cotton production are reviewed, along with the implications of continued overreliance on this technology. Potential scenarios for managing glyphosate-resistant populations, as well as implications on the role of various sectors for dealing with this purported tragedy of the commons, are presented. Benghal dayflower, a glyphosate-tolerant species, continues to spread through Georgia and surrounding states, whereas glyphosate susceptibility in Palmer amaranth is endangered in Georgia and other cotton-producing states in the southern United States. Improved understanding of how glyphosate susceptibility in our weed species spectrum was compromised (either through occurrence of herbicide-tolerant or -resistant weed species) may allow us to avoid repeating these mistakes with the next herbicide-resistant technology.

Nomenclature: Glyphosate; Benghal dayflower, Commelina benghalensis L.; Palmer amaranth, Amaranthus palmeri S. Wats; cotton, Gossypium hirsutum L.

Global expansion in the cultivation of genetically engineered (GE) crops has raised concerns about the adventitious presence of GE seeds in non-GE and organic products. Flax is the second most important oilseed crop in western Canada and is currently being evaluated as a potential platform for the production of bio-products. Before transgenic flax is released for commercial production, mitigation measures must be identified to reduce the adventitious presence in subsequent crops. To quantify adventitious presence of volunteer flax in spring wheat and to identify the efficacy of herbicide treatments on mitigating volunteer flax adventitious presence, research was conducted at four locations during 2005 and 2006 in central Alberta. To simulate artificial volunteer populations, flax was seeded prior to wheat at a target population of 150 plants m⁻². In the untreated control, volunteer flax seed yield was 135 kg ha⁻¹, which resulted in adventitious presence of 8.57% in spring wheat. When left uncontrolled, volunteer flax reduced wheat yields ∼57% and resulted in volunteer flax seed production of 4,755 seeds m⁻². A single PRE treatment of glyphosate or glyphosate plus tribenuron reduced volunteer flax density from 39 to 4 and 6 plants m⁻², respectively, seed production from 4,755 to < 58 seeds m⁻², and volunteer flax seed viability from 55 to < 40%. POST herbicides, fluroxypyr plus MCPA and fluroxypyr plus 2,4-D, reduced volunteer flax seed production as low as 0.6 and 0.0 seeds m⁻², respectively, adventitious presence to 0.64 and 0.03%, respectively, and seed viability to ≤ 10%. Combination of glyphosate applied PRE followed by fluroxypyr plus 2,4-D or by thifensulfuron plus tribenuron plus quinclorac applied POST reduced adventitious presence of volunteer flax in wheat to near 0%. These treatment combinations were also effective for reducing volunteer flax fecundity to 0.0 and 7.1 seeds m⁻², respectively, and volunteer flax seed viability to 0 and 5%, respectively. This study demonstrated that with effective mitigation strategies, seed mediated gene flow from GE volunteer flax can be reduced.

Nomenclature: 2,4-D; fluroxypyr; glyphosate; MCPA; quinclorac; thifensulfuron; tribenuron; flax, Linum usitatissimum L.; wheat, Triticum aestivum L.