Greenhouse studies were conducted to evaluate 14C-glufosinate absorption, translocation, and metabolism in glufosinate-resistant corn, goosegrass, large crabgrass, and sicklepod. Glufosinate-resistant corn plants were treated at the four-leaf stage, whereas goosegrass, large crabgrass, and sicklepod were treated at 5, 7.5, and 10 cm, respectively. All plants were harvested at 1, 6, 24, 48, and 72 h after treatment (HAT). Absorption was less than 20% at all harvest intervals for glufosinate-resistant corn, whereas absorption in goosegrass and large crabgrass increased from approximately 20% 1 HAT to 50 and 76%, respectively, 72 HAT. Absorption of 14C-glufosinate was greater than 90% 24 HAT in sicklepod. Significant levels of translocation were observed in glufosinate-resistant corn, with 14C-glufosinate translocated to the region above the treated leaf and the roots up to 41 and 27%, respectively. No significant translocation was detected in any of the weed species at any harvest timing. Metabolites of 14C-glufosinate were detected in glufosinate-resistant corn and all weed species. Seventy percent of 14C was attributed to glufosinate metabolites 72 HAT in large crabgrass. Less metabolism was observed for sicklepod, goosegrass, and glufosinate-resistant corn, with metabolites composing less than 45% of detectable radioactivity 72 HAT.

In order to clarify the translocation and metabolism of glyphosate in lead tree, a mixture of glyphosate (0.5 mmol) and 14C-glyphosate was injected into mini tree plants (1.5-cm trunk diameter and 120 cm tall), simulating a grown lead tree in a greenhouse experiment. Within 6 d after treatment (DAT), all leaves on the lead tree plants dropped. Analysis of nonlabeled glyphosate in both xylem and phloem every 15 d showed that glyphosate residues accumulated mainly in phloem, similar to the findings in established lead tree. In this experiment, glyphosate concentration in the phloem of three parts of the trunk, i.e., upper (15 cm above the upper injection hole), middle (between the two injection holes), and lower (15 cm below the lower injection hole) parts, increased rapidly within 45 DAT, and decreased thereafter. Obviously, significant glyphosate metabolism occurred during the latter period. Further 14C radioactivity measurement also exhibited similar time-course changes of glyphosate dissipation, except that a low level of radioactivity in the phloem of the upper part and a high level of radioactivity in the lower part were detected from 45 to 90 DAT, suggesting that more glyphosate metabolites in the phloem of the upper part might have translocated with photosynthetic assimilates and reallocated to the lower part of the mini tree. Thin-layer chromatography (TLC) analysis of metabolites derived from 14C-glyphosate also revealed that about 70% of the radioactivity recovered in the phloem of the lower part comprised the unknown metabolites M1 (retention factor [Rf] : 0.83) and M2 (Rf : 0.94), with nearly 28% of the radioactivity from sarcosine (Rf : 0.76), and less than 2% of the radioactivity from aminomethylphosphoric acid (AMPA) (Rf : 0.49) and glyphosate (Rf : 0.36). It is concluded that active metabolism of glyphosate in this 90-d simulation experiment occurred mainly from 45 to 90 DAT, and the primary metabolism was modulated by C-P lyase.

Nitrogen-fixing cyanobacteria are vital photosynthetic microorganisms that contribute to soil fertility by fixing atmospheric nitrogen and are also important for maintaining ecosystem stability. These microorganisms can be very sensitive to herbicides because they possess many characteristics of higher plants. Six days after the application of monosulfuron at 0.03 to 0.3 nmol L−1 under laboratory conditions, growth of the nitrogen-fixing cyanobacteria Anabaena flos-aquae, Anabaena azollae, and Anabaena azotica was stimulated, but at higher concentrations (30 to 300 nmol L−1) protein synthesis was inhibited. The production of 16 amino acids in A. flos-aquae was reduced from 7 to 69% with increasing monosulfuron concentration. Application of monosulfuron at 3 to 300 nmol L−1 substantially inhibited in vitro acetolactate synthase (ALS) activity as indicated by 50% inhibition index values of 3.3, 65.2, and 101.3 nmol L−1 for A. flos-aquae, A. azollae, and A. azotica, respectively. In contrast, extractable ALS activity was not affected in these algal species with monosulfuron treatments ranging from 0.03 to 300 nmol L1 except in A. flos-aquae at higher concentrations (30 to 300 nmol L−1). The most sensitive species to monosulfuron was A. flos-aquae, followed by A. azollae and A. azotica. Molecular analyses showed that the genomic DNA of A. azollae and A. azotica differed in only one amino acid. Results from photogenetic analyses revealed a high degree of homology between these algae. In contrast, the genomic DNA of A. flos-aquae differed from that of A. azollae and A. azotica in 44 and 45 amino acids, respectively. Our findings support the view that monosulfuron toxicity in these three nitrogen-fixing cyanobacteria is due to interference with protein metabolism via inhibition of branch-chain amino acid biosynthesis, and particularly ALS activity.

Studies were conducted under controlled growth conditions to determine whether differential absorption, translocation, or metabolism was the basis for the differential response of grain sorghum hybrids to mesotrione. Mesotrione-tolerant (‘Dekalb DKS35-70’) and mesotrione-susceptible (‘Pioneer 84G62’) grain sorghum hybrids were treated with 14C-labeled mesotrione. At 1 d after treatment (DAT), absorption was 7% in both hybrids; at 7 DAT, however, absorption remained nearly steady in Pioneer 84G62 but increased to 12% in Dekalb DKS35-70. Translocation of 14C-mesotrione in sorghum hybrids was similar with less than 30% of the absorbed herbicide translocated out of the treated leaf by 7 DAT. A distinct metabolite of 14C-mesotrione was separated in both hybrids at 3 DAT. The amount of mesotrione parent compound that remained in Pioneer 84G62 and DKS35-70 was 72 and 65%, respectively. Dekalb DKS35-70 had significantly less mesotrione at 3 DAT than Pioneer 84G62 did, but the amount of mesotrione was similar for both hybrids at 5 and 7 DAT. Rapid metabolism of mesotrione may help explain the differential response of grain sorghum hybrids.

Greenhouse studies were conducted to evaluate absorption, translocation, and metabolism of 14C-glufosinate in glufosinate-resistant cotton, nontransgenic cotton, Palmer amaranth, and pitted morningglory. Cotton plants were treated at the four-leaf stage, whereas Palmer amaranth and pitted morningglory were treated at 7.5 and 10 cm, respectively. All plants were harvested at 1, 6, 24, 48, and 72 h after treatment (HAT). Absorption of 14C-glufosinate was greater than 85% 24 h after treatment in Palmer amaranth. Absorption was less than 30% at all harvest intervals for glufosinate-resistant cotton, nontransgenic cotton, and pitted morningglory. At 24 HAT, 49 and 12% of radioactivity was translocated to regions above and below the treated leaf, respectively, in Palmer amaranth. Metabolites of 14C-glufosinate were detected in all crop and weed species. Metabolism of 14C-glufosinate was 16% or lower in nontransgenic cotton and pitted morningglory; however, metabolism rates were greater than 70% in glufosinate-resistant cotton 72 HAT. Intermediate metabolism was observed for Palmer amaranth, with metabolites comprising 20 to 30% of detectable radioactivity between 6 and 72 HAT.

Variations in the acetolactate synthase (ALS) gene sequence were determined from 28 populations of corn poppy resistant (R) to tribenuron and from 6 populations susceptible (S) to this herbicide. The ALS gene fragment (634 bp) sequence revealed in R populations five point mutations at the codon Pro197, and among them the substitution of Pro197 by Ala was the most common. The sequencing chromatograms revealed that nine R individuals had only the mutant ALS gene and were homozygous (RR), 18 R individuals had both the wild type and the mutant ALS gene and were heterozygous (RS), whereas one R individual was heterozygous but with two different mutant ALS alleles (R1R2). The use of restriction digestion profile analysis to verify the DNA sequence results by detecting the existence of point mutations at the codon 197 managed to distinguish the R and S alleles and confirmed the results obtained from the sequencing chromatograms analysis. The secondary protein structure prediction suggested the formation of novel β-strands for each of the five mentioned amino acid substitutions that was not present in wild type ALS around the mutant site. These findings support the hypothesis that the substitution of Pro197 by Ser, Thr, Ala, Arg, or Leu resulted in altered secondary structure, which stabilizes an ALS tertiary conformation that prevents tribenuron binding and thus confers resistance to this herbicide.

Junglerice is one of the most serious grass weeds of rice in the tropics. Experiments were conducted in the laboratory and screenhouse to determine the influence of environmental factors on seed germination and seedling emergence of junglerice in the Philippines. In the laboratory, germination was stimulated by light, suggesting that seeds of this species are positively photoblastic. The tested temperatures (35/25, 30/20, and 25/15 C alternating day/night temperatures), however, did not influence germination. Germination in the laboratory was not affected by a soil pH range of 4 to 9, but was decreased by salinity (> 50 mM NaCl) and moisture stress (< −0.2 MPa osmotic potential). In the screenhouse, germination of junglerice was greatest (97%) for seeds at the soil surface, but emergence declined exponentially with increasing seed burial depth, and no seedlings emerged from seeds buried at 6 cm. In pots, seedling emergence declined markedly with the addition of rice residue to the soil surface at rates equivalent to 4 to 6 tonnes (t) ha−1. As germination of junglerice was strongly stimulated by light, and seedling emergence was optimal at shallow burial depths, this species is likely to be problematic in reduced tillage systems.

Inhibitory effects of two sesquiterpene lactones, costunolide and parthenolide, isolated from dichloromethane extract of the stem bark of southern magnolia and a parthenolide derivative, 1,10-epoxyparthenolide, were evaluated on germination and seedling growth of wild oat. The sesquiterpene lactones effected a significant reduction of seed germination, particularly at the highest concentrations of 200, 400, and 600 mg L−1, with costunolide being the most active one. Furthermore, the three sesquiterpenes strongly inhibited root and shoot growth of the weed. However, the inhibition of root growth by all compounds was greater than that of shoot growth. Parthenolide inhibited growth of both root and shoot more strongly than the other compounds and a reference herbicide imazamethabenz. At a concentration of 100 mg L−1, parthenolide caused 87 and 41% growth inhibition of root and shoot, respectively. Parthenolide was tested for its effect on acetolactate synthase (ALS) activity. The compound inhibited the enzyme in a concentration-dependent manner, with 50% inhibition of 51.44 µM. The results of this study indicated that the herbicidal activity of the isolated sesquiterpene may be attributed to inhibition of ALS. The promising phytotoxic activity of sesquitepene lactones reported here could be considered a starting point for developing environmentally safer herbicides.

Greenhouse studies were established in Fayetteville, AR, to investigate glyphosate resistance in Arkansas common ragweed populations. Common ragweed seed were collected from plants in Pope and Jackson counties in Arkansas. Plants grown from seed were sprayed with one of seven glyphosate rates. Populations in Pope and Jackson counties were 21-fold and 10-fold more tolerant to glyphosate, respectively, than a known susceptible population. Based on 14C-glyphosate absorption and translocation studies, reduced glyphosate absorption or translocation was not the resistance mechanism in Arkansas glyphosate-resistant common ragweed. Shikimate accumulation did not differ among the known susceptible and the two resistant populations at 3 d after treatment (DAT). However, by 5 DAT, shikimate accumulation in the two resistant populations was lower than the known susceptible population. Data indicate that glyphosate-resistant common ragweed is present in at least two locations in Arkansas, and the resistance mechanism is not an insensitive target site or reduced glyphosate absorption or translocation.

The transgenic corn line 98140 has a high level of resistance to glyphosate and all five chemical classes of herbicides that inhibit acetolactate synthase (ALS). The dual herbicide resistance is due to a molecular stack of two constitutively expressed genes: gat4621, which produces a glyphosate acetyltransferase that rapidly inactivates glyphosate, and hra, which produces a highly resistant ALS. On a rate basis, the positive 98140 isoline with a single copy of the gat4621 gene is over 1,000-fold more resistant to glyphosate than a negative isoline without the transgene. Similarly, the positive 98140 isoline with the hra gene is over 1,000-fold more resistant to ALS-inhibiting herbicides such as chlorimuron and sulfometuron at the whole-plant and enzyme level. The gat4621 and hra genes do not change the natural tolerance of corn to selective herbicides, so new corn hybrids based on 98140 will give growers more options to manage weeds and delay the evolution of herbicide-resistant weeds.

The genetic basis of weedy and invasive traits and their evolution remain poorly understood, but genomic approaches offer tremendous promise for elucidating these important features of weed biology. However, the genomic tools and resources available for weed research are currently meager compared with those available for many crops. Because genomic methodologies are becoming increasingly accessible and less expensive, the time is ripe for weed scientists to incorporate these methods into their research programs. One example is next-generation sequencing technology, which has the advantage of enhancing the sequencing output from the transcriptome of a weedy plant at a reduced cost. Successful implementation of these approaches will require collaborative efforts that focus resources on common goals and bring together expertise in weed science, molecular biology, plant physiology, and bioinformatics. We outline how these large-scale genomic programs can aid both our understanding of the biology of weedy and invasive plants and our success at managing these species in agriculture. The judicious selection of species for developing weed genomics programs is needed, and we offer up choices, but no Arabidopsis-like model species exists in the world of weeds. We outline the roadmap for creating a powerful synergy of weed science and genomics, given well-placed effort and resources.

The joint activity of mesotrione and atrazine can display synergistic effects on the control of both triazine-sensitive and site-of-action-based triazine-resistant (TR) broadleaf weeds. The first objective of this study was to evaluate a PRE application of atrazine followed by a POST application of mesotrione for potential interactions in both site-of-action-based TR redroot pigweed and metabolism-based atrazine-resistant (AR) velvetleaf. Results from these sequential experiments demonstrated that synergism was detected in reducing biomass of the TR redroot pigweed but not in the AR velvetleaf with metabolism-based resistance. The second objective was to evaluate the joint activity of mesotrione and atrazine in a tank-mix application in the AR velvetleaf biotype. Greenhouse studies with the AR biotype indicated that synergism resulted from a tank mix with a constant mesotrione rate of 3.2 g ai ha−1 in mixture with atrazine ranging from 126 to 13,440 g ai ha−1. Chlorophyll fluorescence imaging also revealed a synergistic interaction on the AR biotype when 3.2 g ha−1 of mesotrione was applied with 126 g ha−1 of atrazine beginning 36 h after treatment and persisting through 72 h.

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.

Goatsrue is an introduced perennial plant which has proven to have great invasive potential, leading to its classification as a noxious weed in many states and at the federal level. Very little research has been done on its basic biology. Physical dormancy of mature goatsrue seed was tested through scarification with sulfuric acid for up to 60 min resulting in 100% germination. Comparison of dormancy for 26-yr-old and 6-mo-old goatsrue seed indicated that aged seeds had reduced dormancy levels compared to newly harvested seeds. Maximum germination was similar among the 6-mo old and 26-yr-old seed lots, suggesting no loss of viability had occurred in seed stored dry for 26 yr. Goatsrue seedling emergence was inversely related to burial depth, and decreased as burial depth increased. Emergence of seed buried at 0.5 to 3.0 cm soil depth was 93 to 87%, respectively, and no emergence occurred from 12 and 14 cm. When the soil seed bank of five goatsrue-infested areas was sampled, the largest density of seeds found was 74,609 seeds m−2 while the lowest was 14,832 seeds m−2. Viability and dormancy of seeds recovered from the soil seed bank survey ranged from 91 to 100% and 80 to 93%, respectively. Management, which reduces the soil seed bank and controls emerging seedlings, is as essential as control of mature goatsrue plants in order to avoid seedling establishment and reinvasion of a location.

The ecological significance of species diversity within agroecosystems has become a point of interest in recent years. Although the time and spatial scale at which diversity is measured may influence the interpretation of its functional importance, little research has been conducted on methodological approaches to assess the number and relative abundance of agricultural plant species. In this study, we (1) evaluated the applicability of the species-area curve to examine plant species richness and α and β diversity in conventional no-tillage and organically managed spring wheat systems, and (2) assessed temporal changes in plant species richness across systems. Measurements were obtained at three times during two growing seasons in experimental plots and at three times during one growing season on commercial farms in Montana. In accordance with previous studies, management system affected species richness and diversity. In eight of nine studied cases, we detected a positive relationship between species richness and sampled area. In these eight cases, intercepts (α diversity) were higher for the organic systems than for the conventional no-tillage systems. Slopes (β diversity) were higher for the organic system in six of nine cases studied. Species richness declined as the season progressed for both systems, with the organic systems consistently having more species than the conventional no-tillage systems. Despite differences in the species composition and between the experimental plots and commercial farm field size, the patterns of species richness and α and β diversity were relatively constant, suggesting that the processes responsible for the assembly of plant communities in agroecosystems of the Northern Great Plains are similar at a range of spatial scales.

Recent advances in sequencing technologies (next-generation sequencing) offer dramatically increased sequencing throughput at a lower cost than traditional Sanger sequencing. This technology is changing genomics research by allowing large scale sequencing experiments in nonmodel systems. Waterhemp is an important weed in the midwestern United States with characteristics that makes it an interesting ecological model. However, very few genomic resources are available for this species. One half of a 70 by 75 picotiter plate of 454-pyrosequencing was performed on total DNA isolated from waterhemp, generating 158,015 reads of an average length of 271 bp, or a total of nearly 43 Mbp of sequence. Included in this sequence was a nearly complete sequence of the chloroplast genome, sequences of several important herbicide resistance genes, leads for simple sequence repeat (SSR) markers, and a sampling of the repeated elements (e.g., transposons) present in this species. Here we present the waterhemp genomic data gleaned from this sequencing experiment and illustrate the value of next-generation sequencing technology to weed science research.

A population of shepherd's-purse suspected to be resistant to the triazinone herbicide hexazinone, a photosystem II (PS II) inhibitor, was collected from an alfalfa field in 2007 in Oregon. A whole-plant, dose–response assay confirmed that the putative-resistant population was highly resistant to hexazinone. The resistant population was 22-fold more resistant to hexazinone than the susceptible population. However, the hexazinone-resistant population was susceptible to other PS II-inhibiting herbicides, including atrazine, diuron, and terbacil. DNA sequence analysis of the chloroplast psbA gene encoding the D1 protein of PS II, the target site of PS II inhibitors, identified a point mutation from Phe to Ile at position 255 in the hexazinone-resistant population. Single- and double-point mutations at position 255, which is located in the QB binding niche of the D1 protein, were previously reported in Chlamydomonas reinhardtii, Synechococcus species, and Synechocystis species after site-directed mutagenesis and were associated with decreased binding of PS II inhibitors. To our knowledge, this is the first report of a mutation of the psbA gene at Phe255 in a field-selected, herbicide-resistant plant.

Spiny and slender amaranth are troublesome Amaranthaceae species of direct-seeded rice and other upland crops in many Asian countries. Seed germination and seedling emergence response of these species to various environmental factors was determined in laboratory and screenhouse experiments. Germination in both species was stimulated by 35/25 and 30/20 C fluctuating temperatures and light. Germination of slender amaranth was more sensitive to increasing salt and water stress than spiny amaranth. Spiny amaranth germinated at a NaCl concentration of 100 mM (19%), whereas slender amaranth seeds did not germinate at this concentration. In seed burial trials where the seeds were on the soil surface, emergence was 56 and 68% for spiny amaranth and slender amaranth, respectively. Only 7% of spiny amaranth seedling emerged from a soil depth of 0.5 cm, whereas no emergence was observed for 4 cm or deeper. For slender amaranth, 6 and 0% emergence was observed at 4 and 6 cm, respectively. Seedling emergence of spiny amaranth was affected more by high rates of rice residue than slender amaranth. Greater quantities of residue than those normally found in rice fields were required to significantly reduce weed densities.

The temporal dynamics of spatial heterogeneity was studied for the weed communities in a seashore paspalum turf with the use of a power-law model. Surveys were conducted in January, March, May, July, September, and November in 2007. In every survey, we set 100 quadrats (50 by 50 cm) referred to as L quadrats on a 50-m line transect at the same position in the turf. Each L quadrat was then divided into four S quadrats (25 by 25 cm) and all plant species occurring in each of these S quadrats were identified and recorded. These data were summarized into frequency distributions and the percentage of S quadrats containing a given species, and the variance of each species was estimated. The power law was used to evaluate the spatial heterogeneity (δ) and frequency of occurrence (p) for each species in the weed communities in six survey months. The results showed that weeds emerged more frequently in the summer–spring season than in winter–autumn, and the spatial heterogeneity was much higher in summer–spring than winter–autumn, especially in summer. The Shannon–Wiener diversity indexes (H') from large to small were July (5.9202) > May (5.6775) > September (5.6631) > March (5.5727) > January (5.1742) > November (4.9668). Likewise, the spatial heterogeneity index (δc ) of the whole community was also different in different months. The biggest δc (0.2790) was in July, and the smallest (0.1811) in November. Meanwhile, manilagrass had a high p (= 1.0), indicating that it occurred in all S quadrats in every weed community of every month. However, the turfgrass, seashore paspalum, only emerged in March, May, July, and November, and possessed a low p, indicating the seashore paspalum turf has been naturally replaced by manilagrass.