Weed Biotypes Research Articles

Weed resistance development and management in herbicide-tolerant crops: experiences from the USA

The evolution of weeds in USA agroecosystems predates herbicide tolerant (HT) crops by several decades. However, given the unprecedented adoption of genetically engineered (GE) HT crops, particularly in maize, cotton and soybean and the concomitant use of glyphosate, the evolution of glyphosate-resistant (GR) biotypes of agronomic important weeds now represents a significant threat to the sustainability of the GE HT trait and the herbicide. Notably, GR biotypes of horseweed (Conyza canadensis), common waterhemp (Amaranthus tuberculatus), Palmer pigweed (Amaranthus palmeri), giant ragweed (Ambrosia trifida) and johnsongrass (Sorghum halepense) have increased dramatically in frequency in conjunction with the adoption of crop production systems based on GE HT crops. Furthermore, other shifts in weed communities have been observed. It must be emphasized that the GE HT crops did not directly cause these weed shifts. The industry was responsible initially for these changes in weed communities given their initial positions that the glyphosate-resistant weed problems were unlikely to evolve and the recommendations to use glyphosate exclusively for weed management. However, decisions by growers to use glyphosate must be factored in as imparting selection pressure on the weed communities resulting in weed management problems, particularly when university recommendations strongly suggested not to use glyphosate in this manner. The key is now to provide growers with sufficient information to convince them to adopt integrated weed management programs and utilize best management practices. Given the presumed and real benefits of convenience and simplicity of the GE HT crop production systems; this will be a difficult task.

HERBICIDE RESISTANT WEED MANAGEMENT USING SENSITIVITY ANALYSIS OF THE WEED POPULATION GROWTH CURVE

Strategies to prevent herbicide weed resistance are rarely practiced by farmers. As a consequence, herbicide resistant weed biotypes (HRWB) have been increasing worldwide in the past decades. This paper aims to analyze the weed population growth curve and to propose a strategic plan for prevention and management of HRWB. The existing weed control methods are organized considering the sensitivity analysis of the population growth at each phase of the logistic growth curve. This analysis indicates that tactics directed to reduce the population growth rate are most appropriate for HRWB management, mainly at the initial phase of the resistant weed population growth. This epidemiological approach provides evidence to the importance of early detection and management of HRWB.

Fate and Efficacy of Metolachlor Granular and Emulsifiable Concentrate Formulations in a Conservation Tillage System

Use of genetically modified cultivars resistant to the herbicide glyphosate (N-phosphonomethylglycine) is strongly associated with conservation-tillage (CsT) management for maize ( Zea mays L.), soybean ( Glycine max L.), and cotton ( Gossypium hirsutum L.) cultivation. Due to the emergence of glyphosate-resistant weed biotypes, alternate weed management practices are needed to sustain CsT use. This work focused on metolachlor use (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide) in a CsT system. The fate and efficacy of granular and emulsifiable concentrate (EC) formulations or an EC surrogate were compared for CsT cotton production in the Atlantic Coastal Plain region of southern Georgia (USA). The granular formulation, a clay-alginate polymer, was produced in the authors' laboratory; EC was a commercial product. In field and laboratory dissipations the granular metolachlor exhibited 8-fold greater soil persistence. Rainfall simulation runoff assessments indicated that use of the granular formulation in a common CsT system, strip-tillage (ST), may reduce metolachlor runoff loss when compared to conventional tillage (CT) management or when EC formulations are used in the ST system. Metolachlor leaching assessments using field-deployed lysimeters showed some tillage (ST > CT) and formulation (EC > granular) differences. Overall leaching was generally small when compared to runoff loss. Finally, greenhouse bioassays showed control of two weed species with the granular was greater than or equal to that of the EC formulation; however, the granular formulation suppressed cotton growth to a greater extent. In sum, this metolachlor granular formulation has advantages for CsT cotton production; however, additional research is needed to assess impacts on crop injury.

Effect of herbicide treatments on the weed mass in a long-term experiment in Martonvásár

The weed mass (g m −2 ) recorded in the first 15 years (1965–1979) of a long-term, bifactorial, split-plot herbicide experiment (main plots: two types of soil cultivation, subplots: 7 herbicide treatments, with two control plots) without crops indicated that the best weed control was achieved with 10 kg ha −1 rates of simazine and atrazine. These were followed by 5 kg ha −1 ametryn, 10 kg ha −1 linuron and 2+2 kg ha −1 2,4-D, all with moderate efficiency, while 5 kg ha −1 prometryn and 10 kg ha −1 monolinuron resulted in poorer weed control. Medium deep ploughing once a year in autumn reduced the weed mass by 36.5%. There was a substantial year effect, well illustrated by the annual changes in weed mass both in the herbicide treatments and in the control plots. In plots treated with simazine and atrazine there was an exponential increase in the weed mass from the 17 th year of the experiment, suggesting the multiplication of weed biotypes resistant to triazine. As a result of some herbicide treatments there was a shift in the monocot-dicot ratio.

Herbicidas alternativos para controle de biótipos de Conyza bonariensis e C. canadensis resistentes ao glyphosate

Após sucessivos anos, aplicações do herbicida glyphosate em pomares de citros no Estado de São Paulo selecionaram biótipos resistentes de Conyza bonariensis e C. canadensis. Na ocorrência de plantas daninhas resistentes em uma área agrícola, tornam-se necessárias mudanças nas práticas de manejo para obtenção de adequado controle das populações resistentes, bem como para a redução da pressão de seleção sobre outras espécies. Assim, este trabalho foi realizado com o objetivo de identificar herbicidas alternativos para controle de biótipos de Conyza spp. resistentes ao herbicida glyphosate, com aplicações em diferentes estádios fenológicos da planta daninha. Três experimentos foram conduzidos em campo, em pomares de citros em formação, sobre plantas de buva em estádio fenológico de dez folhas e no pré-florescimento. Para plantas no estádio de dez folhas, controle satisfatório foi obtido com aplicações de glyphosate + bromacil + diuron (1.440 + 1.200 + 1.200 g ha-1), glyphosate + atrazina (1.440 + 1.500 g ha-1) e glyphosate + diuron (1.440 + 1.500 g ha-1). Quando em estádio de pré-florescimento de Conyza spp., a aplicação do herbicida amônio-glufosinato, na dose de 400 g ha-1, isolado ou associado a MSMA, bromacil+diuron, metsulfuron, carfentrazone e paraquat, foi a alternativa viável para controle dos biótipos resistentes ao glyphosate.

Evolution of glyphosate resistance in a Lolium rigidum population by glyphosate selection at sublethal doses

The majority of the documented cases of field-evolved herbicide-resistant weed biotypes established that single major genes confer glyphosate resistance. However, the contribution of minor genes endowing substantial plant survival at sublethal herbicide doses may be a potential complementary path to herbicide resistance evolution in weed populations under selection. Here, we subjected a number of susceptible individuals of Lolium rigidum to recurrent glyphosate selection to test the potential for sublethal glyphosate doses to additively select for glyphosate resistance. After 3-4 cycles of glyphosate selection in two distinct environments, the progenies of the initially susceptible population were shifted toward glyphosate resistance. The results indicate progressive enrichment of minor gene trait(s) contributing toward plant survival in the glyphosate-selected progenies. After three generations of selection, the estimated LD(50) values were doubled compared with the original population and up to 33% plant survival was obtained in the glyphosate-selected progeny at the recommended glyphosate label rate. This level of resistance probably was the maximum shift achievable with sublethal glyphosate dose selection in this small population. Cross-pollination was a crucial factor enabling the rapid rate of accumulation of minor glyphosate resistance gene trait(s) that are likely to be present at a relatively high frequency in a small susceptible population. The mechanistic basis of the moderate glyphosate resistance level selected by sublethal glyphosate doses remains unknown and warrants future research. Studying the main factors influencing the evolution of resistant weed populations is crucial for understanding, predicting and managing herbicide resistance.

In-Field and Soil-Related Factors that Affect the Presence and Prediction of Glyphosate-Resistant Horseweed (Conyza canadensis) Populations Collected from Indiana Soybean Fields

Glyphosate-resistant (GR) crops have been rapidly adopted in the United States and the evolution of GR weeds throughout the world has also been on the rise. With experience, weed scientists and crop advisers develop “intuition” on the basis of field history and current in-field conditions for predicting whether escaped weed biotypes may be herbicide resistant. However, there are no previous reports on the association of in-field crop management factors with the prediction of herbicide resistance. By using in-field survey data, we tested the accuracy of predicting glyphosate resistance in late-season horseweed escapes. We hypothesized that glyphosate resistance in late-season horseweed populations found in soybean fields could be predicted using in-field knowledge of crop residues and the appearance and distribution of weeds in the field. Field survey data were collected to determine the distribution and frequency of GR horseweed populations in Indiana soybean fields during September and October of 2003, 2004, and 2005. After the in-field survey, soil properties for sampled field locations were also collected from the U.S. Department of Agriculture Natural Resources Conservation Service Web Soil Survey. GR horseweed predictions used in-field presence of crop residues and the appearance, abundance, and distribution of weeds in the field. The significance of independent data factors were determined by chi-square statistics. The interactions and relative significance of multiple factors were modeled using classification and regression tree analysis. Our results indicated that the most important factor for predicting GR populations was the identification of an altered plant phenotype after injury from POST glyphosate. This was followed by crop rotation, field distribution, and the presence of other escaped weed species in the field in a model with a classification rate of 0.68.

Interactions of Tillage and Cover Crop on Water, Sediment, and Pre‐emergence Herbicide Loss in Glyphosate‐Resistant Cotton: Implications for the Control of Glyphosate‐Resistant Weed Biotypes

The need to control glyphosate [N-(phosphonomethyl)glycine]-resistant weed biotypes with tillage and preemergence herbicides in glyphosate-resistant crops (GRCs) is causing a reduction in no-tillage hectarage thereby threatening the advances made in water quality over the past decade. Consequently, if environmental gains afforded by GRCs are to be maintained, then an in-field best management practice (BMP) compatible with tillage is required for hectarage infested with glyphosate-resistant weed biotypes. Thus, 1 d after a preemergent application of fluometuron [N,N-dimethyl-N'-(3-(trifluoromethyl)phenyl)urea] (1.02 kg ha(-1)) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] (1.18 kg ha(-1)) to a Dundee silt loam (fine-silty, mixed, active, thermic Typic Endoaqualf), simulated rainfall (60 mm h(-1)) was applied to 0.0002-ha microplots for approximately 1.25 h to elucidate tillage (no tillage [NT] and reduced tillage [RT])and cover crop (no cover [NC] and rye cover [RC]) effects on water, sediment, and herbicide loss in surface runoff. Regardless of tillage, RC delayed time-to-runoff 1.3-fold, reduced cumulative runoff volume 1.4-fold, and decreased cumulative sediment loss 4.7-fold. Cumulative fluometuron loss was not affected by tillage or cover crop. Conversely, total metolachlor loss was 1.3-fold lower in NT than RT and 1.4-fold lower in RC than NC. These data indicate that RC can be established in hectarage requiring tillage and potentially curtail water, sediment, and preemergence herbicide losses in the spring to levels equivalent to or better than that of NT, thereby protecting environmental gains provided by GRCs.

Differential Response of Grain Sorghum Hybrids to Foliar-Applied Mesotrione

The selection of herbicide-resistant weeds in grain sorghum production has prompted researchers to explore alternative herbicides to prevent, delay, and manage herbicide-resistant weed biotypes. Greenhouse and field experiments were conducted to evaluate the differential response of sorghum hybrids to POST application of mesotrione. In a greenhouse experiment, 85 sorghum hybrids were treated with 0, 52, 105, 210, and 315 g ai/ha mesotrione when plants were at the three- to four-leaf collar stage. Sorghum response ranged from susceptible to tolerant sorghum hybrids. ‘Pioneer 84G62’, ‘Pioneer 85G01’, and ‘Triumph TR 438’ were the three most susceptible, whereas ‘Dekalb DKS35-70’, ‘Frontier F222E’, and ‘Asgrow Seneca’ were the three most tolerant hybrids. One week after treatment (WAT), the mesotrione rate causing 50% visible injury ranged from 121 to 184 and 64 to 91 g/ha in the most tolerant and susceptible hybrids, respectively. Mesotrione dose–response studies were conducted under field conditions on four sorghum hybrids. One WAT, injury symptoms were greater (up to 23%) in Pioneer 85G01 than in Asgrow Seneca (< 14%). However, all plants appeared normal by the end of the growing season. In addition, sorghum yields were not reduced by mesotrione treatments as verified by correlation coefficient analysis.

Role of Translocation as A Mechanism of Resistance to Glyphosate

The continuous use of glyphosate has resulted in the selection of glyphosate-resistant (GR) biotypes in 13 weed species. Decreased translocation of glyphosate to the meristematic tissue is the primary mechanism of resistance in horseweed, hairy fleabane, rigid ryegrass, and Italian ryegrass, and the resistance is inherited as a single, semidominant nuclear trait. The question is: What role does decreased translocation play in glyphosate resistance, and what is the actual mechanism(s)? The enzyme 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS), the target site of glyphosate, preferentially accumulates in the active meristems of plants. Inhibition of EPSPS results in the accumulation of shikimate. Leaf disc assays across a number of species show that the maximum accumulation of shikimate occurs in young, rapidly expanding tissue. Gene expression studies have also shown that the gene encoding EPSPS is maximally expressed in meristems. Thus, glyphosate needs to translocate to the growing points of plants to be effective. In some GR weed biotypes, glyphosate moves in the treated leaf via the transpiration stream; but instead of being loaded into the phloem, it is trapped in the distal portion of the leaf. These results suggest that there is some type of inhibition of glyphosate-loading into the phloem in GR plants. However, this mechanism may involve uptake of glyphosate at the cellular level. Shikimate accumulation in isolated leaf discs occurs at high glyphosate concentrations in both susceptible and GR biotypes of horseweed and Italian ryegrass; but at low concentrations, shikimate accumulation occurs only in susceptible biotypes. Decreased cellular uptake of glyphosate might occur by one of four mechanisms: (1) the active uptake system no longer recognizes glyphosate, (2) an active efflux system pumps glyphosate out of the cell into the apoplast, (3) an active efflux system pumps glyphosate out of the chloroplast into the cytoplasm, or (4) glyphosate is pumped into the vacuole and sequestered in the cell.

Interaction effects of two biological control organisms on resistant and susceptible weed biotypes of Chondrilla juncea in western North America

Interactions between natural enemy species may modify their net effect on host plants, but little research has been done to examine how plant resistance influences species interactions in biological weed control. We performed common garden experiments with clonal accessions of Chondrilla juncea to compare a rust-susceptible weed biotype with a rust-resistant biotype, both of which are invasive in western North America. Inoculations with two biological control organisms, a rust fungus pathogen ( Puccinia chondrillina) and an eriophyid gall mite ( Eriophyes chondrillae), were applied separately and in combination to test if plant performance is modified by antagonistic or facilitative species interactions. We found no significant rust × mite interaction effects for several plant traits (shoot length, shoot dry biomass, fecundity, rosette growth, and rosette senescence), and therefore the two natural enemies appear to have independent and complementary effects on plant performance. As expected, genetic resistance of weed biotypes modified the effects of rust disease on shoots and rosettes, although not for plant fecundity. We found no significant biotype × mite effects on plant performance traits. However, rust inoculation reduced the total dry biomass of mite galls in the rust-susceptible biotype by indirect competition due to decreased growth of diseased shoots, but not in the rust-resistant biotype. Our results indicate that rust disease may have the potential to modify mite gall epidemiology and relative plant performance in mixed populations of resistant and susceptible C. juncea biotypes, though at present the geographic distributions of these North American biotypes are mainly separate.

Effect of cowpea intercropping on weed control and corn yieid

Herbicides have simplified weed control, but the use of herbicides, besides being costly, resulted in the selection of herbicide-resistant weed biotypes and has become an environmental contamination factor. Herbicide use reduction is one of the goals of modern agriculture, with several alternatives being investigated, including intercropping. The objective of this study was to evaluate the effects of cowpea and corn cultivar intercropping on weed control and corn green-ear (immature ears with 80% humidity grains) and grain yield. A completely randomized block design with split-plots and four replications was used. AG 1051, AG 2060 and PL 6880 corn cultivars (assigned to plots) were submitted to the four treatments: no weeding, two hoe-weeding (22 and 41 days after planting), and intercropping with cowpea (BR 14 and IPA 206 cultivars, with indeterminate growth). The cowpea was planted (with corn planting) between the corn rows, in pits 1.0 m apart, with two plants per pit. The corn cultivars did not differ from each other as to weed density (WD), fresh above-ground weed biomass (WB), green-ear yield and grain yields. Higher WD and WB mean values were found in no weeding subplots; lower mean values in two hoe-weeding subplots; and intermediate mean values in intercropped subplots, indicating that cowpea plants had, to a certain extent, control over weeds. The no-weeded plots and the intercropped plots had lower green-ear and grain yields. Although the cowpea cultivars had a certain control over weeds (mean reductions of 22.5 and 18.3%, in terms of green matter density and weight of the above-ground part of weeds, respectively), they also competed against the corn plants, leading to yield reduction (mean reductions of 17.0 and 32% in green ear and grain yield, respectively). The cowpea cultivars did not produce grain, certainly due to the strong competition exerted by the corn and weeds on cowpea plants.

A Hydrothermal Seedling Emergence Model for Giant Ragweed (Ambrosia trifida)

Late-season giant ragweed emergence in Ohio crop fields complicates decisions concerning the optimum time to implement control measures. Our objectives were to develop a hydrothermal time emergence model for a late-emerging biotype and validate the model in a variety of locations and burial environments. To develop the model, giant ragweed seedlings were counted and removed weekly each growing season from 2000 to 2003 in a fallow field located in west central Ohio. Weather data, soil characteristics and geographic location were used to predict soil thermal and moisture conditions with the Soil Temperature and Moisture Model (STM2). Hydrothermal time (θHT) initiated March 1 and base values were extrapolated from the literature (Tb= 2 C, ψb= −10 MPa). Cumulative percent emergence initially increased rapidly and reached 60% of maximum by late April (approximately 400 θHT), leveled off for a period in May, and increased again at a lower rate before concluding in late July (approximately 2,300 θHT). The period in May when few seedlings emerged was not subject to soil temperatures or water potentials less than the θHTbase values. The biphasic pattern of emergence was modeled with two successive Weibull models that were validated in 2005 in a tilled and a no-tillage environment and in 2006 at a separate location in a no-tillage environment. Root-mean-square values for comparing actual and model predicted cumulative emergence values ranged from 8.0 to 9.5%, indicating a high degree of accuracy. This experiment demonstrated an approach to emergence modeling that can be used to forecast emergence on a local basis according to weed biotype and easily obtainable soil and weather data.

Difference in ultraweak photon emissions between sulfonylurea‐resistant and sulfonylurea‐susceptible biotypes of Scirpus juncoides following the application of a sulfonylurea herbicide

We examined the ultraweak photon emissions from a paddy weed, Scirpus juncoides, to assess the availability of photon emissions for the identification of weed biotypes resistant to sulfonylurea herbicides. The emission intensity from the plant organs increased when treated with a sulfonylurea herbicide in a concentration‐dependent manner. The increment in emissions was higher in the sulfonylurea‐resistant biotypes than in the sulfonylurea‐susceptible biotypes. The difference between the biotypes was greater in the culms than in the roots and remained so through the vegetative growth stage to the flowering stage. This difference was independent of the seed source or mutations in the acetolactate synthase genes of the resistant biotypes. These results suggest that the determination of ultraweak photon emissions can be a useful method for identifying the sulfonylurea‐resistant biotypes of S. juncoides.

Glyphosate sustainability in South American cropping systems

South America represents about 12% of the global land area, and Brazil roughly corresponds to 47% of that. The major sustainable agricultural system in South America is based on a no-tillage cropping system, which is a worldwide adopted agricultural conservation system. Societal benefits of conservation systems in agriculture include greater use of conservation tillage, which reduces soil erosion and associated loading of pesticides, nutrients and sediments into the environment. However, overreliance on glyphosate and simpler cropping systems has resulted in the selection of tolerant weed species through weed shifts (WSs) and evolution of herbicide-resistant weed (HRW) biotypes to glyphosate. It is a challenge in South America to design herbicide- and non-herbicide-based strategies that effectively delay and/or manage evolution of HRWs and WSs to weeds tolerant to glyphosate in cropping systems based on recurrent glyphosate application, such as those used with glyphosate-resistant soybeans. The objectives of this paper are (i) to provide an overview of some factors that influence WSs and HRWs to glyphosate in South America, especially in Brazil, Argentina and Paraguay soybean cropped areas; (ii) to discuss the viability of using crop rotation and/or cover crops that might be integrated with forage crops in an economically and environmentally sustainable system; and (iii) to summarize the results of a survey of the perceptions of Brazilian farmers to problems with WSs and HRWs to glyphosate, and the level of adoption of good agricultural practices in order to prevent or manage it.

Weed species shifts in glyphosate‐resistant crops

The adoption of glyphosate-based crop production systems has been one of the most important revolutions in the history of agriculture. Changes in weed communities owing to species that do not respond to current glyphosate-based management tactics are rapidly increasing. Clearly, glyphosate-resistant crops (GRCs) do not influence weeds any more than non-transgenic crops. For most crops, the trait itself is essentially benign in the environment. Rather, the weed control tactics imposed by growers create the ecological selection pressure that ultimately changes the weed communities. This is seen in the adoption of conservation tillage and weed management programs that focus on one herbicide mode of action and have hastened several important weed population shifts. Tillage (disturbance) is one of the primary factors that affect changes in weed communities. The intense selection pressure from herbicide use will result in the evolution of herbicide-resistant weed biotypes or shifts in the relative prominence of one weed species in the weed community. Changes in weed communities are inevitable and an intrinsic consequence of growing crops over time. The glyphosate-based weed management tactics used in GRCs impose the selection pressure that supports weed population shifts. Examples of weed population shifts in GRCs include common waterhemp [Amaranthus tuberculatus (Moq ex DC) JD Sauer], horseweed (Conyza canadensis L), giant ragweed (Ambrosia trifida L) and other relatively new weed problems. Growers have handled these weed population shifts with varying success depending on the crop.

Acetolactate synthase inhibitor-resistant stinkweed (Thlaspi arvense L.) in Alberta

Two stinkweed populations from southern and central Alberta were not controlled by acetolactate synthase (ALS)-inhibiting herbicides in 2000. This study reports on their cross-resistance to ALS-inhibiting herbicides, molecular basis of resistance, and inheritance of resistance. Both putative herbicide-resistant biotypes responded similarly to increasing doses of the herbicides. The biotypes were highly resistant to ethametsulfuron and exhibited a low level of resistance to metsulfuron and imazethapyr. However, both biotypes were not resistant to florasulam, a triazolopyrimidine ALS inhibitor, or sulfometuron, a non-selective sulfonylurea ALS inhibitor. The cross-resistance pattern was consistent with the confirmed target-site mutation. Sequence analysis of the ALS gene detected a Pro197Leu mutation in both biotypes. Similar to many other ALS inhibitor-resistant weed biotypes, resistance was conferred by a single dominant gene. This study confirms the first global occurrence of herbicide resistance in this species. Key words: ALS-inhibitor resistance, ALS sequence, herbicide resistance, target-site mutation

A New Mutation in PlantALSConfers Resistance to Five Classes of ALS-Inhibiting Herbicides

Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3,200-fold resistance to all four ALS-Inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison ofALSgenes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of anALSgene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, theALSgene was cloned and expressed inArabidopsis. TransgenicArabidopsisexpressing thisALSgene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-Inhibiting herbicide classes.

Alternative control of littleseed canary grass using eucalypt oil

Globally, huge amounts of synthetic herbicides are used to manage weeds in arable lands. However, their widespread use has resulted in various toxicological effects on the environment and human health, besides resulting in the emergence of herbicide-resistant weed biotypes. To overcome these problems, there is an urgent need to search for novel compounds, particularly natural plant products, with potential herbicidal activity. In this area, we studied the phytotoxic effect of volatile oil from lemon-scented eucalypt on littleseed canary grass, a noxious weed of wheat fields. Our findings show that under laboratory conditions the emergence and earlier growth of the weed decreased and completely ceased using a very low concentration of eucalypt oil (0.0714%, v/v). Treatment with eucalypt oil of the 4-week-old pot-raised weeds caused visible damage such as chlorosis and necrosis, wilting and even plant death. The effect was concentration-dependent. At low concentrations, 2.5 and 5%, v/v of eucalypt oil, plants were damaged but recovered later, whereas at concentrations higher than 5%, v/v, of eucalypt oil plants showed severe injury with little or no sign of recovery, and death. There was a severe effect on the photosynthetic and respiratory ability of treated plants 7 and 21 days after treatment. Eucalypt oil treatment caused a rapid electrolyte leakage in the P. minor leaf tissues, indicating a loss of membrane integrity. The study concludes that lemon-scented eucalypt oil offers a good option for control of littleseed canary grass and could be included as a viable component of integrated weed management under sustainable agricultural practices.

Discovery of single-nucleotide mutations in acetolactate synthase genes by Ecotilling

TILLING (Targeting Induced Local Lesions IN Genomes) is a powerful reverse genetic technique that employs a mismatch-specific endonuclease to discover induced point mutations in genes of interest. The use of the TILLING technique to survey natural variation in genes is called Ecotilling. We report an adaptation of Ecotilling for rapid detection of single-nucleotide mutations in the acetolactate synthase (ALS) genes of sulfonylurea (SU)-resistant (R) Monochoria vaginalis (Pontederiaceae), a paddy weed, in Japan. Genomic DNA of a SU-R plant (target DNA) was mixed with the DNA of a SU-susceptible (S) plant (reference DNA). Ecotilling detected two nucleotide mutations in the ALS gene of SU-R M. vaginalis. These 2 mutations were confirmed by DNA sequencing. A single nucleotide mutation (C to A), in the codon CCT to CAT and another mutation (C to T), in the codon CCT to TCT were identified by sequencing. Both mutations result in the disruption of a Pro codon in the conserved Domain A region with the consequent substitution of a His residue in the first mutation and a Ser residue in the second. Substitution of the Pro residue in Domain A of the ALS gene has been reported to result in insensitivity to SUs in many weed biotypes. This study demonstrates that Ecotilling is a fast, reliable, economical method for detecting single-nucleotide mutations in genes arising from herbicide selection.