Jamshid Ashigh

An Ala205val Substitution in Acetohydroxyacid Synthase of Eastern Black Nightshade (Solanum Ptychanthum) Reduces Sensitivity to Herbicides and Feedback Inhibition

Abstract Twelve populations of eastern black nightshade from different locations in Ontario are resistant to imazethapyr. This study aimed at determining the molecular basis of resistance in these populations and the activity of the resistant acetohydroxyacid synthase (AHAS) enzyme compared to that of the sensitive AHAS in response to different herbicides and branched-chain amino acid concentration. The results of partial AHAS sequencing indicated that all resistant populations had a cytosine331 to thymine substitution coding for an alanine205 to valine substitution. In vitro AHAS enzyme assays of one resistant population showed that the specific activity of the resistant enzyme was 56% less than that of the susceptible enzyme. AHAS from the resistant population was 72-, 70-, and 64-fold less sensitive than that of the susceptible population to imazethapyr, imazamox, and primisulfuron, respectively. Furthermore, the resistant enzyme was less sensitive to feedback inhibition from branched-chain amino acids compared to the susceptible enzyme. Results confirmed that resistance in resistant populations of eastern black nightshade was conferred by target-site modification and that the Ala205Val substitution alters the kinetics and regulation of branched-chain amino acid biosynthesis. Nomenclature: Imazethapyr; imazamox; primisulfuron; eastern black nightshade, Solanum ptychanthum Dunal SOLPT.

Mechanism of Resistance and Inheritance in Glyphosate Resistant Palmer amaranth (Amaranthus palmeri) Populations from New Mexico, USA

Abstract Two populations of Palmer amaranth from New Mexico have been confirmed to be resistant to glyphosate. In the present study, the molecular basis of resistance and the mode of inheritance of resistance in those populations were investigated. Quantitative real-time polymerase chain reaction analysis indicated up to an eightfold increase in genomic EPSPS copy number in glyphosate resistant plants compared with susceptible plants. The relative genomic EPSPS copy number of resistant plants was positively correlated with the relative EPSPS cDNA expression levels. Eight hours after treatment with glyphosate, the shikimate accumulation levels in resistant plants were negatively correlated with the genomic EPSPS copy numbers. Multiple sequencing of the EPSPS cDNA of resistant plants did not reveal any glyphosate resistance-conferring mutations. The evaluation of F1, reciprocal F1, and F2 Palmer amaranth families indicated that resistance to glyphosate does not follow a single-gene segregation pattern. Results suggest that the EPSPS amplification is the primary molecular basis of resistance in glyphosate resistant populations of Palmer amaranth from New Mexico. Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Watts.

Resistance to Glyphosate in Palmer Amaranth (Amaranthus palmeri) Populations from New Mexico Pecan Orchards

Abstract Two populations of Palmer amaranth suspected of being resistant to glyphosate have been reported since 2007 in pecan orchards in Doña Ana County, New Mexico. The objectives of the study were to confirm and evaluate the level of resistance, to evaluate the effectiveness of alternative herbicide mechanisms of action, and to compare the cost of effective alternative herbicides for weed management in pecan orchards. Greenhouse experiments indicated that the resistant populations were able to survive glyphosate at 736 g ae ha−1. Compared with a susceptible (S) population, one of the resistant (R) populations had sevenfold resistance to glyphosate. POST application of 12 herbicides, with five different mechanisms of action, all provided at least 88% control of both R and S populations when applied at their recommended field rates. PRE application of trifluralin and metolachlor also provided more than 99% control of R and S populations. The results of field studies indicated that the financial benefit of season-long weed management with glyphosate, in pecan orchards, was comparable with some of the tested alternative herbicides. Nomenclature: 2,4-D; atrazine; carfentrazone-ethyl; dicamba; flumioxazin; glyphosate; glufosinate; imazethapyr; imazamox; metolachlor; oxyfluorfen; primisulfuron; prosulfuron; pyrithiobac; trifluralin; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pecan, Carya illinoinensis (Wangenh) K. Koch. Resumen Dos poblaciones de Amaranthus palmeri sospechosas de ser resistentes a glyphosate han sido reportadas desde 2007 en plantaciones de pacana en el condado Doña Ana en New Mexico. Los objetivos de este estudio fueron confirmar y evaluar los niveles de resistencia, evaluar la efectividad de herbicidas con mecanismos de acción alternativos, y comparar la eficacia y el costo de herbicidas alternativos efectivos para el manejo de malezas en plantaciones de pacana. Experimentos de invernadero indicaron que las poblaciones resistentes fueron capaces de sobrevivir a glyphosate a 736 g ae ha-1. Comparada con una población susceptible (S), una de las poblaciones resistentes (R) tuvo una resistencia a glyphosate siete veces mayor. Aplicaciones en pos emergencia (POST) de 12 herbicidas, con cinco mecanismos de acción diferentes, proporcionaron un control de al menos 88% para ambas poblaciones R y S cuando se aplicaron a sus respectivas dosis de campo recomendadas. La aplicación en preemergencia (PRE) de trifluralin y metolachlor también brindó mas de 99% de control de poblaciones R y S. Los resultados de estudios de campo indicaron que en plantaciones de pacana, el beneficio monetario de realizar el manejo de malezas a lo largo de todo el ciclo productivo con glyphosate, fue comparable con algunas de los herbicidas alternativos evaluados.

ALS-Inhibitor Resistance in Populations of Eastern Black Nightshade (Solanum ptycanthum) from Ontario1

Populations of eastern black nightshade suspected of being resistant to acetolactate synthase (ALS) inhibitors have been reported since 1999 in different locations in Ontario, Canada. This event has threatened the use of ALS inhibitors for control of this species. The objectives of this study were to evaluate the spectrum of resistance to different ALS-inhibiting herbicides and to examine the effectiveness of alternative modes of action herbicides. Growth room experiments were conducted to determine the response to imazethapyr and atrazine in seven suspected ALS inhibitor– resistant populations. One resistant and one susceptible population were further characterized for their response to ALS inhibitors and chloroacetamides. Seven populations were able to survive imazethapyr at 100 g ai/ha, while there was no resistance to atrazine. Compared to a susceptible (S) population, resistant (R) population SOLPT 1 had 726-, 31-, 6-, and 4-fold resistance to postemergence (POST) applied imazethapyr, imazamox, primisulfuron, and flumetsulam, respectively. Preemergence (PRE) application of imazethapyr, flumetsulam, cloransulam, nicosulfuron, prosulfuron, and rimsulfuron did not provide control of the R population, whereas they totally controlled the S population. The chloroacetamide herbicides metolachlor, dimethenamid, and flufenacet all provided at least 90% control of both R and S populations when applied PRE at the recommended field rates. The ALS inhibitors will not provide adequate control of these resistant populations, but acceptable control could be achieved with chloroacetamides or with atrazine. Nomenclature: ALS-inhibiting herbicides; eastern black nightshade, Solanum ptycanthum Dun. #3 SOLPT. Additional index words: Spectrums of resistance, chloroacetamide herbicides, dose–response curves. Abbreviations: ALS, acetolactate synthase; GR50, dose required to reduce plant dry weight by 50%; POST; postemergence; PRE, preemergence; R, resistant; S, susceptible.

Water and Temperature Stress Impact Fitness of Acetohydroxyacid Synthase―Inhibiting Herbicide-Resistant Populations of Eastern Black Nightshade (Solanum ptychanthum)

Abstract Many substitutions in the herbicide target enzyme acetohydroxyacid synthase (AHAS) confer whole-plant resistance and may reduce plant fitness. This study was done to determine the impact of different watering and temperature regimes on the germination, growth, and seed production of eastern black nightshade populations resistant (R) to AHAS inhibitors as conferred by an Ala205Val substitution in their AHAS. Growth and reproductive ability of four R and four susceptible (S) populations were determined in growth-cabinet and greenhouse studies. The R populations had lower total berry and viable seed production per plant than S under optimal conditions because of slower berry maturation. Seed production of both S and R populations decreased under lower or higher than optimal watering regimes; however, this reduction was more pronounced for the S populations so that seed production was comparable across S and R. The R populations had significantly higher germination and vegetative growth under cooler alternating temperature regimes. Although there were no differences between R and S plants under stress conditions, under optimal growth conditions, the Ala205Val substitution comes at a significant cost in eastern black nightshade. Under optimal growth conditions and in the absence of herbicide selection, S populations should eventually dominate over R; however, the lack of fitness differences under stress conditions could enhance the persistence of the R individuals. Nomenclature: Eastern black nightshade, Solanum ptychanthum Dun. SOLPT.

Mobility of Indaziflam Influenced by Soil Properties in a Semi-Arid Area

Indaziflam, a broad-spectrum, pre-emergence herbicide was the focus of a field investigation conducted after the identification of sporadic injury symptoms on the pecan trees a few months after the application. The study was conducted in two pecan orchards located in southern New Mexico, USA, and southeastern Arizona, USA. The objectives of this study were to evaluate the occurrence and distribution of indaziflam in the soil profile of areas where pecan trees were injured (impacted) and areas where no injury symptoms were observed (unimpacted), and to determine the relationship between indaziflam concentrations and soil properties in those locations. Soil samples were collected, one year after applications, from six depth representing 0–7, 7–15, 15–30, 30–60, 60–90 and 90–120 cm depth to determine the concentration of indaziflam in impacted and unimpacted areas of the two orchards. Soil samples were analyzed to determine texture, bulk density, organic matter content, cation exchange capacity, pH, nitrate, chloride and calcium concentrations. The detection frequency of indaziflam was higher in Arizona than in New Mexico, likely due to the differences between the tillage practices and sand contents of the orchards. No significant correlations were observed between indaziflam and soil properties, however indaziflam was mostly detected in areas where pecan trees were unimpacted probably as result of greater organic matter content and soil porosity. More research is needed to understand the causes of injury to pecan trees by indaziflam application.

Genetics of Resistance to Acetohydroxyacid Synthase Inhibitors in Populations of Eastern Black Nightshade (Solanum Ptychanthum) from Ontario

Abstract Resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides in populations of eastern black nightshade from Ontario has been linked to an Ala205Val substitution in the AHAS enzyme. The aim of this study was to determine the mechanism of inheritance of AHAS inhibitor resistance and the genetic relationships among resistant (R) and susceptible (S) eastern black nightshade populations from Ontario. Homozygous R and S parental populations were crossed and the inheritance was analyzed in F1 (S × R), reciprocal F1 (R × S), F2, and backcross (S × F1) progenies after application of imazethapyr at 150 g ai ha−1. Compared to parental lines, the progenies were rated as R, intermediate (I), and S phenotypes. All the F1 progenies were of the I phenotype. The backcross progenies segregated in a 1:1 (S:I) ratio, and the F2 families segregated in a 1:2:1 (R:I:S) ratio. These results indicate that a single nuclear gene, with incomplete dominance, controls resistance to AHAS-inhibiting herbicides in R population of eastern black nightshade. Random amplified polymorphic DNA (RAPD) markers were screened among 25 R and S populations. The genetic relationship of R and S populations based on RAPD profiles generated from six RAPD primers indicated four groups of populations in which resistance seems to have arisen independently. However, based on similarity coefficients, resistance within three of the groups could have arisen by gene flow. Both similar local selection pressure and gene flow could explain the spread of the Ala205Val substitution in R populations of eastern black nightshade in Ontario. Nomenclature: Imazethapyr; eastern black nightshade, Solanum ptychanthum Dun. SOLPT

Numerical Evaluation of Nitrate Distributions in the Onion Root Zone under Conventional Furrow Fertigation

AbstractHYDRUS (2D/3D) model was used to simulate spatial and temporal distributions of nitrate-nitrogen (NO3-N) within and below the onion root zone under conventional furrow fertigation with the urea-ammonium-nitrate (UAN) liquid fertilizer. The simulated water contents in the furrow irrigated onion field agreed well with the measurements. Simulations produced similar patterns of the measured NO3-N concentration profiles throughout the growing season. NO3-N concentrations remained higher and accumulation of NO3-N was observed within the root zone. Higher NO3-N within the root zone was dependent on the rate of the UAN fertilizer application, quantity of NO3-N removed by root uptake, and NO3-N drainage fluxes below the root zone. Simulations also suggested that NO3-N below the root zone during different growth stages remained much higher than a recommended (for drinking water) standard concentration level (10  mg L−1). This resulted in higher NO3-N drainage fluxes, particularly during the fertigation even...

Effect of application rate and irrigation on the movement and dissipation of indaziflam

Indaziflam is a new preemergence herbicide for the control of annual grass and broadleaf weeds in various cropping systems including pecan orchards. The objectives of this study were to (1) determine the mobility and dissipation of indaziflam and (2) evaluate herbicide efficacy in a flood-irrigated pecan orchard in southern New Mexico, USA. Indaziflam was applied at 0, 36.5, and 73.1g/ha in areas with (impacted) and without (unimpacted) tree injury symptoms. Soil samples were collected at 0-15, 15-30, and 30-46cm depths 26, 63, 90, and 126days after the first herbicide application. Additional soil samples were collected 4, 30, and 56days after the second application. Indaziflam was detected in soil samples collected at each depth, suggesting movement with irrigation water. Indaziflam concentrations decreased with increasing soil depth and time. Indaziflam mass recoveries were greater in the unimpacted area than in the impacted area after the first and second applications. Dissipation half-lives of indaziflam in the soil ranged from 30 to 86days for total indaziflam recovered from the entire soil profile after the first and second applications in both areas. The percent weed control was similar in the impacted and unimpacted areas for both rates of indaziflam on 26 and 63days after application; however, on 90days after the application, percent weed control was lower in the impacted than unimpacted area.