Common waterhemp (Amarathus rudis Sauer) is a frequent weed in glyphosate-resistant (GR) crops in the midwestern USA due, in part, to the delayed emergence of its seedlings. Variable waterhemp emergence was simulated by transplanting seedlings into both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] and bare plot areas at differing crop growth stages during two growing seasons in western Minnesota. Growth and fecundity were measured. As expected, late planted weeds produced little dry matter and few seeds, and competition from corn or soybean reduced waterhemp dry weight and fecundity by $ 90% compared with isolated plants. Interestingly, common waterhemp was affected differently by crop and transplanting date. Common waterhemp grown with corn was always shaded by the crop canopy but produced seeds even when transplanted as late as the V10 growth stage. In soybean, weeds transplanted before the V4 growth stage were taller than soybean and produced more seeds than those transplanted into corn at a comparable growth stage; however, those transplanted after V5 produced no seeds. Consequently, control of late-emerging common waterhemp plants in soybean may not be needed, whereas control of late-emerging plants in corn may be justified because of relatively high levels of seed production.
Abstract:Herbicide resistance is an evolutionary process that may involve changes in physiological and ecological traits related to plant fi tness. This type of resistance occurs in littleseed canarygrass (Phalaris minor). This study was aimed to determine the effect of resistance to ACCase-inhibiting herbicides on fi tness of seed accessions of littleseed canarygrass. Seed characteristics are important because they may determine the outcome of competition between susceptible (S) and resistant (R) biotypes. Experiments were performed to determine whether differences between S and R accessions in embryo size, dormancy, longevity, and germination speed might occur. Differences in plant fi tness associated to herbicide resistance were found in seed physiological characteristics. The embryo size of R accessions was reduced up to 50 % (on a dry weight basis), which resulted in an increase in the number of days required to germinate and a reduction in the germination speed. Damage to membranes, which is related to longevity, was more severe in the R accessions. The observed decrease in seed vigor could affect the establishment of seedlings, which may explain the low success of R accessions under herbicide selection pressure. Key words: ACCase-inhibiting herbicides, fi tness, germination, dormancy, longevity, vigor.Resumen: La resistencia a herbicidas es un proceso evolutivo que puede involucrar cambios en caracteres ecológicos y fi siológi-cos asociados con la adecuación de las plantas. Este tipo de resistencia ocurre en Phalaris minor. El objetivo de este estudio fue determinar el efecto de la resistencia a herbicidas inhibidores de la ACCasa en la aptitud de las semillas de Phalaris minor. Las características de las semillas son importantes debido a que la competencia entre biotipos susceptibles (S) y resistentes (R) inicia en esta etapa. Una serie de experimentos fueron realizados para determinar si existen diferencias entre colectas S y R en aspectos como el tamaño del embrión, latencia, longevidad y velocidad de germinación. Se encontraron diferencias en la aptitud asociadas con la resistencia a herbicidas, las cuales mostraron diferencias en la calidad fi siológica de las semillas. El tamaño del embrión de las colectas R se redujo en un 50 % (con base en la materia seca), lo cual ocasionó un incremento en el número de días a la germinación y la reducción de la velocidad de germinación. El daño en las membranas, que fue usado como una medida para indicar la longevidad, fue más severo en las colectas R. La reducción observada en el vigor de las semillas podría afectar el establecimiento de las plántulas, y podría dar una explicación del bajo establecimiento de las colectas R cuando la presión de selección del herbicida está ausente. Palabras clave: aptitud, germinación, herbicidas inhibidores de la ACCasa, latencia, longevidad, vigor.
Background: Herbicide application to control weeds can promote a rapid selection of resistant phenotypes in small geographic areas. Also, in areas with a spatial heterogeneous management, resistance may evolve independently, promoting local adaptation in weeds. In the mexican region known as "El Bajio," 100,000 ha are cultivated with wheat, and the weed Phalaris minor, resistant to ACCase-inhibiting herbicides is commonly present. Question:We aim to identify the population structure of two genes in four different P. minor biotypes from "El Bajio" and to determine their association with phenology and plant growth differences (biomass and seed yield) that may contribute to survival in the agricultural environment.Studied species: Phalaris minor Retz. Study site and years of study: The study was carried out at a greenhouse and at the molecular biology laboratory of the Colegio de Postgraduados, during 2013 and 2014. Methods:The diversity of the psbA gene and the sequence of two ACCase gene fragments as well as phenology, growth and biomass allocation were evaluated. Results:Results indicated different polymorphism levels for the two genes. There were no differences in the psbA gene between biotypes, although the ACCase gene exhibited high polymorphism level. In addition, each biotype showed differences in phenology, biomass accumulation and fecundity. Conclusions:The ACCase-inhibiting herbicide resistance in "El Bajio" region might be a resistance hotspot leading to the local adaptation of weeds.Key words: Evolution, herbicide resistance, local adaptation. ResumenAntecedentes: La aplicación de herbicidas para el control de las malezas puede fomentar una rápida selección de fenotipos resistentes en áreas geográficas pequeñas. También, en áreas con un manejo heterogeneo espacial, la resistencia puede evolucionar independientemente, promoviendo adaptaciones locales de las malezas. En la region mexicana conocida como "El Bajío", se cultivan 100,000 ha con trigo, y la maleza Phalaris minor, resistente a herbicidas inhibidores de la ACCasa está comunmente presente.Pregunta: El objetivo fue identificar la estructura poblacional de dos genes en cuatro biotipos diferentes de P. minor de "El Bajío" y determinar su asociación con las diferencias en fenología y crecimiento de la planta (biomasa y rendimiento de semilla) que puedan contribuir a la sobrevivencia en campos agrícolas.Especie estudiada: Phalaris minor Retz. Sitio y años de estudio: El estudio se realizó en un invernadero y en el laboratorio de Biología Molecular del Colegio de Postgraduados durante 2013 y 2014.Métodos: La diversidad del gen psbA y la secuencia de dos fragmentos del gen ACCasa fueron evaluados, así como también la fenología, el crecimiento y la asignación de biomasa.Resultados: Los resultados indicaron diferentes niveles de polimorfismo para los dos genes. No hubieron diferencias en el gen psbA entre los biotipos, aunque el gen ACCasa exhibió nivel alto de polimorfismo. Además, cada biotipo mostró diferencias en la fenología, acumulación de ...
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