This is the first report of fitness cost associated with the AHAS Trp574Leu mutation in R. sativus populations. This fitness cost could reduce frequency of the resistant allele without herbicide selection. © 2018 Society of Chemical Industry.

This is the first report of feral radish with resistance to herbicides belonging to all the AHAS-inhibiting herbicide families, conferred by Trp574Leu mutation in the AHAS gene. An appropriate herbicide rotation with alternative herbicides such as fluorochloridone or aclonifen and an increase in the diversity of cropping systems are important for minimising the prevalence of these biotypes.

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Brassica rapa L. is an annual Brassicaceae species cultivated for oil and food production, whose wild form is a weed of crops worldwide. In temperate regions of South America and especially in the Argentine Pampas region, this species is widely distributed. During 2014, wild B. rapa populations that escaped control with glyphosate applications by farmers were found in this area. These plants were characterized by morphology and seed acidic profile, and all the characters agreed with B. rapa description. The dose-response assays showed that the biotypes were highly resistant to glyphosate. It was also shown that they had multiple resistance to AHAS-inhibiting herbicides. The transgenic origin of the glyphosate resistance in B. rapa biotypes was verified by an immunological test which confirmed the presence of the CP4 EPSPS protein and by an event-specific GT73 molecular marker. The persistence of the transgene in nature was confirmed for at least 4 years, in ruderal and agrestal habitats. This finding suggests that glyphosate resistance might come from GM oilseed rape crops illegally cultivated in the country or as a seed contaminant, and it implies gene flow and introgression between feral populations of GM B. napus and wild B. rapa. The persistence and spread of the resistance in agricultural environments was promoted by the high selection pressure imposed by intensive herbicide usage in the prevalent no-till farming systems.

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SummaryThe timing of seed germination may determine the success of a weed species in an agroecosystem, and its expression is modulated by environmental conditions, but also by seed physiology and anatomy. The aims of this study were to investigate the roles of light, pericarp, dry storage and cold stratification on seed dormancy and germination in feral radish, a troublesome agricultural weed in temperate zones of the Americas that reduces crop yields. To this end, we used isolated intact pods and extracted seeds to test germination over time under contrasting temperature, light and storage conditions. Here, we showed that fresh seeds were non‐dormant, but that light and the presence of the pericarp reduced germination, especially under low temperatures. The pericarp reduced the final water content absorbed by seeds inside pods and decreased absorption/dehydration rates. The pericarp showed several small lignified cell layers in the endocarp, and x‐ray images displayed the lack of space between the partially embedded seed and the endocarp. Dry storage and cold stratification were ineffective in breaking the dormancy imposed by the pericarp. The apparent requirement for darkness and the mechanical restriction of the pericarp may have the potential to induce dormancy, spreading the timing of seed germination over a more extended period and hindering the control of feral radish.

Seed dormancy could be a factor related to natural reseeding of hairy vetch (Vicia villosa ssp. villosa Roth.), a winter annual species cultivated for seed, pasture, hay, green manure and cover crop. The presence of combinational dormancy (physical dormancy + physiological dormancy, PY + PD) in hairy vetch was explored by a model using laboratory and field measures. At the stage of natural dispersal, dry seeds of hairy vetch were stored under laboratory conditions at 5, 10, 20 and 30°C (±2°C) or buried at 5 cm depth in an experimental field. Germination at 5, 8, 10, 15, 20 and 25°C was assessed at regular intervals up to 295 days after harvest. Following the hypothesis of the existence of a combinational dormancy mechanism, model development was based on the estimation of: (i) the fraction of non-PY seed as a function of after-ripening thermal-time accumulation, and (ii) seed population thermal parameters associated with a given level of PD. The developed model adequately described the after-ripening thermal-time requirements for PY + PD release of V. villosa. Based on model predictions, under a semi-arid thermal regime, >45% of vetch seeds shed during the summer season would be able to germinate during early autumn. Thus, the seed-bank size threshold at the end of the first growing season should be >65 seeds m–2 in order to reach a minimum stand of 30 plants m–2 necessary for a productive pasture.

Helianthus annuus and H. petiolaris (Asteraceae) are wild sunflowers native to North America but have become naturalized in central Argentina covering an area of about 5 million hectares. Wild H. annuus has been recognized as invader species in several countries, but no research has been done to study the ecological determinants of their distribution. In a survey covering seven provinces between 31858 0 -388S and 60833 0 -698W, we described the ecology of the main wild populations. Wild Helianthus populations were located in three of the 18 ecological regions of Argentina, on five Mollisol and seven Entisol soil groups. The associated plant communities were comprised 60 species belonging to 16 families, all being frequent components of the native flora. Disease symptoms were seldom observed in wild populations, with Alternaria helianthi being the most commonly observed pathogen. Population size varied from less than 100 to more than 100,000 plants, covering from 100 to more than 60,000 m 2 with densities most frequently up to 3 plants/m 2 , but reaching 80 plants/m 2 at certain sites. Intermediate plant phenotypes between wild species and cultivated sunflower were found in one-third of the populations providing evidence of intense gene flow. Hybrid swarms were found at three localities with population sizes between 100 and 10,000 individuals.