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$\\bullet$ We investigated the effect of a herbicide resistance-conferring mutation on fitness in Amaranthus powellii. $\\bullet$ Morphological and histological observations were made. Growth and leaf appearance were recorded for six resistant and six susceptible populations. The competitiveness of a susceptible population was compared with that of a resistant population using a replacement series experiment. $\\bullet$ Leaves of the resistant plants were distorted and much smaller than those of susceptible plants. Additionally, they exhibited an abnormal morphological and structural pattern consisting of a mosaic of heterogeneous areas in the same leaf blade. The roots and stems had similar structures in susceptible and resistant plants, but the former were up to four times more developed. The resistant plants were slower to develop and produced 67% less biomass and 58% lower leaf area than susceptible plants. Under competitive conditions, one susceptible population outperformed one resistant population by 7-15 times. $\\bullet$ The $Trp_{574}Leu$ acetohydroxyacid synthase (AHAS) mutation appears to have considerable pleiotropic effects on the early growth and development of the plants which, in competitive conditions, greatly reduce fitness.

The presence of glyphosate-resistant smooth pigweed (Amaranthus hybridus L.) biotypes has increased in southern Brazil in recent years, presenting the triple amino acid substitution TAP-IVS in 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS) previously found in Argentina. Some of these biotypes have morphological characteristics of A. hybridus and redroot amaranth (Amaranthus retroflexus L.). The present study aimed to identify, through molecular markers, the herbicide-resistant species of Amaranthus from Brazil that have the TAP-IVS substitution and to analyze the occurrence of pollen-mediated gene flow (PMGF) as the source of the TAP-IVS substitution in these biotypes. Six biotypes were evaluated using internal transcribed spacer (ITS) sequences, of which two (AMACHY-S and CAMAQ-R) were molecularly classified as A. hybridus and four (AMACRET-S, AMACVI-S, ARRGR-R, and SAOJER-R) were unclassified. Interestingly, all the glyphosate-resistant biotypes (ARRGR-R, SAOJER-R, and CAMAQ-R) had the TAP-IVS substitution and an increase in EPSPS relative copy number; however, only CAMAQ-R was confirmed as A. hybridus. Although the biotypes ARRGR-R and SAOJER-R are closely related to A. hybridus and green pigweed (Amaranthus powellii S. Watson), their species identity could not be resolved. The biotype SAOJER-R also was resistant to acetolactate synthase (ALS)-inhibiting herbicides due to a tryptophan to leucine substitution at position 574 in ALS. The evaluation of 119,746 seedlings in an intraspecific hybridization study of A. hybridus indicated an outcrossing frequency of 0.09%. In contrast, an absence of interspecific hybridization (A. hybridus × unclassified biotype, AMACVI-S) was found after screening 111,429 offspring. Unclassified biotypes might be derived from one or more ancient hybridization events and subsequently evolved the triple mutation independently. Alternatively, such biotypes could have evolved from recent hybridization events, which occur at a frequency below the level of detection in our study.

Field trials were conducted in New York during 2021 and 2022 to evaluate acifluorfen herbicide as an alternative to postemergence (POST) pigweed management for red beets ( Beta vulgaris L. spp. vulgaris ). Acifluorfen was applied at 0.07, 0.14, and 0.28 kg·ha −1 a.i. to red beets at either the 6- to 8-leaf or 10- to 12-leaf developmental stages. Acifluorfen provided 68% to 96% control of Powell amaranth ( Amaranthus powellii S. Watson), with significantly greater efficacy at higher application rates and earlier timing when weeds were smaller. Conversely, common ragweed control was considerably lower (16%–50%), likely because of the presence of larger plants at the time of application. Red beet herbicide injury, characterized by leaf necrosis, bronzing, and stunting, was significantly influenced by both the application rate and timing. Higher rates caused greater injury; applications at the 6- to 8-leaf stage resulted in substantially more damage than applications at the 10- to 12-leaf stage. Environmental conditions likely influenced injury severity, with higher rainfall and more hours of relative humidity ≥90% in 2021 causing greater leaf loss and crop stunting than in 2022. Applications of acifluorfen at the 6- to 8-leaf stage significantly reduced both red beet leaf biomass and root yield compared with the later application. These results demonstrate that acifluorfen can effectively control Powell amaranth in red beets, although application timing is crucial to minimize crop injury and yield reduction. A later application (10- to 12-leaf stage) is recommended, especially when environmental stress may exacerbate herbicide injury. Additional research is needed to fully evaluate the utility of acifluorfen in red beet production systems across diverse environments and weed pressure scenarios.

Background and aims Due to differential plant resource allocation, changing soil resource availability can alter the competitive advantage of plants. A useful system to test this hypothesis is in row-crop production, where high resource inputs have selected for weed species that are highly responsive to nitrogen (N). We tested whether soil N immobilization can reduce the competitive advantage of nitrophilous weeds over N-fixing soybeans ( Glycine max ). Methods This experiment was conducted in a greenhouse with two soybean varieties and three common nitrophilous weeds: Palmer amaranth ( Amaranthus palmeri S. Watson), Powell amaranth ( Amaranthus powellii S. Watson), and common lambsquarters ( Chenopodium album L.). The plants were grown in monoculture and polyculture in carbon (C) amended and unamended soils. We measured soil microbial growth and community composition, soil N availability, and plant growth after 7.5 weeks. Results Weed species biomass was lower in C amended soils than in unamended soils, but soybean growth was unaffected. In C amended soils we observed increased microbial biomass N and reduced plant-available N, which indicate N immobilization processes. These trends were mirrored in the microbial community, which had a decreased abundance of nitrifying bacteria, an increased abundance of bacteria with low nutrient requirements and a high potential ability to degrade compounds. There were also differences in belowground bio-geo-chemical cycling based on plants grown in polyculture compared with plants grown in monoculture. Conclusion Altering soil resource availability has cascading effects in bio-geo-chemical cycling that result in changes in plant competitive outcomes.

Following the application of MCPA/MCPB at 1.7 kg ae ha–1 at a field site near Dresden, ON, Canada, poor control (<50% visible control) of green pigweed (Amaranthus powellii S. Watson) was observed. Amaranthus powellii is a common weed in Ontario crop production, and its evolution of resistance to synthetic auxin herbicides (SAHs) could pose a risk to crop yields. The suspected resistant A. powellii population (R) was used in dose–response and field experiments to determine resistance to SAHs. The objective of these studies was to determine whether this population of A. powellii is resistant to MCPA and cross-resistant to other SAHs. The GR50 (herbicide dose that causes a 50% reduction in plant aboveground biomass) values were determined by fitting plant dry weight data, obtained following application with seven SAHs, to a four-parameter log-logistic equation and were compared between the suspected-resistant (R) population and a known susceptible (S) population of A. powellii. The field trial was conducted in 2017, 2018, 2019, and 2021 in corn (Zea mays L.) and consisted of 11 postemergence SAH treatments. The GR50 values differed between the R and S populations following application with MCPA, aminocyclopyrachlor, dichlorprop-p, and mecoprop, resulting in resistance factors of 4.4, 3.0, 2.5, and 2.4, respectively. In the field study, dicamba and MCPA ester controlled A. powellii 84% and 30%, respectively, at 8 wk after treatment application (WAA). The control of Amaranthus powellii with all SAHs applied POST in corn was poor (<90% visible control) at 8 WAA. Both studies confirmed resistance to SAHs in this population of A. powellii, which will create limitations for farmers aiming to control this weed.

Leafy green vegetables have been a part of human diets throughout human history. Globally, they are gaining recognition since these wild foods could play an important role in food security. Quelites (the Mexican term for these resources) are dehydrated to produce “quelites pasados” by the Rarámuri in anticipation of the scarcity of food in winter. The diversity of quelites in the state of Chihuahua includes species of the widely consumed Amaranthus , as well as endemic, native, and introduced species that are eaten locally. The present work generated nutritional, sensory, and molecular information on four species that are consumed in the Sierra Tarahumara: Amaranthus palmeri , Amaranthus powellii , Arracacia edulis , and Phacelia platycarpa . Their nutritional analysis exhibited high protein values and a significant concentration of macro- and micronutrients. The acceptance by the public of the species of Amaranthus was high, while that of Arracacia edulis and Phacelia platycarpa was lower. Because of the morphological similarity within the two pairs of quelites, their DNA barcodes were generated as an identification tool which, together with the nutritional and sensory results, provides added value to the four “quelites pasados” of the Sierra Tarahumara. This study could be considered a starting point for sustainable use of native vegetables in future economic programs of regional agrobiodiversity, and even replicated globally.

A Powell amaranth population suspected to be resistant (R) to linuron was discovered in a carrot field in Keswick, Ontario, Canada, in 1999. Dose-response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II were done to confirm the resistance and identify its basis. A calculated resistance factor indicated a 12-fold increased resistance when linuron was applied to an R population compared with a susceptible (S) population. The partial nucleotide sequence of the psbA gene of R populations differed at two locations when compared with S populations. The first mutation coded for a Val sub( 219)Ile substitution in the deduced amino acid sequence of the D1 protein, and the second mutation was silent and encoded for a proline at position 279 in both R and S populations. The Val sub( 219)Ile substitution in the psbA gene is most likely the cause of this Powell amaranth population resistance to linuron and other PSII inhibitors.

A previously unknown glyphosate resistance mechanism, amplification of the 5‐enolpyruvyl shikimate‐3‐phosphate synthase gene, was recently reported in Amaranthus palmeri. This evolved mechanism could introgress to other weedy Amaranthus species through interspecific hybridization, representing an avenue for acquisition of a novel adaptive trait. The objective of this study was to evaluate the potential for this glyphosate resistance trait to transfer via pollen from A. palmeri to five other weedy Amaranthus species (Amaranthus hybridus, Amaranthus powellii, Amaranthus retroflexus, Amaranthus spinosus, and Amaranthus tuberculatus). Field and greenhouse crosses were conducted using glyphosate‐resistant male A. palmeri as pollen donors and the other Amaranthus species as pollen recipients. Hybridization between A. palmeri and A. spinosus occurred with frequencies in the field studies ranging from <0.01% to 0.4%, and 1.4% in greenhouse crosses. A majority of the A. spinosus × A. palmeri hybrids grown to flowering were monoecious and produced viable seed. Hybridization occurred in the field study between A. palmeri and A. tuberculatus (<0.2%), and between A. palmeri and A. hybridus (<0.01%). This is the first documentation of hybridization between A. palmeri and both A. spinosus and A. hybridus.

Three populations of were discovered in Kerala region (SE-India), representing the first record of this species for the national flora. A morphological description based on the Indian plants, as well as ecological data are given. A preliminary list of all the species occurring in India, which was never published during the last 136 years, is also provided.

Aims Agricultural practices often have persistent effects on soil physicochemical properties and soil biota, which can feedback to influence plant performance. We investigated management-induced differences in soil fertility and soil microbiota for their legacy effects on plant nutrient acquisition and crop/weed competition. Methods Sorghum bicolor (L.) Moench (sorghum), Ambrosia artemisiifolia (L) (ragweed), and Amaranthus powellii S. Wats. (pigweed) were grown as monocultures and mixtures in the greenhouse in soils with distinct management histories. Cross inoculations of sterilized soils were used to quantify biotic vs. abiotic influence on plant growth, nutrient uptake and competitive outcomes. Results Differences in management history led to a two-fold difference in soil organic matter (OM), and a 37% increase in N acquisition in all species in the high OM soil. The increase in N availability preferentially benefitted pigweed growth at the expense of sorghum. However, microbiota from the high OM soil alleviated the negative effects of pigweed competition on sorghum. Soil management legacy also affected sorghum tiller production and mycorrhizal colonization. Conclusion Management-induced differences in soil biotic and abiotic factors altered plant performance in a species-specific manner. Management legacy effects on soil microbiota have the potential to increase crop competitiveness against some weed species.