This study aimed to evaluate the influence of CO₂ and temperature on glyphosate-resistant and susceptible biotypes of Amaranthus palmeri (Palmer amaranth) in terms of morphological development. Height (cm), stem diameter (cm), leaf area (cm²), number of leaves, leaf, stem, and root dry matter, plant volume (m³), as well as shoot-to-root allometry were evaluated. The Palmer amaranth biotypes were grown under four different scenarios: 1—low temperature (23/33 °C) and CO₂ (410 ± 25 ppm); 2—low temperature (23/33 °C) and high CO₂ (750 ± 25 ppm); 3—high temperature (26/36 °C) and low CO₂ (410 ± 25 ppm); and 4—high temperature (26/36 °C) and CO₂ (750 ± 25 ppm). Between CO₂ and temperature, the majority of differences observed were driven by CO₂ levels. Palmer amaranth grown under 750 ppm of CO₂ was 15.5% taller, displayed 10% more leaf area (cm²), 18% more stem dry matter, and had a 28.4% increase in volume (m³) compared to 410 ppm of CO₂. GA2017 and GA2020 were 18% and 15.5% shorter, respectively. The number of leaves was 27% greater for GA2005. Plant volume decreased in GA2017 (35.6%) and GA2020 (23.8%). The shoot-to-root ratio was isomeric, except at 14 and 21 DAT, where an allometric growth towards shoot development was significant. Palmer amaranth biotypes responded differently to elevated CO₂, and the impacts of temperature need further investigation on weed physiology. Thus, environmental and genetic background may affect the response of glyphosate-resistant and susceptible populations to climate change scenarios.

This study examined the growth parameters of both glyphosate‐susceptible and glyphosate‐resistant biotypes of Amaranthus palmeri, designated as GA2005 and GA2017, respectively. A two‐year microplot field study was conducted to assess their growth characteristics. Scheduled destructive harvests on named harvest days (HD) were conducted to collect measurements for further calculation of net assimilation rate (NAR; g m⁻² day⁻¹), specific leaf area (SLA), leaf weight ratio (LWR), stem‐to‐leaf ratio (SLR), leaf area index (LAI), leaf area ratio (LAR; cm² g⁻¹), leaf area duration (LAD; days), relative growth rate (RGR; g.g⁻¹ day⁻¹) and plant volume (m³). In addition, stem diameter, number of leaves, and Chlorophyll content (μmol m²) were determined. The main objective was to identify growth parameters that differentiate biotypes along the plant life cycle. While certain growth parameters showed no variation among biotypes, differences in leaf area index (LAI) over HD and chlorophyll content and leaf area duration (LAD) were observed as the main effects. Glyphosate‐resistant biotypes exhibited higher LAD and chlorophyll content, potentially conferring a competitive advantage, especially in heavily used glyphosate environments. The study highlights the complexity of intraspecific genetic differentiation, adaptation, and environmental factors affecting A. palmeri. It may offer insights into biotype distinction and resistance spread while advancing our comprehension of species adaptation and growth strategies for enhanced control.

Sorghum is an important crop to plant in rotation with sugarcane. This is mainly because both are inputs for the ethanol industry. Crop residues of sweet sorghum promote suppression of weed re-infestation, avoiding weed interference to the sugarcane crop due to the strong allelopathic potential of sorghum. In order to determine the suppressive effects of sorghum crop residues on weeds, a field experiment was carried out. Seven vegetation covers were used as options for crop rotation with sugarcane. The treatments were sweet sorghum, velvet bean, sunflower, soybean, sugar cane, fallow, and an area without cover. The experiment was randomized blocks with four replications of 27 m2 plots. The weed community of each plot was evaluated by phytosociological indexes at 60 and 120 days after the formation of vegetation cover. The composition of soil seed bank was also evaluated. The weeds with the highest indexes of relative importance during the evaluations were Cyperus rotundus, Raphanus raphanistrum and Parthenium hysterophorus. The diversity of the weed community, estimated by relative importance indexes, was lower in the area with velvet bean as soil cover. Sorghum, velvet bean and sunn hemp covers reduced the soil seed bank compared to the fallow treatment and the treatment without vegetation cover. Crop residues of sweet sorghum and velvet bean provide a decrease in weed infestation in field, and the weed suppression period can last up to 120 days during the dry season.

Para a tomada de decisão sobre qual estratégia é melhor para manejo de plantas daninhas podem ser usadas ferramentas estatísticas, como análise multivariada, principalmente quando se dispõe de quantidade elevada de dados. Assim, objetivou-se avaliar a viabilidade da utilização da análise multivariada para estudos de seleção da flora de plantas daninhas em decorrência da aplicação de diferentes herbicidas. O experimento foi conduzido em campo, em delineamento experimental blocos ao acaso, com quatro repetições, e a área foi dividida em 32 parcelas com 3 x 5 metros de comprimento, com 8 parcelas por bloco. Os tratamentos experimentais foram: 1-glifosato; 2- 2,4D; 3- glifosato + 2,4D; 4- carfentrazone-ethyl; 5- glifosato + carfentrazone-ethyl; 6- haloxyfop-methyl; 7- cletodim; 8- glufosinato de amônio. Foram feitos os levantamentos da comunidade infestante em cada parcela antes da aplicação e aos 30 dias após a aplicação dos tratamentos. Os dados foram submetidos à análise de agrupamento e componentes principais. As espécies de plantas daninhas mais discriminantes foram Alternanthera tenella e Commelina benghalensis, seguidas das gramíneas Panicum maximum, Cenchrus echinatus e Eleusine indica. Os resultados da análise de agrupamento foram bastante similares àqueles obtidos pela análise dos componentes principais. As técnicas de estatística multivariada foram capazes de agrupar tratamentos químicos de acordo com a composição específica independentemente do índice fitossociológico considerado.

Background: Lettuce-production areas in Brazil are frequently infested by pigweed plants, which can interfere with production by competing for resources or releasing allelopathic compounds in the environment. Objective: The present study evaluated the effect of redroot pigweed (Amaranthus retroflexus) density and distribution on lettuce crop (Lactuca sativa L.). Methods: The cultivars Lídia, Verônica, Lucy Brown and Salad Bowl were used. After transplanting, these plants coexisted until harvest with four pigweed plants (equivalent to 16 plants m-2) located within or outside planting rows, or with one pigweed plant located at the center of the plot (4 plants m-2), as well as a control without weed. The experiment was conducted in an open and semi-controlled area, with treatments arranged in a 4x4 factorial scheme, in a completely randomized experimental design with three replicates. Evaluations included: the number of leaves, fresh matter (lettuce only), height, leaf area and dry matter of lettuce and redroot pigweed plants. Results: The coexistence with redroot pigweed caused yield losses of up to 45% for cultivar Lídia; 41% for Salad Bowl; 33% for Verônica; and 28% for Lucy Brown. Conclusions: The density of 16 plants m-2 of redroot pigweed was the one that most negatively affected the growth of the cultivars, in which the positioning in the planting lines resulted in greater interference to the crop growth. Among the cultivars tested, Lucy Brown showed greater tolerance to coexistence with redroot pigweed plants, while the cultivar Lídia was the most sensitive to the imposed interference.