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Variation in environmental conditions, such as light availability, can affect herbicide efficacy by altering leaf morphoanatomy. Accordingly, the aim of the present study was to evaluate the influence of light level and herbicide application on leaf anatomy and herbicide efficacy, using Macroptilium atropurpureum as a model weed species. The experiment involved a factorial design (3 × 4), using three light levels (0, 50, or 70% shadow) and four herbicide treatments (no herbicide, glyphosate, carfentrazone-ethyl, or a combination of glyphosate and carfentrazone-ethyl). When combined, the herbicides were efficient in controlling M. atropurpureum under all light levels. However, when applied independently, the herbicides failed to satisfactorily control plants cultivated in full sunlight, and plants cultivated in shaded environments were more sensitive to glyphosate application than those cultivated in full sunlight. The greater efficacy of the herbicides on plants grown under shaded conditions can likely be attributed to changes in leaf morphoanatomy. Plants grown under restricted light conditions exhibit greater average leaf area, lower trichome density, reduced epicuticular wax deposition, thinner leaves, and lower dry mass accumulation, characteristics which are closely correlated to chemical control. The combination of the herbicides presents an additional effect in the control of M. atropurpureum. Also, changes in leaf anatomy caused by light restriction reduce the tolerance of M. atropurpureum to the herbicides.

The repetitive use of glyphosate in the control of weeds in eucalyptus plantations has selected tolerant species such as Commelina benghalensis. Therefore, the use of herbicides of other mechanisms of action, such as saflufenacil, in rotation or combination with glyphosate, is necessary to reduce damages caused by C. benghalensis, whose sensitivity to herbicides can be influenced by the shading caused by the eucalyptus crop. This study aimed to assess the efficiency of saflufenacil in the management of C. benghalensis at different shading levels. Three experiments were carried out regarding a certain shading level (cultivation environment with 0, 40, and 55% shade). The experimental design was a randomized block design with five herbicide doses (0, 24.5, 49, 73.5, and 98 g ha-1 of saflufenacil) and four replications. At 28 days after herbicide application, the percentage of control and shoot dry matter were assessed, as well as the biomass allocation to leaves, stems, and roots of C. benghalensis. Plants cultivated at 0% shade presented a higher dry matter accumulation and a better distribution of biomass in leaves, stem, and roots, contributing with the increased tolerance to saflufenacil and not being controlled with effectiveness under the assessed doses. As shading was increased, plants of C. benghalensis allocated most of their biomass to the shoot, especially leaves, reducing their reserve structures and increasing the contact area with the herbicide, making them more susceptible to saflufenacil. RESUMO: O uso repetitivo de glyphosate no controle de plantas daninhas em plantios de eucalipto tem selecionado espécies tolerantes a esse herbicida, como a Commelina benghalensis. Diante disso, torna-se necessária a aplicação de herbicidas de outros mecanismos de ação, como o saflufenacil, em rotação ou em mistura com o glyphosate, para reduzir os danos causados por C. benghalensis, cuja sensibilidade aos herbicidas pode ser influenciada pelo sombreamento imposto pela cultura do eucalipto. Objetivou-se neste trabalho avaliar a eficiência do saflufenacil no manejo de C. benghalensis em diferentes níveis de sombreamento. Foram realizados três ensaios, sendo cada um referente a determinado nível de sombreamento (ambiente de cultivo com 0, 40 e 55% de sombra), seguindo o mesmo delineamento experimental em blocos casualizados com cinco doses de herbicida (0; 24,5; 49; 73,5; e 98 g ha-1 de saflufenacil) e quatro repetições. Aos 28 dias após a aplicação do herbicida, avaliou-se a porcentagem de controle e a massa da matéria seca da parte aérea, bem como a alocação de biomassa para folhas, caules e raízes das plantas de C. benghalensis. Plantas cultivadas a 0% de sombra obtiveram maior acúmulo de matéria seca e melhor distribuição de biomassa em folhas, caule e raízes, contribuindo com o aumento da tolerância ao saflufenacil, não sendo controladas com eficácia nas doses avaliadas. À medida que se aumentou o sombreamento, plantas de C. benghalensis alocaram a maior parte de sua biomassa para a parte aérea, em especial para as folhas, reduzindo suas estruturas de reserva e elevando a área de contato com o herbicida, tornando-as mais suscetíveis ao saflufenacil.