Adequacy of glyphosate doses in the Merremia cissoides (Lam.) Hallier f. control as a function of light intensity in the growth environments
Shading interferes with the weed's biology, which can change their sensitivity to post-emergence herbicides. The objective was to evaluate the control of Merremia cissoides with glyphosate in full sunlight and shade conditions in two plant growth stages (30 and 73 days after sowing (DAS)). Shading promoted 50 and 40% reductions in glyphosate doses at 30 and 73 DAS, respectively. At 73 DAS, M. cissoides is 177.77 and 131.48% more tolerant to glyphosate than 30 DAS in shading and full sunlight, respectively. Due to the increase in glyphosate tolerance as the plant grows, the management of M. cissoides should be carried out until the stage of six fully expanded leaves. Increasing glyphosate doses reduced the quantum yield of photosystem II and electron transport rate (ETR) in both growth environments, with ETR data showing a high negative correlation with the control. The doses reductions promoted by shading and glyphosate application in the initial growth stage of M. cissoides reduces costs and the negative environmental impacts of this herbicide use.
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Materials and methods Site and plant material description Two M. cissoides control experiments were carried out. M. cissoides seeds were collected from different plants in the field and scarified in PA sulfuric acid solution for 20 minutes for dormancy-breaking. The experimental units consisted of 12 L pots containing one plant per pot, and the soil moisture was maintained close to field capacity by irrigation performed once a day. After the emergence, the seedlings were taken to the growth environments in full sunlight and 58.7 ± 1.08% of shading. The shading was produced by a black polypropylene shade screen. Experimental design and treatments The design adopted in both experiments was randomized blocks with four replications. The treatments were established in a 2 x 5 and 2 x 6 factorial scheme in the first and second experiments, respectively. In both experiments, the growth environments in full sunlight and 58.7% of shading constituted the first factor. In the first experiment, the second factor was constituted by doses of 0, 270, 540, 810, and 1080 g/ha of glyphosate, and in the second experiment, doses of 0, 250, 500, 750, 1000, and 1250 g/ha of glyphosate. In the first experiment, glyphosate doses were applied at the initial growth stage, 30 days after sowing (DAS), and 21 days after placing the plants in the growth environments, when the plants had an average of six fully expanded leaves. In the second experiment, the doses were applied in the advanced growth stage, 73 DAS, and 52 days after the plants were placed in the growth environments when the plants were at the beginning of the reproductive stage. Assessments In the first experiment, at 3 and 5 days after glyphosate application (DAA), and in the second experiment, at 3 and 7 DAA, were evaluated the quantum yield of photosystem II and the electron transport rate of plants with the fluorometer device Y (II) meter (OPTI-SCIENCES, Hudson, USA). In both experiments, visual control assessments were performed at 28 DAA, adopting a scale from 0 to 100%. In addition, in both experiments, at 28 DAA, the remaining biomass in the pots was collected and placed in a closed air circulation oven at 55° C until reaching constant weight for dry biomass determination. Statistical analysis Control and dry biomass data were descriptively analyzed using the mean and standard error of the mean. The other results were submitted to variance analysis (p ≤ 0.05). When significant for factors interaction, splits were performed. When significant for simple effects, the qualitative means were compared to the Tukey mean test (p ≤ 0.05) and the quantitative means to linear regression analysis. Pearson's correlation analysis was also performed between the variables: control, dry biomass, ΦPSII and ETR at 5 DAA, and ΦPSII and ETR at 7 DAA, for the initial and advanced growth stages, respectively. Statistical analyzes were performed using the statistical program R Studio and the ExpDes.pt package.