Fitotoxicidade in vitro de extratos de Mikania laevigata Schultz Bip. ex de Baker sobre Lactuca sativa L. e Bidens pilosa L.

Plant extracts have secondary metabolites that can act as bioherbicides, with the advantage of being more environmentally friendly. Mikania laevigata belongs to the family Asteraceae and has coumarin 1,2-benzopyrone as a chemical marker, which is capable of inhibiting or stimulating the germination and growth of adjacent plants. Thus, the objective of this work was to evaluate the phytotoxicity of aqueous and ethanolic extracts of M. laevigata on Lactuca sativa (lettuce) and Bidens pilosa (beggarticks). Five concentrations (0.1, 1.0, 1.5, 2.0 and 3.0 mg.mL-1) of aqueous and ethanolic extracts were tested in separate experiments, with distilled water as the control. Germination percentage, germination speed index, initial growth and the content of hydrogen peroxide (H2O2) and malonaldehyde (MDA) were evaluated. In the presence of the ethanolic extract, germination of the species (lettuce and beggarticks) was reduced by 85% and 90%, respectively. Radicle length, for the lowest concentration of the extracts, was not affected. However, at a concentration of 2.0 mg.mL-1 the reductions were 85% for lettuce and 65% for beggarticks. The increase of H2O2 content depended on the dose, where the greater the concentration of the extracts, the greater the production of peroxide, followed by an increase in MDA for lettuce and beggarticks. Seedlings grown in a concentration of 3.0 mg.mL-1 of the ethanolic extract were subjected to necrosis, making subsequent analyses impossible. The M. laeviagata extracts at concentrations of 2.0 and 3.0 mg.mL-1 demonstrated phytotoxicity, with increased oxidative stress in lettuce and beggarticks.