During the biological evolution, plants developed several biosynthetic routes to produce and storage a wide variety of secondary compounds,which are involved in specific responses according to the environment. Mikaniaglomerata Spreng (Asteraceae), popularly known as guaco,it is known for its ethnopharmacology properties and production of secondary compounds, likecoumarins. This phenolic compound may interfere in the growth of other plants by inhibiting plant hormone metabolism, disturbing membrane function, and delaying seed germination. Thus, the objective of this work wasto analyze the effects of an hydro alcoholic extract of M. glomerate containing coumarin, at the mutagenicity level and the proton (H+) flux of onion (Allium cepa)roots.Allium cepa seeds were grownin the dark, on Petri dishes containing filter paper and 5 mL of guaco extract (0,001; 0,002; 0,004; 0,008; 0,016 mgmL-1ofcoumarin), pure coumarin (0,002 mgmL-1, CuP), methyl metane sulfonate (MMS, positive control, PS) or water (negative control, NP). The mitotic index and possible chromosomic aberrations were microscopically analyzed at the root tips. The H+ flux was determined at the elongation zone of the roots, using a selective-ion microprobe vibrating system. The mutagenic data were analyzed by Chi square test. The 0,001 and 0,004 mgmL-1 of guaco extract were more aneugenic than clastogenic when compared to CuP. Both concentrations also promoted the mitotic index. On the other hand, CuP produced clastogenicity indicating that the isolated form of coumarin might cause more damages than mixed with other compounds in a plant extract.Analyzing the H+ fluxat the dose-response curve, the concentration of 0,002 mgmL-1of coumarin in the guaco extract was chosen to be compared with the CuP treatment, due to its effect on H+ flux inhibition. It was possible to observe that all treatments promoted a H+ flux inhibition when compared to the NP, and the highest inhibition of the H+ flux was provoked by the hydro alcoholic extract containing coumarin.Thus, the inhibition of the H+ flux caused by the CuP and, especially, the hydro alcoholic extract containing coumarin can be related to inhibition ofseed germination, abnormal cellular differentiation and reduced plant growth.