Genetic, biochemical and physiological parameters can be changed by applying seaweed extract-based products. However, there is scarce information about the influence of seaweed extract on yarrow performance (Achillea millefolium L.), which is widely used in the folk medicine. Therefore, this study aimed to evaluate the effects of Ascophyllum nodosum extract on plant development (leaf and root biomass, and leaf area), physiological indexes (leaf weight ratio, and root: leaf ratio), secondary

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... bolite (phenolic compounds) content and antioxidant activity of yarrow. The experiment was carried out in a completely randomized design with 4 treatments (seaweed extract concentrations 0, 3, 6 and 9 mL L-1) and 10 replications. The higher concentration of seaweed extract caused higher total dry weight of plants (from 17.8 to 19%), especially due to increases in the root biomass (up to 28.5%). Only plants that received the highest concentration of seaweed-based product presented increments in the number of leaves when compared to the control plants (18.3 %). Furthermore, the use of A. nodosum extract 9 mL L-1 provided increases in the antioxidant activity and content of phenolic compounds in leaves (up to 30.44%). In conclusion, application of A. nodosum is a potential tool strategy to improve the quality of raw material from yarrow plants, since it increased the phenolic compound content and antioxidant activity in leaves, which are the plant organs commonly used in folk medicine.