Effects of Seaweed-Derived and Microbial Biostimulants on Growth, Nutritional Composition, and Antioxidant Activity of Field-Grown Sonchus oleraceus L.

Biostimulants are increasingly used to improve plant growth, nutrient use efficiency, and crop quality and to mitigate abiotic stress; however, most evidence comes from conventional crops, and little is known about how wild edible species respond to them. Common sowthistle (Sonchus oleraceus L.) is a nutrient-rich wild edible leafy species with potential as an alternative crop; however, its response to biostimulants under field conditions has not been evaluated. This study assessed the effects of two biostimulant formulations (a seaweed-derived extract and the microbial formulation Bactiva, applied in liquid form directly to the soil) on plant growth, leaf proximate composition, mineral content, phenolic compounds, and antioxidant activity of common sowthistle in a single-season field trial. None of the formulations significantly affected the morphological or physiological traits compared with the untreated control. Bactiva significantly increased leaf protein (11.8%) and nitrogen content (11.4%) compared to seaweed extracts, whereas ash, fat, and carbohydrate contents and leaf phosphorus and potassium were not significantly affected by any formulation. Energy content also increased for the seaweed extracts, mostly due to a numerical increase in fat content, compared to untreated plants. Total phenols and total flavonoids did not differ significantly among treatments, and antioxidant activity depended on the formulation and assay. In conclusion, under the conditions of this single-season trial, biostimulant application did not improve plant growth, and only Bactiva increased leaf protein and nitrogen content, without clear effects on phenolic compounds. Multi-season and multi-site studies with different application rates are needed to confirm these results.