Biostimulant Extracts Obtained from the Brown Seaweed <i>Cystoseira barbata</i> Enhance the Growth, Yield, Quality, and Nutraceutical Value of Soil-Grown Tomato

The use of seaweed-derived biostimulants has gained attention as a sustainable strategy to enhance crop production. Brown seaweeds, in particular, are rich in bioactive compounds that can improve plant growth, yield, and quality parameters. This study investigated the biostimulant potential of extracts derived from <i>Cystoseira barbata</i> for promoting tomato growth and improving fruit quality. Three different extracts (water, alkali, and acid), applied as soil drenches, were tested on a determinate tomato cultivar under greenhouse conditions. In young plants, alkali and acid extracts increased stem length by 40% and 60%, respectively, while water and acid extracts accelerated early flowering. Alkali and acid extracts also improved fruit yield by approximately 65%. Additionally, all extracts enhanced fruit quality by increasing fruit EC and Brix values, soluble carbohydrate levels, total phenolic content, total antioxidant capacity, lycopene and β-carotene concentrations, and vitamin C content, albeit to varying degrees. Along with increases in fruit K concentration in response to water and alkali extracts, all seaweed extract-treated groups showed elevated fruit S concentrations, accompanied by increases in reduced glutathione levels. These results indicate that <i>C. barbata</i> extracts can enhance plant performance while improving the nutritional and nutraceutical properties of tomato fruits. The observed effects were strongly influenced by the extraction method, which alters the extract composition. Extracts from sustainably sourced <i>C. barbata</i> may contribute to improved productivity and quality in horticulture; however, further research is needed to enable the standardized production of <i>C. barbata</i>, optimize biostimulant formulations, and validate their effectiveness under field conditions.