In the context of promoting sustainable agriculture, this research explores the biostimulant potential of aqueous extracts from three brown seaweed species, Sargassum trichocarpum, Cystoseira tamariscifolia and Fucus spiralis, on the germination and early seedling growth of Solanum lycopersicum.
Extracts were prepared by cold maceration and sonication and applied at different concentrations (0.1–1.5%) in a completely randomized design, with positive and negative controls and ten replicates per treatment. The extracts were applied at different concentrations. Significant treatment effects were observed in the results for all evaluated parameters. F. spiralis at a 0.2% concentration significantly enhanced germination, outperforming C. tamariscifolia treatments and both controls at higher concentrations. The extract's high content of phenolics, polysaccharides and essential minerals is likely responsible for this positive response. However, both F. spiralis and C. tamariscifolia at higher concentrations (1–2%) showed inhibitory effects, which could have been caused by osmotic stress due to excessive salinity. Macroscopic observations revealed that higher concentrations of S. trichocarpum enhanced hypocotyl growth and overall seedling vigor; however, root growth responses depended on species and concentration. Overall, this study highlights the concentration and species specificity of seaweed-based biostimulants. F. spiralis at low concentrations and S. trichocarpum at high concentrations have great potential to improve early tomato seedling growth, supporting their specific application in sustainable crop management practices.