Comparative Effects of Macroalgal Extracts on Tomato Plant Growth and Fruit Quality

Macroalgal extracts are widely recognised as biostimulants that enhance crop productivity and plant growth under both optimal and stressful conditions. They offer a sustainable approach to mitigating the adverse effects of abiotic stress on crop development. This study investigates the efficacy of macroalgal-based fertilisers in enhancing tomato (<i>Solanum lycopersicum</i> L.) growth, yield, and fruit quality, as sustainable alternatives to chemical fertilisers. Different seaweed species (<i>Sargassum muticum</i>, <i>Ulva ohnoi</i>, <i>Furcellaria lumbricalis</i>, <i>Ascophyllum nodosum</i>, and a commercial <i>A. nodosum</i> extract) were evaluated as foliar treatments. The results showed that while the leaf fresh weight and chlorophyll content were not significantly affected, the fruit morphology and biochemical composition exhibited notable variations. <i>Sargassum muticum</i>-treated fruits displayed the highest °Brix (6.57), indicating superior sugar accumulation, while <i>Ulva ohnoi</i> maintained near-neutral pH levels (avg. 3.94), suggesting balanced acidity. <i>Ascophyllum nodosum</i> extracts induced the highest proline concentrations (peak: 63.77 µmol/g), but also caused extreme acidity (pH 1.39–2.58). <i>Furcellaria lumbricalis</i> enhanced the fruit size (axial length up to 41.4 mm), but reduced the pH sharply (1.69–2.13). The commercial product underperformed in regard to sugar content and flavour complexity. The integrative analysis revealed species-specific flavour profiles: <i>Sargassum</i> yielded sweet, mildly acidic fruits; <i>Ascophyllum</i> produced intensely aromatic, acidic tomatoes; and <i>Ulva</i> resulted in bland flavours. These findings underscore the importance of algal species and extraction methods in tailoring biofertilisers for target fruit qualities. This study advocates for the use of macroalgal fertilisers in sustainable agriculture, but highlights the need for optimised formulations to balance crop yield, taste, and stress adaptation.