Brown Algae Extracts Increase the Tolerance of Tomato Plants to High Temperatures by Improving Morphological, Physiological, Metabolomic, and Transcriptional Parameters

Currently, biostimulants in the horticultural sector are a tool that is being used to improve the yield and quality of vegetables under optimal and stressful growth conditions. In the present study, we evaluate the effects of foliar application of a hydroethanolic extract of <i>Sargassum</i> spp., a commercial extract based on <i>Ascophyllum nodosum</i>, and a control with distilled water on growth and biomass, stomatal conductance, photosynthetic pigments, enzymatic and non-enzymatic antioxidants, protein content, and the expression of defense genes in tomato plants (<i>Solanum lycopersicum</i> L.) without stress and with high-temperature stress (45 °C). The results showed that <i>Sargassum</i> spp. extract only increased the height of tomato plants under stress-free conditions (2.71%) in the last evaluation. The aboveground and total dry biomass of the plants were increased by <i>Sargassum</i> spp. extract under stress-free conditions by 9.56 and 8.58%, respectively. Under stress conditions, aboveground dry biomass was increased by 6.66% by <i>Sargassum</i> spp. extract. Stomatal conductance, photosynthetic pigments, protein content, enzymatic and non-enzymatic antioxidants, and defense gene expression of tomato plants were positively modified with the use of <i>Sargassum</i> spp. and <i>A</i>. <i>nodosum</i> extract under high-temperature stress conditions. Under stress-free conditions, the described variables were positively modified except for gene expression, where some genes were expressed and others were repressed. The results indicate that extracts of <i>Sargassum</i> spp. and <i>A</i>. <i>nodosum</i> are effective in mitigating high-temperature stress, making their use a promising alternative for inducing resistance in plants to the daily adversities of climate change.