Efficacy of Ascophyllum nodosum and Ecklonia maxima extracts in enhancing heat stress tolerance of chilli pepper

• This study shows the impact of seaweed biostimulants at alleviating heat stress in chilli pepper • Simulated heatwaves were used to elucidate the mode of action of seaweed-based biostimulants • Integrated trait and gene analyses revealed HSFA2 and HSP101 as biomarkers of efficacy • Application of A. nodosum extract Martello® improved pollen quality, ovule development and yield • Biostimulants reduced heat stress impact during reproduction, increasing crop yield Climate change negatively impacts farming with rising temperatures and heatwaves, damaging crops. Biostimulants are a promising tool to deal with these effects. This study evaluated two seaweed extracts from Ascophyllum nodosum (Martello®) and Ecklonia maxima (EME) for their efficacy to alleviate the impact of high temperatures during the reproductive phase of chilli pepper plants. Both extracts claim to improve tolerance to stress during the reproductive phase, however Martello® demonstrated an enhanced performance when compared to the Ecklonia maxima extract (EME) after exposure to high temperatures (36/18°C) for fifteen days. It improved pollen viability by a 94% while the number of developed ovules in pistils increased by a 48% when compared to untreated plants exposed to high temperatures. Similarly, Martello® efficiently reduced oxidative stress in leaves when compared to the untreated plants or EME, maintaining a high expression of HSFA2 and HSP101 in leaves and flowers after 15 days of stress, likely protecting photosynthesis and increasing fructose in leaves by 48%. Moreover, Martello® increased the number of xylem vessel elements and ray parenchyma in the main stem of chilli peppers by 35 and 60% respectively, improving water transport and the storage of starch. All these parameters combined led to a 50% yield increase in plants treated with Martello® and exposed to high temperatures, while no improvement was observed after the EME treatment. The results presented in this study indicates that Martello® alleviates the impact of high temperatures during the reproductive phase of plants, improving plant fitness, thermoadaptation and, ultimately, yield.