Biostimulant effects of <i>Rugulopteryx okamurae</i> aqueous extracts on radish growth

Abstract In recent years, the invasive algae Rugulopteryx okamurae has spread along the Mediterranean and Atlantic coasts, causing ecological and economic damage. However, upwelling algae could provide a valuable source of carbon biomass for circular economy applications. Marine algae, particularly brown algae, have a long history of use in agriculture as biostimulants and biofertilizers, demonstrating their effectiveness on various crops and underscoring their potential as a valuable resource for sustainable agricultural practices. This study aimed to evaluate the agronomic effects of aqueous extracts from R. okamurae on radish growth using a rapid, cost‐effective method, with the goal of exploring potential applications for upwelling biomass. Two groups of seaweed were used: one washed with distilled water and the other unwashed. Both groups were macerated in water for 10 days, with and without a mixture of activators (chickpea flour, poultry manure, brown sugar, and fertile soil). Four liquid extracts, along with a water control, were tested on Raphanus sativus . The results showed a significantly higher biostimulant effect on germination index and plant growth (root and shoot length, fresh and dry weight) compared to the control. The greatest increase in shoot and root length was obtained with non‐washed seaweed (NWS), with improvements of + 40.6% and +68.2%, respectively. The best performance in fresh and dry plant weight was achieved with non‐washed seaweed + activators (NWS + A), which increased root fresh weight by + 160.9% and root dry weight by + 146.2%. These findings highlight the potential agronomic use of low‐cost aqueous extracts from R. okamurae as biostimulants.