Exploring the biostimulant potential of Gelidium processing residues to improve plant resilience to drought

Abstract The industrial extraction of red macroalgae generates significant quantities of residue that remain underexploited, despite the fact that they retain valuable biochemical compounds. This issue is of particular concern for Gelidium sesquipedale , a key agarophyte employed in the production of phycocolloid, for which the residual biomass is commonly discarded. The reuse of this material in agricultural contexts is a promising approach to the transformation of industrial waste into biostimulant products within a circular bioeconomy. The present study evaluated the impact of utilizing macroalgae residue extracts as biostimulants on the growth of faba bean ( Vicia faba L.) under conditions of drought stress. Four extraction methods were employed: hot water, acid, acid combined with ultrasound, and enzymatic treatment. The extracts were diluted to 5% (v/v) and applied as a soil drench at 25 days after sowing (DAS), followed by foliar sprays (10 mL per plant) at 40 and 50 DAS. The imposition of drought stress occurred from 40 to 50 DAS, after which the plants were rewatered until harvest. The results showed that, at 74 DAS, treated plants exhibited two‐ to threefold higher stomatal conductance, a 10–67% increase in water content, and augmented biomass production. Notably, dry root mass increased by 71–135%, and fresh node weight increased by up to 126% in comparison with stressed controls. Although the chlorophyll index increased during drought stress, no significant differences in chlorophyll index or normalized difference vegetation index were observed among treatments after recovery. The results obtained demonstrate the biostimulant potential of macroalgae residues to enhance crop resilience to water deficit, supporting their use as sustainable agricultural inputs in semi‐arid regions.