Layered double hydroxide-seaweed composites promote seed germination and seedling growth: a new generation of biostimulants.

Introduction: Agricultural producers worldwide face increasing pressure to ensure food security while contending with the adverse effects of climate change and unsustainable farming practices. Layered double hydroxides (LDHs) have been applied in agriculture due to their versatile properties as slow-release and efficient carriers, while seaweed extracts are widely used as plant biostimulants. However, their integrated use as nanostructured composites for enhancing seed germination and seedling growth remains largely unexplored. Methods: In this study, we addressed this gap by synthesizing composites that combine cationic LDH nanosheets with anionic compounds present in alkaline extracts of the brown seaweed Results: Physicochemical analyses confirmed that approximately 12% of the composite mass corresponded to seaweed-derived molecules adsorbed onto the LDH surface. Both LDH-Sargassum and LDH-Ulva significantly improved germination and seedling growth at lower concentrations (1.57, 3.14, and 6.28 mg·mL⁻¹), while higher doses (12.56-50.25 mg·mL⁻¹) produced effects that were either comparable to those of the control or slightly better. Notably, both composites enhanced root architecture in mung bean cuttings by increasing root number, length, and dry weight. These results highlight the potential of LDH-seaweed composites as effective biostimulants, particularly in promoting early-stage root development by improving root branching, size, and biomass. Importantly, LDH-Sargassum at 6.28 mg·mL⁻¹ emerged as a promising natural alternative to synthetic root-promoting agents. Discussion: This study demonstrates, for the first time, the feasibility of LDH-seaweed composites as next-generation nanostructured phycobiostimulants, providing insights into their plant interactions and identifying optimal application dosages. Overall, these findings provide a foundation for implementing LDH-seaweed composites as a sustainable strategy to reduce agrochemical inputs and advance towards food security through bio-based nanotechnologies.