Biostimulant-Mediated Enhancement of Growth, Yield, and Abiotic Stress Tolerance in Wheat and Barley: Mechanisms, Research Gaps, and Future Perspectives

Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) are crucial cereal crops worldwide, facing increasing challenges from abiotic stresses such as heat, cold, drought, salinity, and waterlogging. Biostimulants present a promising strategy to enhance stress tolerance by influencing interconnected physiological, biochemical, and molecular mechanisms. This review synthesizes evidence on biostimulant-mediated stress tolerance in these cereals using five functional categories: phytohormones and signaling molecules; osmoprotectants and antioxidants; silicon, mineral nutrients, and nanoparticles; humic substances, seaweed, and plant extracts; and beneficial microorganisms. Across stress conditions, biostimulants generally improved photosynthetic performance, redox homeostasis, osmotic and ionic balance, and membrane stability. Molecular evidence further implicates antioxidant defense, hormone signaling, ion transport, aquaporin, and cold-responsive pathways, although mechanistic validation remains limited. Salicylic acid, melatonin, and silicon were among the most supported agents, whereas evidence for waterlogging and microbial biostimulants was comparatively scarce. Treatment efficacy varied with genotype, developmental stage, dose, and stress intensity and duration. Overall, biostimulants offer promising opportunities to improve stress tolerance and climate resilience in wheat and barley; however, multi-location field trials and omics-based investigations are needed to validate their effectiveness and elucidate underlying mechanisms for sustainable, stress-resilient crop production.