Plant Biostimulants in Synergy: Unlocking Their Full Potential

The global agricultural sector faces mounting challenges stemming from climate change, population growth, and environmental degradation, necessitating the transition to more sustainable and resilient crop production systems. Plant biostimulants, defined as substances and microorganisms that enhance physiological processes in plants, are increasingly recognized as tools for improving crop growth, stress tolerance, and nutrient use efficiency without acting as fertilizers or pesticides. This chapter presents a comprehensive synthesis of current knowledge on plant biostimulants, including their definitions, classification schemes, and modes of action. Particular emphasis is placed on the phenomenon of synergy, in which combinations of biostimulants produce effects greater than the sum of their individual contributions. Synergistic interactions are discussed involving arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), and various organic compounds, with reference to their capacity to enhance photosynthesis, water use efficiency, nutrient uptake, biomass accumulation, and resistance to abiotic and biotic stressors. Although promising, these interactions are still not adequately understood at the mechanistic level. Variability in results depending on crop species and environmental context presents a significant barrier to field-level predictability. The chapter highlights the need for standardized assessment protocols, field-based validation, and integrative approaches that combine molecular, physiological, and agronomic data. Advancing understanding in this area is essential for the rational design of synergistic biostimulant formulations tailored to specific agroecosystems.