Plant-derived biostimulants and plant water relations: current knowledge and the emerging roles of aquaporins.

Plant-derived biostimulants (PdBs) have emerged as innovative agricultural tools to address the increasing restrictions on chemical agro-inputs and the challenges imposed by climate change, particularly water scarcity. These products are progressively being used to enhance crop performance and resilience under abiotic stress conditions. This review examines how PdBs can optimize plant water balance through the regulation of key physiological processes involved in water relations. Given the central role of cellular water transport in these processes, particular attention is given to aquaporins (AQPs), membrane proteins that facilitate water transport across biological membranes. Current evidence suggests that PdBs may influence AQP expression and activity through multiple signalling pathways, including ABA-, calcium-, and pH-dependent mechanisms. However, these effects appear to be highly context-dependent, and the molecular mechanisms linking PdB application to AQP-mediated hydraulic regulation remain largely unresolved. The scientific literature on PdBs has expanded considerably in recent years, but studies specifically addressing their effects on plant water relations and AQP modulation remain relatively limited. Significant gaps persist regarding the identification of the bioactive compounds responsible for these responses, the potential synergistic interactions within complex extracts, and the precise mechanisms linking biostimulant application to aquaporin-mediated hydraulic regulation. This review synthesizes current knowledge on these topics, highlights major methodological and conceptual limitations, and proposes future research directions aimed at establishing standardized characterization strategies and unified evaluation frameworks. Such advances will be essential for unlocking the full potential of plant-derived biostimulants as sustainable technologies for improving crop water management under real agricultural conditions.