International Journal of Plant & Soil Science

Water deficit during germination and early establishment restricts imbibition, delays reserve mobilisation and impairs stand formation, making the seed stage an attractive target for low-input stress management. Melatonin-based seed treatment has consequently moved from an experimental curiosity to a widely studied chemical-priming approach. This critical narrative review evaluates the physiological and molecular mechanisms through which melatonin applied before sowing may mitigate water stress, while separating mechanistic plausibility from agronomic proof. Literature published from 1995 to 1 June 2026 was identified through accessible scholarly indexes, citation searching and DOI-level verification. Evidence was appraised according to stress realism, priming controls, dose definition, genotype coverage, developmental duration, mechanistic depth and field relevance. Across cucumber, rapeseed, soybean, safflower, triticale, rice, maize, peanut, wheat, cotton and foxtail millet, melatonin treatment generally improved germination, seedling vigour, membrane stability and antioxidant capacity under polyethylene glycol-induced osmotic stress or restricted water supply. The most reproducible mechanism is redox buffering: melatonin moderates damaging reactive oxygen species accumulation while preserving the signalling functions required for germination, supported by enhanced enzymatic and non-enzymatic antioxidant systems. Osmotic adjustment, reserve mobilisation, root development, stomatal regulation, chloroplast protection and crosstalk with abscisic acid, gibberellins, nitric oxide and sugar metabolism provide additional, but unevenly substantiated, explanations. Emerging evidence implicates phytomelatonin receptor-mediated signalling, methylglyoxal detoxification and autophagy. Confidence in broad agronomic claims remains limited because most studies use short-term laboratory assays, heterogeneous concentrations and polyethylene glycol models that do not reproduce soil–plant–atmosphere drought. Rare field studies indicate possible gains in yield and water productivity, but independent multi-environment validation, treatment standardisation, storage testing, cost analysis and regulatory assessment are inadequate. Melatonin seed treatment is therefore best regarded as a promising, biologically credible priming technology whose practical value depends on crop-specific dose optimisation and evidence extending beyond early seedling biomarkers.