Exogenous Polyamines and Plant Salinity Tolerance: A Systematic Review, Evidence Map, and Family-Aware Quantitative Synthesis

Salinity disrupts plant redox balance, ion homeostasis, and water relations, and exogenous polyamines have been widely investigated as potential mitigators of these effects. However, it remains unclear which polyamine-mediated responses are reproducible across independent studies and whether redox protection consistently translates into downstream physiological recovery. We therefore conducted an integrated systematic review, evidence map, and publication-family-aware quantitative synthesis to evaluate the effects of exogenous putrescine, spermidine, and spermine in salt-stressed plants. We systematically searched Scopus and Web of Science through 9 August 2026, and eligible studies compared exogenous-polyamine-treated plants with corresponding salt-stressed controls. The review was not prospectively registered. Of 1093 records, 647 unique records were screened, and 179 reports were assessed in full text, yielding 34 retained reports and 444 complete quantitative contrasts. Multilevel random-effects models estimated reductions of 19.2% in malondialdehyde (95% confidence interval [CI], 12.9–25.1%) and 25.6% in hydrogen peroxide (H₂O₂; 95% CI, 15.5–34.6%). Superoxide showed a protective mean response but remained uncertain because its 95% CI included the null. In contrast, antioxidant-enzyme responses varied widely across studies, with no consistent cross-family increase in superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), or peroxidase (POD/POX) activity. Conservative independent-family syntheses supported favorable ion balance, modestly higher relative water content, and greater dry biomass, whereas total chlorophyll remained uncertain and net photosynthesis was highly heterogeneous. No clear superiority of putrescine, spermidine, or spermine was detected. Overall, exogenous polyamines most consistently reduced oxidative damage and stabilized ion–water homeostasis, whereas antioxidant–enzyme activity and photosynthetic recovery were more context-dependent. Interpretation is limited by small independent-family counts, incomplete reporting in some studies, and the absence of a formal study-level risk-of-bias assessment. By integrating systematic review, evidence mapping, and family-aware quantitative synthesis, this review provides an evidence-based hierarchy that distinguishes reproducible polyamine-mediated responses from context-dependent outcomes and helps define priorities for future salinity-tolerance research.