Exogenous Spermidine Enhances Salt Tolerance in Nicotiana rustica: Physiological Mechanisms and Implications for Phytoremediation

Salt stress disrupts plant physiology by impairing photosynthesis, osmotic balance, and ion homeostasis. This study evaluated the effect of exogenous spermidine (Spd, 1 mM) on Nicotiana rustica exposed to moderate (100 mM NaCl) and severe (200 mM NaCl) salinity. Salt stress significantly reduced chlorophyll a, carotenoids, protein contents, and biomass, while inducing proline and soluble sugar accumulation and disturbing K⁺/Na⁺ and NO₃⁻/Cl⁻ balances. Spd application markedly alleviated these effects under moderate salinity, restoring chlorophyll a and protein levels, sustaining biomass production, limiting excessive proline and sugar accumulation, and reducing Na⁺ and Cl⁻ toxicity while maintaining K⁺ and NO₃⁻ uptake. However, its protective effects were largely absent under severe stress, with only marginal improvements in biomass and osmolyte regulation. These findings indicate that spermidine confers substantial tolerance to moderate but not severe salinity, basically through stabilization of photosynthesis, preservation of protein metabolism, and regulation of ion homeostasis.