Several reports are available and have shown that spermidine (a polyamine) is associated in the regulation of plant response to abiotic stresses such as drought or osmotic stress, salinity, heat and chilling by binding with membrane phospholipids, osmotic adjustment, scavenging of free radicals and maintaining a cation-anion equilibrium. Many biochemical and physiological effects are provoked by exogenous application of spermidine under stress conditions. They maintained antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase etc.) in plants which are able to moderate radical scavenging system to reduce oxidative stress. Please check this attached review and research paper for evidence in detail.