Abstract: Two cultivars of tomato (Solanum lycopersicum, cvs. ‘Jinpengchaoguan’ and ‘Zhongza No. 9’, with the former being more tolerant to salinealkaline stress) seedlings grown hydroponically were subjected to salinityalkalinity stress condition (NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃=1:9:9:1) without or with foliar application of 0.25 mmol·L^-1spermidine (Spd), and the root morphology and physiological characteristics of mitochondrial membrane were analyzed 8 days after treatment, to explore the protective effects of exogenous Spd on mitochondrial function in tomato roots under salinityalkalinity stress. The results showed that the salinityalkalinity stress increased the concentrations of both mitochondrial H₂O₂ and MDA as well as the mitochondrial membrane permeability in the roots of the two cultivars, while it decreased the mitochondrial membrane fluidity, membrane potential, Cyt c/a and H⁺-ATPase activity, which impaired the mitochondria and therefore inhibited the root growth; and these effects were more obvious in ‘Zhongza No. 9’ than in ‘Jinpengechaoguan’. Under the salinityalkalinity stress, foliar application Spd could effectively decrease the concentrations of mitochondrial H₂O₂ and MDA and mitochondrial membrane permeability, while increased the mitochondrial membrane fluidity, membrane potential, Cyt c/a and H⁺-ATPase activity. These results suggested that exogenous Spd could effectively mitigate the damage on mitochondria induced by salinityalkalinity stress, and the alleviation effect was more obvious in ‘Zhongza No. 9’ than in ‘Jinpengchaoguan’.