Abstract: To report the hypothesis that irreversible winter injury of Korean pine (Pinus koraiensis Sieb.et Zucc.) seedlings is caused by photooxidation,the following experimental evidences are provided:1.The CO₂ release of exposed seedling needles under strong PFD is coupled with O₂ absorption and it can not be inhibited by photorespiration inhibitor NaHSO₃,indicating the released CO₂ comes from photooxidation rather than photorespiration.2.Exposed seedling needles contain more product of lipid peroxidation and their thylakoid membranes have lower fattyacid unsaturation.3.The SOD activity in exposed needles is lower than that in shaded needles.4.The SOD activity in 1-year needles is lower than that in 2-year needles.5.Needles from adult trees have an second band of SOD is ozyme may provide an extra protection mechanism.