Abstract: Three hypotheses in the cause of winter injury in Korean pine(Pinus koraiensis Sieb.et Zucc.) seedlings—winter desiccation,late frost and winter solar radiation were tested.The results indicate that the injury is not related to winter desiccation or late frost but to wintet solar radiation.It enhances photosynthesis inhibition,resulting in irreversible photooxidation injury of the needles.'Negative' photosynthesis is also reported and discussed in relation to the injury.