Porphyra haitanensis is an economically important marine crop in southern China.Due to global warming it is highly desirable to select or breed new lines of P.haitanensis that can tolerate higher water temperatures; and to use these for cultivation.We observed the physiological responses of gametophytic blades of an F₄ high temperature tolerance line(Z-61)and a wild-type line.These were grown at normal temperature(21 ℃),and at high temperatures(26 ℃ and 30 ℃),for 0,2,4,6,8,and 10 days.Changes in physiology were measured,including free radical content,H₂O₂,malondialdehyde(MDA)and proline levels,and also the activities of superoxide dismutase,peroxidase,and catalase.Physiological changes within the Z-61 gametophytic blades were as follows:high temperature stress→levels of reactive oxygen species(ROS)increased→cell membranes were damaged by oxidation and MDA levels increased→cells received signals of excessive ROS→antioxidation and osmoregulation systems began to clean out excessive ROS→free radical levels declined.We compared the responses of Z-61 and wild-type lines to high temperatures.The Z-61 line is able to initiate both antioxidation and osmoregulation.The wild-type line,however,can only initiate osmoregulation when under high temperature stress.The background activities of the antioxidation systems also show notable differences between the two types.These differences may be the cause that the wild-type is not able to tolerate high temperatures.Our results may lay a foundation for understanding the principles of high temperature tolerance in P.haitanensis,and may suggest some directions for future breeding.