Abstract:Two years old of Citrus sinensis(L.) Osbeck cv.Fengjie 72-1 navel oranges grafted onto Poncirus trifoliate(L.) Raf.were planted in containers filled with artificial nutrient soil(sand ∶ peat ∶ rice husk=5∶4∶1).Different duration of continuous drought stress,0,2,4,6,8,10,12,or 14 day,was imposed on different treatments.For each drought treatment,soil sample and five plants were picked out periodically to determine the soil water content and water potential(Ψl),relative water content(RWC),activities of catalase(CAT),peroxidase(POD) and superoxide dismutase(SOD),and content of malondialdehyde(MDA) of the leaves.Meantime,physiological responses of protective(enzyme) activities and content of MDA to soil and plant water changes were studied.Results showed that activity of CAT was depressed,while activity of POD and SOD was promoted and content of MDA changed little under drought stress.At the earlier stage of drought stress or under light drought stress,the activities of protective enzymes are of evident adaptive(response).Being in a condition of moderate stress,POD and SOD retained high activity.POD and SOD together played a dominant synergetic effect in restraining the produce of MDA,while CAT contributed little.