Abstract:It is necessary to study the relationship between aphid stress and oxidases in different alfalfa varieties for breeding of resistant alfalfa varieties and sustainable control of aphids.The changes in the activity of superoxide dismutase(SOD),peroxidase(POD),and polyphenol doxidase(PPO) were studied in seedling leaves of three alfalfa varieties,HA-3(High aphid resistance),Gannong No.3(aphid resistance),and Hunter River(low aphid resistance),infested by Therioaphis trifolii Monell through the artificial inoculation in lab.The results show that the activities of SOD,POD,and PPO in seedling leaves of three alfalfa varieties increased significantly along with the increasing of aphid densities.The changes in the activity of SOD,POD,and PPO of alfalfa seedling leaves after infestation by T.trifolii were significantly different among three alfalfa varieties.The SOD activities in seedling leaves of HA-3 and Gannong No.3 after infestation were higher than that of Hunter River,but the activities of POD and PPO were highest in HA-3,lowest in Hunter River,and middle in Gannong No.3.The differences of protective enzyme activities in three different aphids resistant alfalfa varieties were consistent with their resistant performances in the field.