The aim of this work was to study the relationship between low potassium stress and the reactive oxygen species system.The tomato plants were grown in hydroponic systems to determine the role of reactive oxygen species (ROS) in the leaves response to low potassium stress.Parameters of oxidative stress superoxide (O^-·₂),malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) concentration and activities of antioxidant enzymes,such as superoxide dismutase (SOD),catalase (CAT),ascorbate peroxidase (APX) were investigated.The results indicated that:(1)The protective enzyme activities in the potassium-sensitive tomato were significantly depressed in the low potassium treatment compared to controls.While reactive oxygen species and MDA content showed significant differences between control and low potassium treatment.(2)The protective enzyme activities were increased in potassium-tolerant tomato compared with the control,and its reactive oxygen species and MDA content were increased.Relative expression levels of Cu/Zn-SOD,CAT and APX were analyzed in two kinds of tomato genotypes.The results showed that potassium-tolerant tomato had high protective enzyme expression under the low potassium stress,while potassium sensitive tomato was the opposite.These results show that higher protection enzyme activities probably scavenge H₂O₂ and O^-·₂,resulting in preventing the membrane permeability,and this can increase the adaptability to low potassium.