AbstractAbscisic acid (ABA) is a plant hormone that regulates tolerance to abiotic stress. In this study, we evaluated the effect of exogenous different concentrations of ABA (0, 0.5, 1, 10, 100 and 200 μM) to mitigate oxidative stress in Platycladus orientalis (L.) Franco exposed to 100 mM hydrogen peroxide (H2O2), and its effects on reactive oxygen metabolism. Results indicated that exposure to 100 mM H2O2 significantly increased the contents of H2O2, malondialdehyde (MDA), ascorbic acid, total glutathione, and proline, and the activities of superoxide dismutase (SOD) and catalase (CAT) in P. orientalis leaves, while peroxidase (POD) activity decreased. The transcript levels of Cu/Zn-SOD, CAT, GR, APX, MDAR, and GST were upregulated in P. orientalis after 48 h of H2O2-induced stress. Compared with 100 and 200 μM ABA, lower ABA concentrations (0.5, 1, and 10 μM) significantly enhanced the activities of SOD, POD and CAT, and increased the contents of ascorbic acid, total glutathione, and proline in H2O2-treated seedlings at 48 h, accompanied by reduced H2O2 and MDA contents. Among the three lower concentrations of ABA, the 1 μM ABA concentration had the strongest effect on enzyme activities and the contents of antioxidant compounds. Among all the ABA concentrations, 1 μM ABA had the strongest effect to increase the transcript levels of Cu/Zn-SOD, CAT, GR, APX and MDAR in P. orientalis under H2O2-induced stress. This study revealed that an appropriate concentration of ABA effectively mitigated the damage caused by reactive oxygen species to P. orientalis via regulating the antioxidant defense system.