Abstract: n order to investigate the inductive effect of exogenous reactive oxygen species（ROS）on hyperhydricity of plantlet in vitro，as well as the mitigative effect and mechanism of exogenous ROS scavenger on hyperhydricity，using garlic variety‘Ershuizao’as test material，the effects of exogenoushydrogen peroxide（H2O2）and H2O2 + Ascorbic acid（AsA，a ROS scavenger）on hyperhydricity and changes of reactive oxygen metabolism in plantlet in vitro were studied. The results showed that exogenous H2O2 treatment led to the increase of superoxide anion（）production rate and H2O2 content，as well as the rise of hyperhydricity rate and degree in garlic plantlet in vitro. The occurence of hyperhydricity lagged behind substantial generation of endogenous ，and was in synchrony with H2O2 accumulation. Between day 0 to 8 of exposure to H2O2，superoxide dismutase（SOD），peroxidase（POD）and catalase（CAT）activities were on the rise and significantly higher than that of the control. But after 8 d，antioxidant enzymes activities decreased（SOD，CAT）or increased slowly（POD）. Whereas internal antioxidant compound，AsA，content on 8 d was remarkably lower than that of the control. Exogenous H2O2 + AsA treatment could reduce internalproduction rate and H2O2 content caused by exogenous H2O2；Antioxidant enzymes（SOD，POD and CAT etc.）activities kept rising，and were higher than that of the H2O2 treatment and control at the later stage；Endogenous AsA maintained a high level and were significantly higher than that of the exogenous H2O2 treatment；At the same time，hyperhydricity rate of garlic plantlet in vitro raised slowly，and was significantly lower than that of the exogenous H2O2 treatment. In sum，exogenous H2O2 treatment could lead to the increase of production rate and H2O2 accumulation；Reduce the antioxidant capacity of antioxidant system，resulting in the rise of hyperhydricity rate. The addition of exogenous antioxidant，AsA，could relieve the increase of production rate and H2O2 content，and then effectively control the occurrence of the hyperhydricity rate of plantlet in vitro.

Key words: llium sativum L., plantlet in vitro, hyperhydricity, ascorbic acid, hydrogen peroxide, reactive oxygen species