Abstract:The effects of exogenous spermidine (Spd) and inhibitor of polyamine oxidase (PAO) on growth of roots, endogenous polyamine levels, hydrogen peroxide (H2O2 ) content and activity of polyamine oxidase in roots of cucumber seedlings under root-zone hypoxia grown hydroponicallywere studied. The results showed that exogenous polyamines could imp rove roots growth of cucumber seedlings, and levels of free putrescine Put) , spermidine (Spd) and spermine (Spm) in cucumber roots obviously increased under hypoxia stress. Exogenous Spd and inhibitor of PAO induced an increase in free polyamine levels and decreased H2O2 content in cucumber roots, which could alleviate damage of hypoxia stress to cucumber seedlings. Polyamines are products responding to hypoxia stress and PAO belongs to adaptive enzyme whose activities change with levels of polyamines.