In the present study,we evaluated the protective effect of exogenous heme oxygenase-1(HO-1 EC 1.14.99.3) inducer hematin against mercury-induced oxidative damage in the roots of Alfalfa(Medicago sativa L).Plants exposed to mercury(HgCl_2) exhibited a significant increase of lipid peroxidation,as well as inhibition of root elongation.However,hematin(50μM) supplementation to HgCI_2(100μmol/L) treated plants effectively reduced the lipid peroxidation and partially increased the root elongation.These responses were mimicked by the application of aqueous solution of carbon monoxide(CO) with 50%saturation.Also,treatment with hematin could result in the potent induction of HO-1 transcript in the root tissues, as detected 12 h following treatment.Moreover,the activation of anti-oxidant enzyme,including glutathione reductase, monodehydroascorbate reductase and superoxide dismutase activities,and the decrease of lipoxygenase activity,were induced by hematin at 12 h or 24 h,which was further confirmed by histochemical staining for the detection of lipid peroxidation and loss of membrane integrity.Whereas,ascorbate peroxidase and guaiacol peroxidase isozyme activities or their transcripts were reduced,respectively,indicating that hydrogen peroxide might act as a signal to mediate Hgtolerance at the beginning of treatment.The ameliorating effects of hematin were specific,since the CO scavenger hemoglobin differentially reversed the above actions.Taken together,our results suggested that hematin exhibits a vital role in protecting the plant against Hg-induced oxidative damage. In the present study, we evaluated the protective effect of exogenous heme oxygenase-1 (HO-1 EC 1.14.99.3) inducer hematin against mercury-induced oxidative damage in the roots of Alfalfa (Medicago sativa L) .Plants exposed to mercury (HgCl 2 (50 μM) supplementation to HgCI_2 (100 μmol / L) treated plants effectively reduced the lipid peroxidation and partially increased the root elongation. These responses were mimicked by the application of aqueous solution of carbon monoxide (CO) with 50% saturation. Also, treatment with hematin could result in the potent induction of HO-1 transcript in the root tissues, as detected 12 h following treatment. More over, the activation of anti -oxidant enzyme, including glutathione reductase, monodehydroascorbate reductase and superoxide dismutase activities, and the decrease of lipoxygenase activity, were induced by hematin at 12 h or 24 h, which was further co nfirmed by histochemical staining for the detection of lipid peroxidation and loss of membrane integrity. Thereas, ascorbate peroxidase and guaiacol peroxidase isozyme activities or their transcripts were reduced, respectively, indicating that hydrogen peroxide might act as a signal to mediate Hgtolerance at the beginning of treatment The ameliorating effects of hematin were specific, since the CO scavenger hemoglobin differentially on the above actions.