Involvement of reactive oxygen species generated from melanin synthesis pathway in phytotoxicty of L-DOPA

(L)-DOPA is an active allelochemical that inhibits plant growth. To determine whether the phytotoxicity is due to the reactive oxygen species generated during its oxidation to melanin, oxidative damage, melanin accumulation, and the effect of antioxidants on its phytotoxicity were examined in (L)-DOPA-tolerant (barnyard grass) and -susceptible (lettuce) plants, and in suspension-cultured carrot cells. (L)-DOPA suppressed root elongation in lettuce compared to barnyard grass. Levels of melanin and thiobarbituric acid reactive substances (TBARS) increased remarkably in (L)-DOPA-treated lettuce roots, but not in barnyard grass. (L)-DOPA also suppressed carrot cell growth to 60% of the control at 1 mM. Melanin content in 1 mM (L)-DOPA-treated carrot cells increased continuously; however, ascorbic acid and alpha-tocopherol suppressed accumulation. When melanin formation was inhibited by ascorbic acid and alpha-tocopherol, growth of (L)-DOPA-treated cells was restored. TBARS levels were higher in 1 mM (L)-DOPA-treated carrot cells than in untreated control cells 2 d after treatment, but not at 4 or 6 d. Ascorbic acid and alpha-tocopherol suppressed the production of lipid peroxide during the initial 2 d. These results suggest that the phytotoxicity of (L)-DOPA is due to oxidative stress caused by reactive oxygen species from the melanin synthesis pathway.