Mechanism of Selective Phytotoxicity of l-3,4-Dihydroxyphenylalanine (l-Dopa) in Barnyardglass and Lettuce

l-3,4-Dihydroxyphenylalanine (l-dopa) is one of the few allelochemicals in which the phytotoxic action mechanism has been studied. Excess exogenous l-dopa suppresses root elongation in some plant species, and the inhibitory action is species-selective. The main factor of phytotoxicity of l-dopa is considered to be oxidative damage by reactive oxygen species (ROS) and/or free radical species (FRS). This study was performed to elucidate the mechanism of species-selective phytotoxicity. The involvement of ROS/FRS and polyphenol oxidase (PPO) in species-selective phytotoxicity was examined with barnyardgrass (Echinochloa crus-galli L.) and lettuce (Lactuca sativa L.), tolerant and susceptible species, respectively. Lipid peroxidation and melanin accumulation correlated with growth inhibition by L-dopa. Antioxidants, ascorbic acid and α-tocopherol, decreased lipid peroxidation and melanin accumulation and rescued lettuce root from growth inhibition. The oxidation of L-dopa by PPO was much greater in lettuce than in barnyardgrass. From these results, the phytotoxicity of L-dopa is considered due to the oxidative damage caused by ROS/FRS generated from the melanin synthesis pathway. PPO activity might be involved in the mechanism of species-selective phytotoxicity between barnyardgrass and lettuce.