Biochemical responses of the aquatic higher plant Lemna gibba to a mixture of copper and 1,2-dihydroxyanthraquinone: synergistic toxicity via reactive oxygen species.

Metals and polycyclic aromatic hydrocarbons (PAHs) are known to be toxic to plants. Because metals and PAHs often are cocontaminants in the environment, plants can be subjected to damage caused by their combined effects. We recently found that copper and an oxygenated PAH (1,2-dihydroxyanthraquinone [1,2-dhATQ]) synergistically are toxic to plants. This synergistic toxicity was linked indirectly to production of reactive oxygen species (ROS). In this study, plant growth, chlorophyll pigments, protein accumulation, and ROS production were chosen as endpoints to assess the mechanism of toxicity of copper and 1,2-dhATQ to Lemna gibba in more detail. Because copper and PAHs can generate ROS, we assayed for specific antioxidant enzymes: Superoxide dismutase (SOD), glutathione reductase (GR), and ascorbate peroxidase (APX). Copper treatment at a concentration that did not cause growth inhibition resulted in upregulation of Mn SOD, Cu-Zn SOD, and APX. At a level that moderately was toxic to plants, 1,2-dhATQ did not alter significantly the levels of these antioxidant enzymes. However, a synergistically toxic mixture of copper plus 1,2-dhATQ upregulated Cu-Zn SOD, Mn SOD, and GR, although APX activity was downregulated. When plants were treated with the ROS scavenger dimethyl thiourea (DMTU), enhanced toxicity and formation of ROS caused by the mixture both were diminished substantially. However, 1,2-dhATQ toxicity was not affected significantly by DMTU. Based on this study, the toxicity caused by the mixture of copper plus 1,2-dhATQ directly can be connected to elevated levels of ROS.