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As an essential element in plant nutrition, copper (Cu) can promote or inhibit plant growth depending on its concentration. However, the dose-dependent effects of copper, particularly on DNA damage associated with reactive oxygen species (ROS) homeostasis, are much less understood. In this work, we analyzed the dual effect of Cu (5, 20, and 60 μM) on the reproductive performance of Arabidopsis plants. Whereas Cu5 promoted inflorescence initiation and increased kilo seed weight, two higher concentrations, Cu20 and Cu60, delayed inflorescence initiation and negatively affected silique size. Excess Cu also induced changes in cellular redox homeostasis, which was examined by in situ visualization and measurements of ROS, including superoxide (O2•−), hydrogen peroxide (H2O2), malonyldialdehyde (MDA), and plasma membrane damage. The most dramatic increases in the production of O2•− and H2O2 along with increased activity of superoxide dismutase (SOD) and glutathione peroxidase (GPX) and decreased activity of catalase (CAT) and ascorbate peroxidase (APX) were observed in roots with Cu60. Oxidative stress also modulated the expression levels of a number of genes involved in the DNA damage response (DDR), particularly those related to DNA repair. The Cu-induced chlorosis of Arabidopsis seedlings could be alleviated by exogenous addition of glutathione (GSH) and ascorbate (Asc), as the chlorophyll content was significantly increased. Overall, internal homeostasis ROS and the associated DDR pathway and the corresponding scavenging mechanisms play a central role in the response of Arabidopsis to oxidative stress induced by inhibitory Cu concentrations. Our results have shown, for the first time, that the biphasic responses of Arabidopsis seedlings to increasing Cu concentrations involve different DNA damage responses and oxidative reactions. They provide the basis for elucidating the network of Cu-induced DDR-related genes and the regulatory mechanism of the complex ROS production and scavenging system. [ABSTRACT FROM AUTHOR]