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This present study examined excess copper (Cu) effects on seedling growth, leaf Cu concentration, gas exchange, and protein profiles identified by a two-dimensional electrophoresis (2-DE) based mass spectrometry (MS) approach after <i>Citrus sinensis</i> and <i>Citrus grandis</i> seedlings were treated for six months with 0.5 (control), 200, 300, or 400 &#956;M CuCl₂. Forty-one and 37 differentially abundant protein (DAP) spots were identified in Cu-treated <i>C. grandis</i> and <i>C. sinensis</i> leaves, respectively, including some novel DAPs that were not reported in leaves and/or roots. Most of these DAPs were identified only in <i>C.</i> <i>grandis</i> or <i>C. sinensis</i> leaves. More DAPs increased in abundances than DAPs decreased in abundances were observed in Cu-treated <i>C. grandis</i> leaves, but the opposite was true in Cu-treated <i>C. sinensis</i> leaves. Over 50% of DAPs were associated with photosynthesis, carbohydrate, and energy metabolism. Cu-toxicity-induced reduction in leaf CO₂ assimilation might be caused by decreased abundances of proteins related to photosynthetic electron transport chain (PETC) and CO₂ assimilation. Cu-effects on PETC were more pronounced in <i>C. sinensis</i> leaves than in <i>C. grandis</i> leaves. DAPs related to antioxidation and detoxification, protein folding and assembly (viz., chaperones and folding catalysts), and signal transduction might be involved in <i>Citrus</i> Cu-toxicity and Cu-tolerance.