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Selecting plant species, especially woody species, that can tolerate and accumulate high levels of heavy metals is crucial for the purpose of phytoremediation. In the present study, seven willow species/clones were evaluated for their variations in copper (Cu) tolerance, Cu accumulation, and their relative physiological responses, when exposed to different doses of Cu (control, 15, and 120 μM) in a hydroponic system for 40 days. Upon Cu exposure, all tested willow species/clones (Salix, S.) remained relative normal growth, albeit with some visual evidence of Cu toxicity observed. Seven willow species remained relative high total biomass with tolerance index > 0.6 when being exposed to 120 μM Cu, suggesting their high Cu tolerance. Exposure to 120 μM Cu resulted in notable declines (16.3-76.1%) in photosynthesis in all willow species. Increases in the soluble sugar content and decreases in the soluble protein content in the leaves of five willow species (S. integra "Yizhibi", S. jiangsuensis "J172", S. matsudana 14, S. matsudana 25, S. matsudana 89) were found in the 120 μM Cu treatment. The majority of Cu mainly accumulated in the roots, ranging from 1916 to 26,244 mg kg DW. Principal component analysis and membership function analysis suggested that S. matsudana 89 and S. matsudana 25 showed much higher biomass and accumulation ability than the other species. This suggests that these two willow clones could be used as potential candidates for the phytostabilization of Cu in contaminated soils.

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