Abstract: 【Objective】The objective of this study was to gain better insight into the physiological mechanisms of copper (Cu) on photosynthesis and active oxygen metabolism in three-coloured amaranth plants (Amaranthus tricolor). 【Method】The experiments were performed to investigate the parameters of growth, photosynthesis, mineral elements contents and active oxygen metabolism. Three-coloured amaranth seedlings were subjected to different Cu levels in soil at the whole experimental period. 【Result】2.0-4.0 mmol•kg-1 Cu treatment decreased the whole plant dry mass to 90%-73%. The net photosynthetic rate (Pn) and stomalal conductance (gs) in the third leaves of three-coloured amaranth seedlings treated with 2.0-4.0 mmol•kg-1 Cu were similarly reduced. None of the investigated Cu levels decreased internal CO2 concentration (Ci). The effect of Cu on potential efficiency of photosystem Ⅱ, PSⅡ (Fv/Fm) was negligible. The Cu dependent reduction of PSⅡ quantum efficiency (ΦPSⅡ) after plant adaptation in actinic irradiation was more noticeable. This could imply that reduced demand for ATP and NADPH in a dark phase of photosynthesis caused a down-regulation of PSⅡ photochemistry. Furthermore, in third leaves of Cu-treated plants the decrease in water percentage as well as lower contents of photosynthetic pigments and mineral elements including Fe, K and Mg were observed. Superoxide dismutase (SOD) and guaiacol peroxidase (POD) activities and proline (Pro) content in third leaves of three-coloured amaranth seedlings treated with 2.0-4.0 mmol•kg-1 Cu significantly increased, while catalase (CAT) and ascorbate peroxidase (APX) activities and the contents of carotenoid (Car), glutathione (GSH) and ascorbic acid (AsA) decreased accompanied by the contents of hydrogen peroxide (H2O2), superoxide (O2－•) and malondialdehyde (MDA) and electrolyte leakage increased. 【Conclusion】 Treated with 2.0-4.0 mmol•kg-1 Cu, Pn and ΦPSⅡ decreased, and peroxidization aggravated by unbalance of active oxygen metabolism, so the growth of three-coloured amaranth were significantly inhibited.

Key words: Copper toxicity, Growth, Photosynthesis, Active oxygen metabolism, Three-coloured amaranth plants (Amaranthus tricolor)