Multifaceted Adaptive Strategies of Alternanthera philoxeroides in Response to Soil Copper Contamination

Excessive copper (Cu) content in soil can affect plant growth and also cause damage to the soil ecosystem, making it one of the risk control projects for agricultural land soil pollution in China. Alternanthera philoxeroides exhibits stronger colonization ability in heavy metal-contaminated soil, but its physiological and ecological mechanisms of tolerance to excessive Cu remain unclear. A greenhouse incubation experiment was conducted to study the multifaced responses of A. philoxeroides to Cu stress at artificially augmented concentrations of 0, 250, 500, and 1000 mg kg−1. The results showed that A. philoxeroides exhibited high tolerance to Cu2+, with a tolerance index (TI) exceeding 60%. As the Cu concentration increased from 0 mg kg−1 to 500 mg kg−1, root biomass and Cu concentration in the root increased. Additionally, soluble sugar (SS) and malondialdehyde (MDA) contents, and catalase (CAT) and peroxidase (POD) activities of A. philoxeroides continued to increase, whereas superoxide dismutase (SOD) enzyme activity, branch number, leaf area, and net photosynthetic rate kept declining. However, the trend of these indicators was opposite when Cu2+ concentrations were above 500 mg kg−1, while the canopy area and underground root system of A. philoxeroides increased. These results suggested A. philoxeroides displayed a standing “silent” tolerance strategy to survive when soil copper was lower than 500 mg kg−1 concentrations, and an “escape” strategy to avoid high copper stress by expanding the above- and below-ground areas of plants when soil copper concentrations were higher than 500 mg kg−1. This study recommends the controlled utilization of A. philoxeroides for pollution remediation in Cu-contaminated soil areas where most local native plants are unable to survive.