Combined effects of elevated CO2 and Cd-contaminated soil on the growth, gas exchange, antioxidant defense, and Cd accumulation of poplars and willows

• Elevated CO2 (EC) stimulates leaf photosynthesis, and then increases plant growth.
• EC enhances the phytoremediation efficiency, especially under high Cd treatment.
• EC decreases oxidative damage by increasing photosynthesis.
• EC decreased oxidative damage by increasing antioxidant enzyme activities.
• Photosynthesis well maintained under Cd treatment avoids membrane lipid peroxidation.

Few studies have explored the combined effects of elevated CO2 (EC) and Cd treatments on poplars and willows. The objective of this study is to determine how growth and phytoremediation efficiency are enhanced by EC. For this purpose, this study investigated the combined effects of EC and Cd treatments on the growth, gas exchange, antioxidant defense, and Cd accumulation in one poplar genotype (Populus × euramericana (Dode) cv. ‘Nanlin-95’ (NL95)) and one willow genotype (Salix jiangsuensis CL. ‘172’ (J172)), which were grown on three Cd-contaminated soil in six open-top chambers. Under Cd treatment, plant growth was decreased, Cd accumulation was increased, and the photosynthesis and malondialdehyde concentration were unchanged in leaves of two tree species. At EC levels for both species, plant growth, total Cd uptake, CO2 assimilation rate, and intrinsic water use efficiency were increased; stomatal conductance and transpiration rate were decreased; and Cd concentrations were unchanged. EC also decreased malondialdehyde content in two species grown in Cd-contaminated soil and increased antioxidant enzymatic activities in J172 grown in high Cd-contaminated soil. At EC, plant growth and total Cd uptake exhibited greater increase in high Cd-contaminated soil than in low Cd-contaminated soil. These findings suggest that EC stimulated plant growth by increasing leaf photosynthesis and enhanced phytoremediation efficiency, particularly at high levels of Cd exposure. EC decreased oxidative damage by stimulating photosynthesis and increasing antioxidant enzyme activities. Cd treatment inhibited the growth of two tree species, and this suppression was unrelated to photosynthesis. Under Cd treatment, the well-maintained photosynthesis is assumed responsible for decreasing reactive oxygen species accumulation and avoiding membrane lipid peroxidation.