Growth, gas exchange, root morphology and cadmium uptake responses of poplars and willows grown on cadmium-contaminated soil to elevated CO2

Abstract

Little information is available about effects of elevated CO2 (EC) on growth of poplars and willows grown on Cd-contaminated soil and their potential use for Cd phytoremediation. Plant growth, gas exchange, root morphology, and Cd uptake responses to EC were assessed for one poplar genotype [Populus euramericana cv. ‘74/76’ (P107)] and two willow genotypes [Salix jiangsuensis CL. ‘799’ (J799) and Salix jiangsuensis CL. ‘172’ (J172)]. Rooted cuttings were grown on Cd-contaminated soil in six open-top chambers supplied with ambient and elevated CO2. EC increased leaf, stem, root and total biomass and total Cd uptake, but did not change Cd concentration in the tree tissues. Although stomatal conductance declined remarkably, EC stimulated leaf photosynthesis and intrinsic water use efficiency. There were differences among tree genotypes in growth and photosynthesis response to EC, with photosynthetic acclimation occurring only in P107. EC increased root lengths, root surface areas, root volumes and numbers of root tips of the three tree genotypes grown on Cd-contaminated soils. It was concluded that the biomass increase was closely correlated with stimulation of leaf photosynthesis and root growth induced by EC. EC increased the root surface areas of small-diameter roots and consequently, the ability to capture Cd in root systems, which led to increased total Cd uptake in all plant parts. The increase in total Cd uptake by the tree genotypes due to increased biomass under EC suggested an alternative way of improving the efficiency of phytoremediation of contaminated soil.