Influencing factors of Bidens pilosa L. hyperaccumulating Cadmium explored by the real-time uptake of Cd2+ Influx around root Apexes under different exogenous nutrient ion levels

Though Bidens pilosa L. has been confirmed to be a potential Cd hyperaccumulator, the accumulation mechanism is not yet clear. The dynamic and real-time uptake of Cd2+ influx by B. pilosa root apexes was determined using non-invasive micro-test technology (NMT), which partly explored the influencing factors of the Cd hyperaccumulation mechanism under the conditions of different exogenous nutrient ions. The results indicated that Cd2+ influxes at 300 μm around the root tips decreased under Cd treatments with 16 mM Ca2+, 8 mM Mg2+, 0.5 mM Fe2+, 8 mM SO42− or 18 mM K+ compared to single Cd treatments. The Cd treatments with a high concentration of nutrient ions showed an antagonistic effect on Cd2+ uptake. However, Cd treatments with 1 mM Ca2+, 0.5 mM Mg2+, 0.5 mM SO42− or 2 mM K+ had no effect on the Cd2+ influxes as compared with single Cd treatments. It is worth noting that the Cd treatment with 0.05 mM Fe2+ markedly increased Cd2+ influxes. The addition of 0.05 mM Fe2+ exhibited a synergistic effect on Cd uptake, which could be low concentration Fe2+ rarely involved in blocking Cd2+ influx and often forming an oxide membrane on the root surface to help the Cd uptake by B. pilosa. The results also showed that Cd treatments with high concentration of nutrient ions significantly increased the concentrations of chlorophyll and carotenoid in leaves and the root vigor of B. pilosa relative to single Cd treatments. Our research provides novel perspectives with respect to Cd uptake dynamic characteristics by B. pilosa roots under different exogenous nutrient ion levels, and shows that the addition of 0.05 mM Fe2+ could promote the phytoremediation efficiency for B. pilosa.