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.

Bidens tripartita L. is a cadmium (Cd) accumulator. However, the real-time influx or efflux of Cd²⁺ around its root apex has not yet been performed. The object of this experiment was to compare the roles of added ions in solution on dynamic Cd extraction by B. tripartita root tip. Quartz sand was used to grow the seedling of B. tripartite. The Cd concentrations of all samples were determined by using ICP-OES after digestion. The Cd²⁺ influx around the root apex was measured in vivo, i.e., using non-invasive micro-test technology (NMT). The results showed that the Cd²⁺ influx was found to be decreased by 35.9%, 43.7%, 20.6%, and 57.5% under 10 μM Cd combined with high content Ca²⁺, Mg²⁺, Fe³⁺, or K⁺ (16 mM, 8 mM, 0.5 mM, 18 mM, respectively), compared to that under 10 μM Cd stress. But Cd treatments with low content ions with 0.05 mM Fe³⁺ or 0.5 mM S increased the Cd²⁺ influx in roots by 20.5% and 34.6%, respectively. It was also found that Cd treatment with high concentrations of Ca²⁺ or K⁺ increased the shoot biomass of B. tripartita seedlings. Chl a and b contents were significantly decreased in the Cd treatments with low concentrations of Fe³⁺ or S compared to those under Cd stress alone, and the dehydrogenase activity of the roots decreased in the treatment of Cd with 0.05 mM Fe³⁺ or 0.5 mM S. Our results indicate that the addition of 0.05 mM Fe³⁺ or 0.5 mM S promoted Cd²⁺ influx and Cd uptake by B. tripartita. Unlike traditional measurement, the Cd²⁺ movements of three-dimensional space around the B. tripartita root tip had been performed by NMT. It was suggested that the effects of S and Fe³⁺ on the remediation potential of B. tripartita need to be further researched in the future. The results of this study provided a real-time and micro-dynamic theoretical basis for phytoremediation mechanisms.