Synergistic Effects of Exogenous Nutrient Ions on the Real-Time Cadmium Extraction by an Accumulator

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.