Exploring lead uptake, transport mechanisms in roots of different Sedum alfredii ecotypes as a way of enhancing its hyperaccumulative capacity.

Lead (Pb) contamination poses an extensive environmental challenge. Sedum alfredii, as a Zn/Cd co-hyperaccumulator, also exhibits a considerable capability for Pb tolerance and accumulation, which has great potential for phytoremediation of Pb-contaminated soil. However, the mechanisms of Pb uptake and accumulation in Sedum alfredii roots remain opaque. In this study, synchrotron-based micro X-ray fluorescence and X-ray absorption spectroscopy were used to investigate the localization and speciation of Pb in the roots of Pb-accumulating ecotype and non-accumulating ecotype of Sedum alfredii. The results showed that non-accumulating ecotype accumulated large amounts of Pb predominantly in the meristematic and elongation zones of the root tips after treatments of 100 μM Pb for 24 h. Pb-accumulating ecotype accumulated less Pb in roots than non-accumulating ecotype, but transported more Pb to the shoots through the root stele. Moreover, lateral roots, including lateral root primordia, as well as the apexes and bases of emerged lateral roots, were observed to be the hotspots of Pb distribution in both Pb-accumulating ecotype and non-accumulating ecotype roots, indicating their important role in Pb uptake and accumulation. Pb-cell wall complex was found to be the main form of Pb (>65%) in both ecotypes, although the proportion of the Pb-cell wall complex in Pb-accumulating ecotype roots is lower, and a small amount of Pb-GSH complex (8.6%) was observed, which may be engaged with the Pb accumulation and translocation in Pb-accumulating ecotype.