Accumulation and histological location of heavy metals in Phragmites australis grown in acid mine drainage contaminated soil with or without citric acid

• Metal storages in reeds from an AMD contaminated site were assessed.
• Fe and Al mainly located in the exodermis and endodermis of roots.
• Epidermis, cortex and central cylinder of rhizomes stored Fe and Al.
• Citric acid increased metal accumulations and translocations in reeds.
• Citric acid enhanced Fe and Al entering the steles of roots and rhizomes.

Acid mine drainage (AMD) is a serious environmental issue. This study investigated the heavy metal accumulation and histological alterations caused by heavy metals and citric acid (CA) in reeds grown in AMD contaminated field or spiked soil. The results indicated that the roots and rhizomes accumulated more metals from soil containing higher level of metals. Most of Fe was stored in the exodermis of roots grown in sites that contained less Fe, while Fe had extended to the endodermis of roots grown in sites with higher levels of Fe. Al was found in the exodermis and endodermis of roots collected from sites with lower Al concentrations, but the stele of roots grown in sites with higher level of Al also contained Al. Low level of CA caused Fe enter the pith of roots, while higher level of CA resulted Fe being in the entire stele of roots. CA also altered the Al storage location in roots by enhancing Al entering stele. For rhizomes, the epidermis, cortex and central cylinder were the main sites to store Fe and Al. CA increased the storage of Fe and Al in the tissues around vascular bundles of rhizomes. The higher concentrations of Fe or Al in roots or rhizomes, the stronger intensity of the staining was observed. The quantitative data of heavy metal content also supported the hypothesis that CA can increase the bioavailability of heavy metals in soils and thus enhance the accumulation and translocation of Fe and Al. In general, the higher level of CA added into the soil, the more metals uptake by reeds.