Tissue-specific bioaccumulation of heavy metals in ammopiptanthus mongolicus, the only evergreen shrub in the desert of northwest China

The concentration of heavy metals (HMs) in plants is determined by the background values in soil and/or the expression of genes related to HM metabolism. To determine if the bioaccumulation of HMs in Ammopiptanthus mongolicus, an evergreen desert shrub commonly used as a traditional medicine, is related to environmental background and/or tissue (organ) specific, the HM concentrations (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn) in the soil, roots, stems, and leaves of A. mongolicus in the desert of northwestern China were measured. Efficiencies of bioconcentration, bioaccumulation, and translocation to different portions of the plant were calculated. A linear mixed-effects model was used to test the effects of environmental background and tissue specificity on bioaccumulation. The results showed that most HMs did not exceed the pollution threshold in the soil samples, and the accumulation in plants did not meet the standard of worldwide averages. The phytoaccumulation of HMs was independent of the background concentration in soil but was related to plant tissues, which reflects the low mobility of HMs in the desert, and suggesting that the phytoaccumulation is related to the metabolic capacity and/or differential expression of HM-related genes in tissues of A. mongolicus. The high translocation efficiency of mercury from the underground portion to the leaves makes A. mongolicus a potential phytoremediator for mercury contamination. However, the removal efficiency in high-dose contamination soil still needs to be further examined. This study suggests that the desert environment may lack appropriate microorganisms to decompose immobile HMs effectively. In addition, when using A. mongolicus as a medicinal plant, special attention should be paid to the accumulation of HMs in different tissues, even if they have not reached the risk threshold.