Abstract: Common street trees Platanus hispanica in Nanjing were used to study the distribution of six heavy metal elements (Pb, Cd, Cr, Cu, Ni and Zn) in their leaves and annual branches tissue on both sides of the road in the heavy traffic areas and the relatively clean areas, in order to determine the distribution mechanism of heavy metal elements in tissues and their possible mechanisms to resist heavy metals. The results showed that leaves and annual branches of P.hispanica could absorb heavy metal pollutants to different extent depending on the kinds of heavy metals and tree tissues. Heavy metal contents in tissues of two kinds of organs in pollution regions were obviously higher than control sites (except Ni). Accumulation ability of leaf tissues to heavy metals is lower epidermis>upper epidermis, and sponge tissue>palisade tissue. Annual branches of P.hispanica have picon, wax and trichomes, therefore it can absorb heavy metal elements. The surface systems of branches that contacts directly with air pollutants are the main tissues to monitor and instruct, and heavy metals accumulated in the epidermis entered into the cortex and marrow by picon. It is related with the transport pathway of heavy metal elements, and the specific transport mechanism is to be further investigated. Finally, based on the experimental results, this study suggested that leaves and epidermis of stems were suitable for heavy metal pollution monitoring in the atmosphere, and both materials were easily collected with small damage. Therefore, it is strongly suggested to use leaves and barks of P.hispanica in air pollution monitoring.

Key words: street trees, Platanus hispanica, heavy metal elements, tissue, distribution mechanism