Effects of Urban–rural Atmospheric Environment on Heavy Metal Accumulation and Resistance Characteristics of Pinus tabulaeformis in Northern China

The washed and unwashed current (C) and previous year (C + 1) needles, branches and top soils of Pinus tabulaeformis trees were sampled at five sites Haitai industrial district (HT), Puji River (PJ), Fukang road (FK), Residential area (RA) and Baxian Mountain (BX) in Tianjin along an urban–rural gradient and analyzed for heavy metals (Cu, Mn, Zn, Pb and Cd) concentrations via ICP. C + 1 needles generally had higher Mn, Pb, Cd than C needles while the opposite was for Cu and Zn. Total Cu, Zn, Pb, Cd in soils peaked at HT and decreased at RA and BX. Heavy metals were generally higher in the unwashed needles than the washed needles at all sites. Meanwhile MDA, soluble sugar and free proline concentration in needles were increased with the increasing of heavy metal contents along the urban–rural gradient, further correlated with the heavy metal contents.