Response of Platanus orientalis leaves to urban pollution by heavy metals

Atmospheric pollution is an important concern in urban environments. The impact of urban pollution on the physiological, biochemical and anatomical properties of leaves of oriental plane (Platanus orientalis L.) was studied. The leaves were collected from an urban site (Tehran, Iran) and a non-urban forest park (Chitgar Forest Park). Anatomical (stomata, parenchyma, and cuticle) and physiological (chlorophyll content, enzyme activities) properties were analyzed. The concentrations of Cd (cadmium), Pb (lead), Ni (nickel), and Cr (chromium) in leaves were significantly higher and Zn (zinc) lower at the urban site relative to the forest park. Chlorophyll a, total chlorophyll, and carotenoid content in leaves of the urban site were significantly less than those of the forest park, but there was no significant difference in chlorophyll b. In addition, the activities of superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidase in urban leaves were significantly higher than in the leaves of the forest park. In the urban area, leaves experienced a significant decrease in leaf surface area, stomata density, stomata pore area, epidermis, and spongy mesophyll thickness, but a significant increase in cuticle and palisade thicknesses relative to forest park leaves. The increase in enzyme activities may indicate that the trees are attempting to cope with increased reactive oxygen species (ROS) because of pollution-induced stress. Our study suggests that oriental plane trees alter their physiological and anatomical properties when living in a polluted urban environment.