The influence of air pollution on Plane (Platanus orientalis L.)

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Air pollution is a serious problem in many heavily populated and industrialized areas in the world. Plants growing in urban areas have to face continuously various environmental constraints, of which an important and increasingly concerning one is the pollution by heavy metals. During the last few years, especially the number of vehicles in Iran, mostly operated by leaded fuel, has increased rapidly leading to increasingly high amounts of some heavy metals in the soil, air and trees near highways. The aim of this study was to assess the level of contamination of plants and air by some heavy metals. As a plant species, Platanus orientalis was chosen because it is a tree adapted to different ecological conditions and a widely distributed dominant tree in parks as well as along roads and streets of Iran. As investigation area, Mashhad City was chosen as it is the second largest city in Iran, and the population has experienced an important demographic increase during the last years. This has led, in addition to some industrialization processes, to an exponential increase of small and large vehicles, and motorcycles emitting a high level of air pollution.
For the assessment of the air pollutants and their effect on Platanus orientalis, two sites, one in the urban (heavily polluted) and another in the rural area (less polluted), were chosen in Mashhad. The investigations were focused on measuring Pb, Cu, Cr, Co, Cd, Ni and Zn in the air and soil and their effect on chemical, anatomical and morphological characteristics of Platanus leaf. For the chemical analysis of air, twenty-four hours sampling was done, using a high volume sampler. Soil samples were collected from the bases of 5 trees at a depth of 0-10 cm at each site. Leaf samples for chemical analysis were collected from 10 trees at each site. To assess the amount of emitted aerosol and dust on leaf surface area the leaves were separated into blade and veins and analyzed separately. In addition, leaves were washed thoroughly, using shaking in 100 ml deionised water. The total amount of heavy metals in air, soil and leaf samples as well as wash out solutions was measured by Inductively Coupled Plasma Atomic Emission Spectrophotometry (ICP-AES, Spectro Ciros CCD) and Graphite Furnace Atomic Absorption Spectrometry, respectively.
Microscopical analyses concerning effects of pollution on leaf epidermis concentrated on stomatal frequency, epidermal cell frequency, stomatal index and stomatal size. Measurements were done with the aid of Image J version 1.33 u. The influence of pollutants on leaf parenchyma was studied on leaf cross hand sections viewed in the fluorescence microscope. Morphological observations included leaf length, width, and leaf area and colour. Measurements were done by planimetry.
The results showed a significantly higher amount of Pb, Cr, Zn and Cd in the air of the urban than in that of the rural areas. A higher amount of Pb and Cr was also found in soil samples collected in the urban area than in those collected in the rural area. In contrast, the amount of soil Cu and Ni was lower in the urban than in rural area. The results of the calculated enrichment factor and high correlation between elements indicate that the concentration of Pb and Zn in the air is (highly enriched) largely attributed to anthropogenic activities (vehicular traffic) and that of Cu and Ni, further to crusted erosion. A comparison of data with those of other studies showed that the air and soil of Mashhad City is generally less polluted than those of other large cities around the world. This could be due to (a lack of big) the absence of large factories and also to the fact that the number of vehicles in Mashhad has only rapidly increased during the last five years. In the leaf samples, the amount Pb, Zn, Co and Cr was higher in the urban area as compared to the rural area. In contrast, the total content of Cu and Ni in both years and during all sampling periods was higher in the rural area than in the urban area, indicating an uptake of Cu and Ni by root. The results of the anatomical investigation showed that the accumulation of particles on leaves has an effect on epidermis cell and stomata frequency, epidermis cell area, stomata pore and leaf size. It also influences leaf surface properties like cuticle thickness and the structure of the outer epidermis cell wall. The aggregation of particles has modified the normal epidermal and stomata characteristics of Platanus orientalis.

The results of the examination of the leaf morphology and anatomy indicate a convincing interference between inherited morphological and anatomical traits and the amount of dust captured. Through washing the leaves with deionised water, only a low amount of heavy metals from the Platanus leaf surface was removed. The wash out method recommended by other authors is therefore not suitable for assessing the amount of emitted aerosol and dust on leaf surface area of Platanus orientalis. In contrast, the difference between the amount of heavy metals in blade and veins seems to be appropriate for assessing the amount of deposited aerosol and dust on leaf surface area.
It is well documented that air pollution can injure trees but, within certain concentration levels, plants can effectively act as a sink for airborne pollutants and reduce air quality problems. The present study provides clear evidence that Platanus orientalis is capable of curtailing lead and particulate concentrations in the air and plays therefore an important role in air cleaning, especially in arid regions.