Air pollution is a serious problem in many heavily populated and industrialized areas in the world (Kambezidis et al. 1996). A growing share of the world’s population is living in urban centers and is demanding a cleaner environment. In many urban areas of the world, motor vehicle traffic is a major source of air pollution contributing 57%–75% of total emissions (WHO 2006). In general, metropolitan areas have higher pollution than rural areas (Lagerwerff and Specht 1970; Sawidis et al. 1995, 2001). Air pollutants from motor vehicle exhausts have both direct and indirect effects on the metabolism of roadside plants even before visible symptoms appear (Viskari et al. 2000). Urban forests and trees in the urban environment can improve air quality through filtering and uptake of gases and particles (Beckett et al. 2000). Therefore, urban trees are of high importance for the inhabitants, but may also be endangered by exposure to pollution. For example, in leaves of a typical urban roadside tree species, Platanus orientalis air pollution caused changes in chlorophyll content and peroxidase activities (Alaimo et al. 2000; Baycu et al. 2006). In various roadside plants exposure to road traffic emissions resulted in changes in foliar anatomy and morphology and caused visible injury (Ghouse et al. 1980; Jahan and Iqbal 1992; Pandey and Agrawal 1994; Verma et al. 2006; Joshi and Abhishek 2007). Foliage from trees near air pollution sources can even be ‘coated’ with particulates (Ricks and Williams 1974; Lerman and Darley 1975), which may cause stomatal occlusion, thus, leading to reduced photosynthesis (Williams et al. 1971).

Plane (Platanus orientalis) trees are widely planted in parks and along streets in southern Europe and middle Asia to improve the microclimate. The purpose of this study was to determine air and soil pollution in the center of the megacity Mashad (Iran) and its periphery (Torghabeh) and to investigate functional properties of plane leaves under these conditions.

Materials and Methods

The urban study site was in the city of Mashhad (36° 41′ N, 52° 39′ E, elevation 985 m above sea level, Iran) with a population of ca 2.1 million. The city is visited each summer by several hundred thousand pilgrims. The rural site was Torghabeh, an area at a distance of about 20 km from the city centre. The climate in this area is semi-arid with an annual average temperature of 13.5°C and an annual sum of precipitation of 248 mm. At each site 10 Platanus orientalis (L.) trees of a height of about 8–10 m were selected. Leaf samples were collected about 4–5 m above ground in May and September 2004 and 2005, air samples about 12 m above ground three times a season in 2005 and soil samples from 0 to 0.1 m depth in September 2005, respectively.

Leaves were either immediately fixed in FAE (37% formaldehyde, 96% acetic acid, 70% ethanol) at a ratio of 5:5:90 (v/v) and later preserved in 70% ethanol or dried. Leaf area was measured by an Image Analysis System (WINDIAS, Delta-T Devices Ltd, Burwell, Cambridge, UK). Cross sections (30 μm thick) were prepared from fixed leaf material with a freezing microtome (Reichert, Wien, Austria). The specimen were photographed under a light microscope (Axioplan, Zeiss, Oberkochen, Germany) with a digital camera (Nikon Coolpix 4500, Nikon, Tokyo, Japan) and used to measure the length and width of palisade cells and area of palisade and spongy parenchyma cells with the software Image J (NIH, Wayne Rasband; http://rsbweb.nih.gov/ij/). In addition leaf cross sections were stained with the lipid fluorochrome fluorol yellow 088 (0.01% fluorol yellow 088 (Sigma F-5520) in polyethylene glycol 400 (Merck 8.07485) (Brundrett et al. 1991) and photographed under a fluorescence microscope (Axioplan; UV-filter combination G 365 FT 395 LP 420; Zeiss, Oberkochen, Germany) with a digital camera (Zeiss Axiocam MRC, Software AxioVision Rel. 4.6). Leaf pieces were excised from fixed material, cleared in 10% NaOH (65°C, 24 h, washed twice with water), stored in 70% ethanol, documented under a light microscope (Axioplan) using a digital camera (Nikon Coolpix 4500) and used to measure length, width and area of stomatal pores with Image J.

Dry leaf samples from three selected trees per site were studied under a scanning electron microscope (SEM 515, Philips, The Netherlands); specimen were sputtered with gold in a vacuum coating unit (SC 500, Emscope, Ashford, United Kingdom) as described by Robinson et al. (1987).

Aliquots of dry leaves and soil were powdered and wet-ashed at 170°C in HNO3 for 12 h (Feldmann 1974). The heavy metal concentrations were measured by inductively coupled plasma atomic emission spectroscopy (Heinrichs et al. 1986). To determine air pollution four 24 h-collections of air were conducted per site with a high volume air sampler (Dehm & Zinkeisen, Frankfurt, Germany) which sucks air, aerosols and particles through Teflon filters (Fluopore membrane filters, Millipore). The filters were extracted in HNO3 and analyzed as above.

Data are means of n = 10 ± SD trees per site, if not indicated other wise. Means were compared by Student’s t-test using STATGRAPHICS Plus (St. Louis, ML, USA). p-Values ≤ 0.05 were considered to indicate significant differences and are shown by * for p ≤ 0.05, ** for p ≤ 0.01 and by *** for p ≤ 0.001.

Results and Discussion

Soils from the urban site contained higher concentrations of lead, zinc, chromium, and copper than that of the rural site (Table 1). The concentrations were still within regulatory limits defined for soils in the European community (Polle and Schützendübel 2003). Elevated concentrations of lead, cobalt and chromium were present in urban air (Table 1). The highest enrichment in air was found for lead. Only lead was accumulated to a higher extent in leaves from the urban than in those from the rural site (Table 1). As lead is a heavy metal with low plant mobility (Günthardt-Goerg and Vollenweider 2007), uptake of lead into plane leaves from the soil is unlikely. It can be assumed that the enhanced lead concentration in leaves from urban planes was filtered from the air. In other cities, e.g. Istanbul, even higher lead concentrations were found in Platanus orientalis leaves than in our study (Baycu et al. 2006).

Table 1 Heavy metal concentrations in soil, air and leaf blades of Platanus orientalis trees from an urban and a rural site
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To find out if exposure to the urban environment affected functional leaf properties, we determined leaf area, stomatal density and opening (Table 2). Young expanding leaves showed no differences in mean leaf areas (84 ± 23 cm2). Mature leaves from the urban area were smaller than those from the rural site (Table 2). This suggests an influence of air pollution on leaf expansion. Similarly, significant growth reductions have also been reported for leaves from other tree species in heavily polluted city areas (Platanus acerifolia, Ficus bengalensis, Guaiacum officinale and Eucalyptus sp., Ninova et al. 1983; Jahan and Iqbal 1992).

Table 2 Leaf properties of Platanus orientalis trees from an urban and a rural site
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Furthermore stomatal density and stomatal widths were lower on leaves from the urban than on those from the rural site (Table 2) but not stomatal lengths (12.5 ± 2.0 μm). Negative effects of air pollutants on stomatal densities and opening have also been found in other species such Trifolium repens, Trifolium pratense, Cicer arietenum, Acer saccharum and Ipomea pes-tigridis growing in polluted areas (Sharma and Butler 1973, 1975; Garg and Varshney 1980; Ghouse et al. 1980; Verma et al. 2006). A reduction of stomata is also found in response to elevated CO2 concentrations, frequently present in city centres (Williams et al. 1986). The reduction in stomatal densities and their pore size may be important for controlling absorption of pollutants (Verma et al. 2006), but will limit photosynthesis at the same time.

In spite of exposure to higher pollution at the urban site, the anatomy of stomata from both study sites appeared normal (Fig. 1a, b). Each stoma had a raised rim over the guard cell region, which is typical for plane (Carpenter et al. 2005). Notably, the guard cells appeared more shrunken on the polluted leaves (Fig. 1c, d). A collapsed lower epidermis, as described by Günthardt-Goerg and Vollenweider (2007) for translocated Zn in Populus tremula leaves, was not be detected.

Fig. 1
figure 1

Anatomical leaf properties of Platanus orientalis trees from an urban and a rural site. Stomata with guard cells from an urban (a) and a rural site (b), scale bars = 10 μm. Lower surface of plane leaves from an urban (c) and a rural site (d), scale bars = 100 μm. Upper surface with particles of planes from an urban (e) and a rural site (f), scale bars = 100 μm. Cross sections through leaves from an urban (g) and a rural site (h), scale bars = 50 μm

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Foliage close to pollutant sources can be ‘coated’ with particles (Ricks and Williams 1974; Lerman and Darley 1975; Godzik et al. 1979). Studies of the diffusion resistance of leaves of Quercus petraea covered with such particulates suggested that they interfere with the control of stomatal closing (Ricks and Williams 1974). In contrast, Godzik et al. (1979) reported that the leaves of plants covered with particulates showed no pollution-dependent effects on stomatal closure. Our SEM examinations showed that the stomata of P. orientalis leaves were not plugged by particulates (Fig. 1a–d), although dust particles were present on the leaf surface. At the urban site the particles were abundant and clumped together, whereas they were less frequent and more scattered on leaves from the rural site (Fig. 1e, f).

In spite of differences in surface properties, cross sections of leaves did not reveal anatomical abnormalities (Fig. 1g, h). Cytological parameters such as lengths, widths and projected cell area of palisade parenchyma cells (means: 57 ± 20, 12.7 ± 2.9 μm, 548 ± 176 μm2) and area of the spongy parenchyma cells (243 ± 64 μm2) did not differ between the urban and the rural sites, respectively. However, the cuticle covering the outer epidermal layer was much thinner in leaves from trees at the urban than on those of leaves from trees grown at the polluted site (Fig. 1g; Table 2). The yellow lipid staining also indicated incorporation of less cutin and the blue autofluorescence pointed to higher accumulation of phenolics at the urban compared with the rural site (Fig. 1g, h). A thinner cuticle may enhance transpiration.

In conclusion, our study shows that urban conditions affected structural leaf properties, which may lead to lower photosynthesis through lower leaf area, lower stomatal densities and pore widths and probably higher sensitivity to drought because of a thin cuticle. However, mainly leaf surface features, but not internal functional anatomy, were affected. These results support that plane trees can cope with pollution (Dickinson et al. 1991) and therefore, are suitable roadside trees for megacities. The observation that leaves in urban environments were strongly covered by dust particles suggests that plane may contribute significantly to improving air quality (Smith and Staskawicz 1977). To determine the extent of air filtration and tolerance to traffic exhaust, controlled studies have to be performed in future.