Elemental concentrations in deposited dust on leaves along an urbanization gradient☆
Introduction
The urban environment is a dominant habitat for humans (Duzgoren-Aydin et al., 2006). The rapid urbanization has resulted in the emission of several pollutants in the urban environment (Ahmed and Ishiga, 2006). Pollutants may enter to the urban environment from the atmosphere as gases, particles or aerosols, or by evaporation of liquids, from water by coevaporation of dissolved solvents and erosion of soil by wind (Ordónez et al., 2003). Many studies have focused on the elemental concentration of heavy metals and other contaminants (Al-Khashman, 2004, Apeagyei et al., 2011). Urbanization results in the deposition of pollutants and other toxic substances causing the degradation of environmental conditions (Duzgoren-Aydin et al., 2006).
Leaves are sensitive and highly exposed to air pollution (Prusty et al., 2005). Thus, in many studies tree leaves were used as indicators to assess the quality of air in urban environments (Aksoy et al., 2000, Al-Khlaifat and Al-Khashman, 2007, Olowoyo et al., 2010). Leaves can trap various airborne particles such as trace elements, pollens, spores and salts. Thus, they are good accumulators of atmospheric contaminants (Lohr and Pearson-Mims, 1996). The capacity of leaves as dust trap depends on such factors as surface geometry, phyllotaxy, epidermal and cuticular features, leaf pubescence, and height and canopy of trees (Singh et al., 2005). Leaves collect dust on their leaf surface and trichomes (Lohr and Pearson-Mims, 1996). They could absorb gaseous pollutants via their stomata which interact between plants and their environment (Morani et al., 2011, Kardel et al., 2010); in this aspect leaves are active air contaminant collectors. On the other hand, leaves collect and deposit dust via their trichomes; thus they are also passive dust traps (Schulze and Hall, 1982, Pallardy, 2008).
In air pollution studies the leaf surface with trichomes, the size of stomata and stomatal density are the most important factors (Abbruzzese et al., 2009). Brighigna et al. (2002) studied peculiar peltate trichomes in Tillandsia species. These trichomes are qualified to catch aerosols and these increase the plain-tair interferences. Their studies also demonstrated that leaves by their trichomes are suitable for air pollution monitoring (Brighigna et al., 2002). Dust trap and air contaminant accumulation capability are highly dependent on the morphological and anatomical parameters of leaves. This aspect is demonstrated in an earlier study with the biomonitoring of epiphyte species of the genus Tillandsia (Brighigna et al., 2002). Therefore, the purpose of this study was to analyze the amount of dust and the elemental concentration of air contaminants in the deposited dust (Ba, Cu, Fe, Mn, Ni, Pb, S, Sr and Zn) along an urbanization gradient. By scanning electron microscopy (SEM) analysis we explored the interaction between morphological and anatomical parameters as well as the dust trapping and air contaminant accumulation capability of leaves. Our hypothesis was that the species best catch dust on its leaves and they could be useful biomonitors of atmospheric particulate matter.
Section snippets
Sampling sites and species
Sampling areas were in and around the city of Debrecen (Hungary). Debrecen is at a height of 120 m above the sea level on a nearly flat terrain of the Great Hungarian Plain (Kircsi and Szegedi, 2003). Mixed meteorological effects are typical in Debrecen because the city is located near to the border of different climatic zones. The prevailing wind direction is northwestern in Debrecen which could bring aerosol in the form of interregional transport (Dobos et al., 2009).
Along an urbanization
Amount of deposited dust on leaves
The effect of urbanization and tree species on the amount of deposited dust on the leaves was analyzed by a two way ANOVA. The tree species had a significant effect on the amount of dust (F14,45 = 64.661, p < 0.001). The degree of urbanization had no significant effect on the amount of deposited dust (F14,45 = 4.591, p = 0.068) while the interaction of the urbanization and species was significant (F14,45 = 3.786, p < 0.01). The amount of deposited dust was the highest in all areas for the A. negundo and C.
Discussion
The amount of deposited dust is widely used in environmental pollution studies (Prusty et al., 2005, Freer-Smith et al., 2005). Prusty et al. (2005) observed an increased amount of deposited dust around sites with elevated traffic flow. A significant difference was found in the overall loaded amount of deposited dust on leaves between an urban park and a pasture land in London, where the particle uptake was higher at the urban site than at the rural area (Freer-Smith et al., 2005). They
Conclusions
Our study demonstrated that A.negundo, C. occidentalis and Q. robur were useful biological indicators because of their large stomata size and high stomata density We found that the leaves' surface of A. negundo and C. occidentalis is covered by a large number of trichomes, which made them especially suitable to decrease the level of air pollution. Our results show that the density of trichome, stomata size and density were important in dust deposition. Leaves with a high trichome density can
Acknowledgment
The work is supported by the TÁMOP 4.2.1/B-09/1/KONV-2010-0007 and TÁMOP 4.2.2/B-10/1-2010-0024 projects. The project is implemented through the New Hungary Development Plan, co-financed by the European Social Fund and the European Regional Development Fund. E. Simon was supported by the European Union and the State of Hungary, co-financed by the European Social Fund in the framework of TÁMOP 4.2.4.A/2-11-1-2012-0001 ‘National Excellence Program’.
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Stomata size and distribution are important factors influencing the accumulation of air contaminants in leaves.