اثر اکولوژیکی اجسام ریز معلق در هوا بر گیاهان Ecological effect of airborne particulate matter on plants

Response against particulates stress begins with changes in the population of sensitive individual organisms at single or multiple trophic levels (Bazzaz, 1996). As a minimum three levels of biological interaction are involved between plants and particulates: (a) the individual plant and its environment, (b) the population and its environment, and (c) the biological community and its environment (Billings, 1978). The response of individual organisms against stress is based on its genotype, stage of growth, existing resources, and microhabitat (Levin, 1998). Competition among individuals and species during ecological succession may improve ecosystem tolerance to the challenge of particulates deposition (Rapport and Whitford, 1999; Guderian, 1985). Succession in unpolluted (favorable) environment is progressive while, under harsh conditions, due to intermittent natural disturbance, energy is diverted from growth and reproduction to maintenance, and return succession to an earlier stage (Waring and Schlesinger, 1985). Such disturbances disrupt normal physiology and biochemistry of plants, the determinants of energy flow and nutrient cycling, food chain structure, and nutrient inventory (Odum, 1993). These disturbances, nevertheless, sets the stage for revival, which permits the disturbed ecosystem to acclimatize to changing environments. Therefore, these perturbations may yield a temporary setback and recovery can be rapid (Odum, 1969). On the contrary, anthropogenic stresses, such as those due to particulate matter and other anthropogenic deposition, may be more harsh and devastating, with stressed ecosystems recuperating less readily and often undergoing further degradation (Odum, 1969; Rapport and Whitford, 1999), e.g. presence of heavy metal exposure causes tree injury and contributes to forest decline in the northeastern United States (Gawel et al., 1996). Sayyed and Sayadi (2011) have studies the variations in the heavy metal accumulations within the surface soils from the Chitgar industrial area of Tehran. Effects of particulate matter can result from direct deposition or indirectly by deposition onto the soil. Particulate deposition reduces growth, yield, flowering, and reproduction of plants (Saunders and Godzik, 1986). Tolerant individuals, present in low frequencies in populations when growing in undisturbed areas, have been selected for tolerance at both the seedling and adult stages when exposed to trace metal or nitrate deposition (Ormrod, 1984; U.S. Environmental Protection Agency, 1993). Tolerant individuals within a plant population exhibit a wide range of sensitivity that is the basis for the natural selection of tolerant individuals. The rapid evolution of certain populations of tolerant species, at sites with heavy trace element and nitrate deposition, has been observed (Saunders and Godzik, 1986). Chronic pollutant injury to a forest community may result in the loss of susceptible species, loss of tree canopy, and safeguarding of a residual cover of pollutant-tolerant herbs or shrubs that are recognized as successional species (Smith, 1974; Miller and McBride, 1999). The effects of dust deposited on plant surfaces are more likely to be linked with their chemistry rather than simply with the mass of deposited particles (Farmer, 1993). Alkaline particles may injure plant surfaces, such as limestone particles (Brandt and Rhoades, 1972, 1973). There has been reduction in growth of the dominant trees owing to crust formation on leaves which reduces photosynthesis and bringing premature leaf fall and destruction of leaf tissues (Brandt and Rhoades, 1973). Alkaline dust containing high levels of MgO deposited on leaf surfaces disrupted the epicuticular waxes (Bermadinger et al., 1988). Cement dust on hydration liberates calcium hydroxide which can raise leaf surface alkalinity in some cases to pH 12. This level of alkalinity can hydrolyze lipid and wax components, penetrate the cuticle, and denature proteins finally plasmolyzing the leaf (Guderian, 1986; Czaja, 1960, 1961, 1962). Limestone dust coating of lichen thallus damaged its photosynthetic apparatus (Arianoutsou et al., 1993). All this leads to change in community structure and function. It is reported that deposition of particulate matter affects the microbial community living in the phyllosphere. This microbial community plays an important role in decomposition of litter fall (Miller et al., 1982; Jensen, 1974; Millar, 1974). Since fungi are important decompose, changing the fungal community on the needles finally weakens the decomposer community, decrease the rate of litter decomposition. All these processes alter nutrient cycling (Bruhn, 1980). Slowly decomposing litter influences nutrient availability within the ecosystem because of accumulation of carbohydrates and mineral nutrients (Cotrufo et al., 1995). Epiphytic lichens and mosses, because of their nutritional dependence upon and continued contact to particulate deposition, are at risk. There is various indirect and significant effect of particulate matter on ecosystem. Indirect plant responses of greatest interest are chiefly soil-mediated and depend primarily on the chemical composition of the individual elements present in particulate matter. Changes in the soil may not be observed until accumulation of the pollutant has occurred for 10 or more years, except in the severely polluted areas around industrialized point sources (Saunders and Godzik, 1986). The soil environment is an active site of poorly characterized biological interactions (Wall and Moore, 1999). Rhizosphere organisms play a crucial role in creating chemical and biological transformations, decomposing organic matter and making inorganic minerals available for plant uptake (Wall and Moore, 1999). Indirect effects are usually chronic and occur over time and are difficult to determine because the changes are subtle (Garner, 1994).