Fangmeier, A; Bender, J; Weigel, HJ; Jager, HJ
Plants in their natural environment are important mediators in the exchange of a wide variety of different gaseous and particulate compounds between the atmosphere and the biosphere (Table 14.1), i.e. the vegetation may either act asa source or a sink for these compounds. For example, as part of the overall biogeochemical cycles of the major elements several trace gases (CO, CO2. CH4, NMHC, or VOC, N2, NO, NO2, N2O, SO2, H2S etc.) are emitted from terrestrial ecosystems via the plant canopy into the atmosphere where they act as greenhouse gases or where they may contribute to the causes of regional or local air pollution problems. Once airborne compounds are deposited to the biosphere they may affect plant performance and ecosystem properties. Referring to their impact on vegetation they can be broadly divided (see also Table 14.1) into (1) essential nutrient compounds which act as macro- or micronutrients (e.g. the gases CO, SO2, NO, NO2, NH3 and particulate NH4, N03-N, S04-S, P, Ca, Fe, Mg) and (2) compounds which may cause adverse or toxic effects (e.g. the gaseous pollutants 03, SO2, NO2, HF, PAN, NMHC or VOC, metals like Pb, Cd, Hg) or excess nutrient substances (e.g. N, 5, Zn, Al) which alter normal patterns of growth and development in ecosystems (Dammgen and Weigel, 1998). Plants as well as other organisms in ecosystems are seldom exposed to single airborne chemicals or pollutants, respectively, but almost always to a number of compounds together. The pattern of these pollutant mixtures varies spatially and temporally, e.g. as co-occurrences can be simultaneous and/or sequential. Due to the occurrence of such mixtures any assessment of air pollutant effects on plants and ecosystems should consider possible interactions of pollutants with regard to their effects. Past and current air pollution impact research has been dominated by single pollutant studies and rela- tively few attempts have been made to address the issue of effects of pollutant mixtures. This chapter will review our current knowledge of effects of mixtures of air pollutants on higher plants (crops, wild plants and forest trees). Emphasis will be on gaseous pollutants but examples of interactions of gases with other types of pollutants will also be described. Whenever possible only studies relevant to ambient conditions with respect to the pollutant exposure concentration and the plant growth conditions are considered.