scope:

Introduction

Air pollution is an irritant to organisms, through which responses are initiated in the effected organisms. These responses can lead to various changes in their metabolism and appearance.

It is possible to determine concentrations of important pollution components individually with chemico-physical methods. These components, however, represent only a limited selection of all factors that occur in the environment and are relevant to the effect on plants (VDI 3957 Part 1).

Pollution control is aimed at avoiding harmful effects, which can only be reproduced indirectly by pollution measurements, whereas the direct detection of effect can only take place with sensitive (effect-) organisms (Arndt et al., 1985).

The prediction of an integrated total effect using physico-chemical concentration measurements of individual pollution components suffers from substantial uncertainty, even if the parallel influence of climatic factors is considered.

By contrast the results of biological measurement methods are incomparably more reliable than predictions of effects derived from technical pollution measurements because the determined effects provide direct information on the detrimental effects on the effected object, of which some are plants (VDI 3957 Part 1).

Bioindicators respond to the biologically effective proportion of air pollution. The response can be recorded as integral of the past and - with a certain delay - the present environmental conditions (Prinz and Scholl, 1975).

For a long time the indication of photooxidant effects has been of special importance for the effect meas urements with bioindicators (Erhardt et al., 1995). Photooxidants occur in the atmosphere as gaseous substances that have an oxidising effect and whose formation in the troposphere is light dependent. Precursor substances are NO_x and volatile hydrocarbons.

As the key component of photooxidants, ozone plays a central role. Ozone is a phytotoxically active gas, which can also lead to growth losses in plants and therefore to reduced yields in agriculture (Fuhrer et al., 1989; v. Tiedemann, 1991; Tonneijck and Posthumus, 1991; Rabe and Richter, 1993; Köllner et al., 1995). Photooxidants also contribute to the development of the new type of forest damage (Arndt et al., 1982; Prinz et al., 1985). Effects of ozone on wild plants are subject of numerous investigations (e.g. Ashmore et al., 1995; Pleijel and Danielsson, 1997; Bergmann et al., 1998; Bergmann et al., 1999; Bungener et al., 1999; Franzaring et al., 2000). Additionally, ozone is an aggressive gaseous irritant, which especially affects the respiratory tract in humans (VDI 2310 Part 15; Marquard and Schäfer, 1994; Greim and Deml, 1996).

A major cause of the far-reaching ozone pollution is car traffic, which emits precursor substances. On account of the reducing effect of nitrogen monoxide, hydrocarbons and further substances, however, ozone concentrations occuring in the vicinity of roads are lower than in the more distant surroundings (UBA, 1991; Beilke 2000).

Especially affected are rural areas, in which often higher concentrations of photooxidants occur than in conurbations themselves. The reason for this is partly that through the photochemical reaction during the substance transport away from the conurbations the photooxidants and their key component ozone are formed from precursor substances. Furthermore, the breakdown processes are reduced in rural areas due to the lower NO concentrations.

Photooxidants do not accumulate in plant organs. Instead, they initiate through chemical reactions pollution- caused injury, which can become visually apparent in the leaves (necrosis). For that reason, such leaf injury of sensitively reacting plants can be used for the effect measurement of photooxidants (Rabe, 1982; Schlüter, 1984; Keitel et al., 1985; Nobel, 1987; LfU, 1989; Franzaring, 1994).

The tobacco variety Nicotiana tabacum variety Bel W 3 reacts to Ozone very sensitively and specifically with visible injury symptoms (VDI 2310 Part 6; Knabe et al., 1973; Arndt et al., 1985; Nobel et al., 1986; Erhardt et al., 1995; Wäber and Peichl, 1995; Wäber 1996; Bay. LfU, 1999). This reaction, however, cannot be classified as strictly component-specific, as synergistic and antagonistic effects can occur through the simultaneous influence of various substances. Furthermore, it was shown that the degree of injury in the subtropical tobacco variety correlates to that of indigenous wild and cultivated plants (Tingey et al., 1975; Cornelius et al., 1985; Franzaring and Weigel, 1997; Glombitza et al., 1998; Kostka-Rick, 2002; see Tables 1 and 2).

This representativeness makes it possible to derive from the results of the tobacco injury a risk prognosis for other plant species.

Already the first investigations into effect detection of photooxidants used tobacco (Haagen-Smit et al., 1952; Ledbetter et al., 1959; Glater et al., 1962; Menser et al., 1963; Mac Dowall et al., 1964; Heggestad, 1966; Dean, 1967; Heck and Dunning, 1967; Cantwell, 1968; Heggestad, 1968; Hibben, 1969; Reinert et al., 1970; Knabe et al., 1973; Steubing et al., 1983). Today, tobacco is often exposed along with other indicators that vary in their degree of reaction. Such a combination of different bioindicators is called an indicator array (Arndt et al., 1987).