Plants produce a number of volatile organic compounds (VOCs), and this release plays a significant role in atmospheric chemistry. Although certain factors controlling the emission rates of VOCs from plants are reasonably well understood, the influence of abiotic stress, such as elevated ozone concentrations, is unknown. Therefore, the emission of VOCs from tobacco plants (Nicotiana tabacum L. cv. Bel B and Bel W3) and Scots pine (Pinus sylvestris L.) were studied in continuously stirred tank reactors under ozone exposure. A pulse treatment (5 h) with 120-170 nmol/mol ozone induced visible damages in the ozone-sensitive tobacco cv. Bel W3, while the more tolerant cv. Bel B seemed to be unaffected. The total amount and dynamics of the emission were studied. Both cultivars emitted methyl salicylate and a series of sesquiterpenes after the ozone treatment, but the response was less pronounced for Bel B plants. C 6 -volatiles that are thought to be derived from the lipoxygenase pathway were emitted only from Bel W3 plants. The results give further support to the hypothesis that the ozone-induced reactions of the ozone-sensitive Bel W3 plants resemble the hypersensitive response found after pathogen attack. Longterm ozone treatment (50 nmol/mol, 8 h/d) of pine led to 40% increased emissions of monoterpenes, while no damage was visible on the needles. Since VOCs are precursors of ozone, an increased VOC emission as a consequence of elevated ozone concentrations in the troposphere may lead to feedback mechanisms in photooxidant formation.

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In experiments where mung beans {Vigna radiata L.) and peas {Pisum sativum L.) have been pre-exposed to ethylene and afterwards treated with ozone, it has been shown that such ethylenepretreated plants may become more resistant to ozone. Further experiments with hydrogen peroxide (^HjOj) and the herbicide paraquat suggest that this increased resistance against ozone depends on the stimulation of ascorbate peroxidase activity which provides cells with increased resistance against the formation of HjOj which is also formed when plants are fumigated with ozone. These results explain why increased production of ethylene can be observed in plants exposed with ozone or other oxidative stress and clearly demonstrate that in plants, as well as animals, peroxidases protect cells against harmful concentrations of hydroperoxides.

Production of antioxidants was investigated in needles of fir (Abies alba Mill.) and spruce (Picea abies (L.) Karst) after exposure to low concentrations of So2, 03, and a combination of both pollutants. Glutathione reacted most sensitively to pollutants followed by vitamin E and vitamin C. In spruce needles, the overall increase of antioxidants after exposure to air pollutants was lower than in needles of fir. SO2 was more potent than 03. Maximum increase of antioxidants was found in needles after exposure of trees to SO2 + 03.Oxidation of both proteins and lipids is among the major phytotoxic consequences of 03 and SO2 action (1, I1, 13). Oxidative degradation of lipids can be easily detected by decomposition products, such as ethane and malonaldehyde (17). Although ethane is a sensitive indicator, a significant increase in gas production is generally associated with visual injury of plants (4). This limits the specific use of ethane as an early indicator of oxidative stress in plants. Antioxidants, such as the vitamins C and E or glutathione, determine the extent of oxidative degradation of cell components (4,17,18 RESULTS AND DISCUSSIONIn spruce, we found a 3-fold higher glutathione level in the filtered air-treated control needles of the youngest shoot than in control needles of fir (Fig. 1). After long-term exposure to different air pollutants, needles neither had visual injury nor elevated lipid oxidation, measured as ethane production. Treatment of fir and spruce with SO2 gave a higher increase of antioxidants in needles than treatment with 03 or charcoal-filtered air. Further, exposure to the pollutants alone significantly increased (P < 0.01) the glutathione content in fir needles compared to filtered air-treated needles. A combination of both gases gave maximum response and significantly enhanced (P < 0.01) the level of vitamin E and glutathione in needles of both fir and spruce compared to the antioxidant levels in needles either to 03 alone or to filtered air. Needles of fir had a 1.9-fold and 3-fold higher content ofvitamin E and glutathione, respectively, after exposure to S02 + 03 than needles exposed to filtered air. In comparison, needles of spruce had only a 1.3-fold higher content of both antioxidants after treatment with S02 + 03 than needles treated with filtered air.The increase of antioxidant levels above the filtered air-treated control was higher in 2-year-old spruce needles, exposed to air pollutants for 2 years, than in the youngest needles, exposed to pollutants for one vegetation period (Fig. 2). In general, the level ofantioxidants above the control was higher after treatment with SO2 or S02 + 03 than after treatment with 03 alone. Increase of glutathione was the most sensitive reaction to air pollutant treatment. No significant increase (P > 0.05) was found for vitamin C after exposure of 1-year-old needles to different pollutants. However, in 2-year-old needles the amount of all antioxidants tested was significantly higher (P < 0.05) after exposure to S02 + 03 than to 03...

Ascorbate-dependent detoxification of hydrogen peroxide by guaiacol-type peroxidases is increased considerably in the presence of 3,4-dihydroxyphenolic compounds, suggesting that ascorbate is the natural substrate for many types of peroxidase in situ and not just the ascorbate-specific peroxidases. The ascorbate-dependent destruction of hydrogen peroxide in the more acidic cellular compartments such as the vacuole may be an important function of such non-specific peroxidases. The stressinduced production of phenolic compounds would render the guaiacol peroxidases in other less acidic-cellular sites effective as ascorbate-dependent H202-detoxifying enzymes.