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zone is rarely out of the news these days. Depletion of ozone in the stratosphere and the biological consequences of enhanced fluxes of UV ra-diation to the Earth's surface have, together with the greenhouse effect, brought global air pollution firmly into the international political arena. At the same time, there is concern about high levels of ozone in the lower atmosphere and its effects on plant productivity and human health. Detrimental effects of tropospheric ozone on plant and animal populations are well documented, and numerous government bodies have established air quality standards for this pollutant. A European Community Directive now passing through the legislature establishes procedures for the collection and dissemination of information on ozone pollution in the 12 member states, guarantees a minimum amount of public information on concentration thresholds that are exceeded, and requires research on the causes of ozone episodes in Europe. T h e cited concentration thresholds are frequently exceeded in many areas of Europe and in the United States, d e s p i t e recent progress in controlling ozone formation. Tropospheric ozone will likely remain a problem for some time to come. Although considerable information exists about the types of damage that ozone can cause to plants and animals, much less is known about the chemical mechanisms involved. The atmospheric chemistry of ozone is complex, and some of the biological damage attributed to ,ozone is probably caused by its reaction products. Much emphasis has been placed on the harmful effects of ozone reaction products and other air pollutants, especially gaseous hydrocarbons. Reactive hydrocarbons in the atmosphere may be manmade or emitted by certain plants. Those emitted by plants are formed as precursors of biologically important compounds: act as pollinator attractors, herbicides, and pesticides: may be a means of losing surplus energy under high light conditions; or are emitted as a response to stress factors such as freezing, drought, and air pollution. Interactions of ozone with biogenic
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1890 Environ. Sci. Technol., Vol. 26, No. 10, 1992
0013-936x19210926-1890$03.0010 Q 1992 American Chemical Society
stress hydrocarbons are particularly important because they can lead to further injury in plants that are already damaged (I). Many uncertainties exist in our understanding of ozone damage mechanisms in plants. These include the relative importance of primary ozone attack and secondary attack by ozone-hydrocarbon products, the site of action of these agents, and biological consequencesfor the plant. Gaseous pollutants enter plants through the stomata-pores in the leaf surface through which gas exchange with the underlying tissues occurs. Little, if any, gas permeation through the leaf cuticle occurs, although some direct damage to this layer may be seen with some pollutants such as acid mists. Beneath each stomate lies a substomatal cavity that constitutes an air space in which reactions in the gaseous phase may occur. Beneath the cavity lie layers of mesophyll cells in which the bulk of the ulant’s metabolic activities occur. Each cell is bounded bv a cellulose-based cell w within which 1 the cell membr
gives a primary ozonide (the Criegee reaction). From the primary ozonide, different pathways are possible. The best understood sequence involves the production of a short-lived Criegee diradical followed by a secondary ozonide that leads indirectly to the production of alcohols, aldehydes, and acids. However, in an aqueous environment, organic hydroperoxides, ROOH, may be produced in place of the Criegee diradical. They may also be formed by hydrolysis of the diradical. One ultimate fate of ROOH is to give hydrogen peroxide, which may initiate lipid breakdown via radical peroxidation and thereby produce malonaldehyde as an end-product in a step-by-step degradation of the lipid. It is possible that damage to lipids via ROOH formation will be more extensive than that caused by direct reaction with ozone, because more hydrogen peroxide is produced in the former case. It is also possible that some hydroperoxidesreact more efficiently with plant membranes than does ozone. The partitioning If the primary ozone decay between e formation of adicals and that
reaction rate constants under the relevant conditions and on internal hydrocarbon concentrations makes the evaluation of alternative models impossible at present. However, a satisfactory explanation must be found for the presence of organic hydroperoxides in hydrocarbonemitting plants exposed to ozone. The role of ozonehydrocarbon reactions in determining plant damage is a crucial one. In addition to the reactions occuring within the plant, we must consider analogous reactions that might take place in the atmosphere outside of it. Hydrocarbons from anthropogenic and biogenic sources can and do react with ozone to give labile products, some of which are known to be toxic. The atmospheric chemistry of polluted air and its influence on agricultural productivity, ecosystem diversity and sustainability, and human health in a changing climate are likely to form the focus of air pollution research for the forseeable future. Acknowledgments We gratefully acknowledge Ray Fall, University of Colorado, Boulder, CO, and Leo Salter, University of Natal, for many hours of discussion on ozoneplant interactions. We also appreciate the financial support of the U.K. Department of Environment and the Natural Environment Research Council.
tive attack with of its polyunsatura.,, content. Other membranes within the cell (e.g.,those associated with photosynthesis in the chloroplasts) may also be open to attack. There are two pathways for the primary attack of ozone. The first pathway involves the breakdown of ozone under aqueous conditions, which yields the hydroxyl radical (OH). This mechanism is not considered important because the rate of OH production in the aqueous phase is low and because the likelihood of OH diffusion through the plant cell wall as far as the plasmalemma is small. The second pathway involves the reaction of ozone with unsaturated hydrocarbonseither the structural hydrocarbons of cell membranes or the volatile hydrocarbons that are produced and emitted by plants. In either event the initial reactions are the same and are dominated by attack on the C-C double bond, which ~~JuLuu.-yuLL
ROOH in ozoneinduced plant damage, at least in hydrocarbon-emitting plants, is very strong. Experimentally, hydroperoxides were detected in isopreneemitting plant leaves exposed to ozone, but not in nonemitting leaves hom the same plant (2).In addition, emitting leaves showed visible signs of pollution damage whereas nonemitting leaves did not. This result suggests that ozone-hydrocarbon products produced outside the plant were not the source of the damage. This model is not without its critics. Some doubt has been expressed about the fraction of incoming ozone that can react with emitted hydrocarbons before it arrives at the plasmalemma. It has also been suggested that ozone passing through plant cell walls will be largely prevented from reaching the plasmalemma because of its reaction with the antioxidant ascorbic acid (3). Lack of data on ozone-hydrocarbon vvIIyI..I..L
versity, U n i t e d Kjngdom. His research interests are in atmospherebiosphereinteractions.
sity and is funded by the U.K. Deportm e n t of Environ-
ment to study the biological impacts of reactive hydro-
References (11 Mehlhorn, H.;O’Shea, J. M.; Wellburn, A. R.1. Exp. Bot. 1991, 42, 17-
c, N,; Kok, G,L,; Fall, R, Noture 1990, 344,56-58. (3) Chameides, w. L. ~ ~scj. Tech. ~ no!. 1989,23.595-600. (2) Hewitt, 24.
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