Peroxides such as peroxyacetyl nitrate (PAN) and H_2 O_2 are present in polluted air.Of the corresponding ozone concentrations, PAN may yield up to 5% -20% in centers of “photosmog”, for example, the Los Angeles region. The observation of the effects of PAN on plants first occurred in 1945 in the Los Angeles area as a silvery-bronze glazing on leaf crops like spinach (Spinaceaoleraceae) and lettuce (Lactuca sativa). PAN, a constituent of smog, the acidic, eye-irritating haze, fills the air periodically over certain industrial regions and large cities. It is photochemically formed via the action of UV and visible radiation on a complex mixture of 〖NO〗_x , oxygen and volatile unsaturated hydrocarbon generated in exhaust of automobile. Injury to sensitive species can be caused by concentrations ranged from 0.02 to 0.1µL/L for one to two hours. Concentrations above 0.1µL/L result in cell collapse and introduce large air spaces into the leaves. Collapse of cells leads to visible bronzing and glazing of leaves.
Besides PAN, other peroxides that are considered as plant-toxic species are peroxybenzyl nitrate, peroxybutyryl and peroxypropionyl. The half-life of PAN is 4.4min at physiological pH (7.2). Acetate and nitrite are mainly yielded from its base-catalyzed decay.〖CH〗_3-〖C(O)OONO〗_2 + 〖2OH〗^- → 〖CH〗_3 〖COO〗^- + 〖NO〗_2^- + O_2 + H_2 O. Peroxyacetyl nitrate may react with thiol groups, particularly in enzymes, like the SH groups of glucose-6-phosphate (Glu-2-P) dehydrogenase; the substrates, Glu-6-P and nicotinamide adenine dinucleotide (NAD), SH oxidation is partially prevented. Oxidation of smaller S-containing molecules, conversion of methionine into methionenesulfoxide and lipoic (thioctic-) as well as the conversion of coenzyme A into the corresponding disulfides occur. As for glutathione, one mole of PAN converts three moles of SH groups.
Peroxyacetylnitrate reacts with NADPH. It seems that there is no further reaction between PAN and the oxidized compounds such as NAD and nicotinamide adenine dinucleotide phosphate (NADPH). The reaction between peroxyacetyl nitrate and olefins forms epoxides and further products of lipid peroxidation whereas the reactions with primary amines leads to their acetylation.
Figure 5: Typical injury of peroxyacetyl nitrate (PAN).A glazy bronzing on the underside of newly expanded potato leaves.
(Source from: http://www.omafra.gov.on.ca/english/crops/facts/91-015.htm)
Ethene (formerly ethylene) is a retarding phytohormone. It acts as an antagonist to the growth stimulators auxins and gibberellins as well as inducing stem thickening, fruit ripening, senescence, increased resistance and general responses to stress and wounding. Under natural conditions, the formation of ethene occurs via S-adenosyl methionine (SAM) and 1-amino-cyclopropane carboxylic acid (ACC), from which it is oxidatively released.Under acute stress situations like mechanical wounding and freezing,the origin of ethene in small amounts...