The chemical processes that take place within plants are among the most complex and efficient chemical processes known. A plant’s ability to transform toxic substances into benign products or useful nutrients can give great insight into helping pollution problems encountered in today’s world. Ozone (O3) is a substance that collects in the troposphere (the lowest part of the earth’s atmosphere) when nitrogen oxide and volatile organic compounds in polluted air interact with photons and break down in a process called photolysis. The presence of excess ozone remains a great threat to various plants and other organisms, as it can cause oxidative damage when it reacts with molecules in living tissues.
Current research evidence shows, however, that many plants have a way to combat ozone’s toxicity using an antioxidant compound known as ascorbate (ASC), or vitamin C. ASC molecules are found within plant apoplasts, the internal aqueous space of the cell wall. Research suggests that when ozone passes into the cell wall on the leaf, some of it oxidizes ASC. This reaction produces non-toxic products that can then be managed by the plant cell. This way, ASC in the cell wall is part of a first line of defense against ozone, before the ozone can oxidize and damage sensitive molecules in the plant cell’s underlying plasma membrane. Experimental studies have found that higher concentrations of ASC are associated with higher resistance to oxidative stress.Edit Summary
“Since many of the products of the interaction between O3 and constituents of the apoplast are ostensibly harmless, it is feasible that the sacrificial oxidation of certain compounds may serve as an important first line of defense against O3.” (Lyons et al. 1999:186)
“Although the pool of ASC located in the apoplast represents only a small fraction (~1%) of that in the bulk leaf, there is evidence to suggest that the size and rate of turnover of this pool may be sufficient to afford a significant degree of protection against O3 and/or its toxic reaction products. Ascorbate is readily oxidized by O3, and several other ROS [reactive oxygen species]…” (Lyons et al. 1999:187)
“The phytotoxicity of O3 arises primarily as a result of the oxidative damage it causes to the plasmalemma (Heath 1980, 1987, 1988). The pollutant is taken up into the leaf interior, via the stomates (Kerstiens and Lendzian 1989), where it is believed to react with constituents of the aqueous matrix associated with the cell wall (i.e. the apoplast) to yield a suite of reactive oxygen species (ROS) which, in addition to ozone, result in the oxidation of sensitive components of the plasma-lemma, and subsequently the cytosol (Heath 1980; Chameides 1989; Moldau 1998). In this sense, the oxidative stress induced by ozone shares similarities with the initial events associated with other plant pathologies…Since the plasmalemma is the principle site of oxidative attack, the interception of O3, and/or its reactive products, by constituents of the apoplast may play a crucial role in averting cellular damage (Heath 1988). Thus, considerable attention has recently focused on antioxidative systems in the cell wall, and in particular, the role of apoplastic ascorbate (ASC) in the scavenging of O3 (Kelly et al. 1995; Dietz 1997; Cross et al. 1998; Lyons et al. 1999a).” (Plöchl et al. 2000:454)