Production Practices and Quality Assessment of Food Crops. Vol. 1

194 S. del Valle-Tascon and J. L. Carrasco-Rodriguez
radicals account for the phytotoxicity since they react rapidly with proteins, lipids,
and DNA causing rapid cell damage. Damage of leaves due to the air pollutant
O 3 is also mediated through the production of ROS (Kangasjärvi et al., 1994; Foyer
and Mollineaux, 1994; Foyer et al., 1995). Extracellular space is actually considered
the site of action of O 3.
7. DEFENSES AGAINST THE OXIDATIVE STRESS
Since most ROS are highly reactive and lead to perturbation in enzyme activities
and membrane damage, they are not compatible with cell function and their
generation is frequently considered to be deleterious and harmful. To protect against
damage caused by oxidative stress, cells possess a variety of antioxidant systems.
Cell antioxidants include non-enzymatics (ascorbic acid, carotenoid pigments,
glutathione and α-tocopherol) and enzymes (glutathione reductase, superoxide
dismutase, ascorbate peroxidase, and catalases).
To limit plasma membrane damage and subsequent injury responses, O 3 and associated
oxygen intermediates formed during O 3 decomposition must be neutralized
in apoplast space (Figure 1).
The spontaneous disproportionation of superoxide radicals is dependent on pH
values and the maximum rate is observed at the pH of 4.8 (Asada, 1994b). At
high pH values the spontaneous reaction becomes low.
Superoxide dismutases (SOD) are enzymes that eliminate superoxide radicals:
2 O 2 · – + 2 H + → H 2O 2 + O 2
Three types of SOD have been found which differ in their prosthetic metals (Inzé
and Montagu, 1995).
Apoplast contains peroxidases and SOD activities (Castillo et al., 1986; Rainieri
et al., 1996; Lyons et al., 1999) (Figure 2). The importance of these enzymes in
scavenging O 3 reaction products is at the present moment unclear.
Ascorbic acid is the most abundant antioxidant in leaf apoplast space (Horemans
et al., 2000). Ascorbic acid in the apoplast could minimize ozone injury by two types
of reactions. Direct ozonolysis between ascorbic acid and O 3 has been postulated
(Chaimenides, 1989), although a recent report suggests that this reaction is not
the major pathway (Jakob and Heber, 1998). Ascorbic acid is also involved in the
enzymatic deactivation of peroxides generated during O 3 decomposition (Castillo
and Greppin, 1988). Ascorbic acid is oxidized by H 2O 2 to monodehydroascorbate
(MDHA) radical, which disproportionates into AA. The role of AA in protecting
plants against oxidative stress is based on the positive relationship between leaf
AA and O 3 resistance (Menser, 1964; Lee et al., 1984) and the hypersensibility of
a mutant of Arabidopsis thaliana defective in AA biosynthesis (Conklin et al., 1997).
Monodehydroascorbate oxidase activity has been described in apoplast (Polle et
al., 1990; Luwe et al., 1993), which reduced DHA to AA. However, contradictory
data have been observed regarding the physiological significance of this enzyme.
In the ascorbate-glutathione (Halliwell and Asada) cycle (Halliwell and