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

192 S. del Valle-Tascon and J. L. Carrasco-Rodriguez
Figure 1. The postulated mechanism of ozone phitotoxicity within cell wall and cytosol and the enzymes
that can react with the O 3-generated reactive oxygen species (after Mhelhorn, 1990; Alscher and Hess,
1993).
production of active oxygen species (Grimes et al., 1983; Pryor and Church, 1991;
Byvoet et al., 1995).
5. OXIDATIVE STRESS
During normal metabolism of plant cells a variety of activated oxygen species are
formed (Rubinstein and Luster, 1993; Asada, 1994; Fridovich, 1995). Oxygen
toxicity is due primarily to activated oxygen species rather than to molecular oxygen
itself (Scandalios, 1994). Oxidative stress is caused by exposition to reactive oxygen
intermediates, such as superoxide anion (O 2 · – ), hydrogen hydroxyl radicals (OH·)
and hydrogen peroxide (H 2O 2). Oxidative stress is an unavoidable by-product of
aerobic life style, because activated oxygen species are formed whenever molecular
oxygen oxidizes electron carriers chemically.
ROS are important metabolites, participating in the metabolism, growth and development
of plant cells (Alscher and Hess, 1993). ROS production is stimulated by
environmental stresses such as exposure to high temperature, heavy metals, herbicides,
extremes of temperature, UV radiation, and air pollutants including O 3, and
they are produced in response to invasion by various pathogens (Alscher and Hess,
1993; Foyer and Molineaux, 1994; Dangl et al., 1996; Hammond-Kosack and Jones,