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

Impact of Ozone on Crops 191
observed in spring and summer and under these conditions (high irradiance, high
temperature, low wind speed) boundary layer and stomatal resistances are high
and, thus, limit O 3 uptake (Musselman et al., 1994).
Only the O 3 that are absorbed inside the plant directly affect metabolism. Analysis
of O 3 uptake by plants is the key to understanding the phytotoxicity impact of O 3
on physiological and growth processes in plants. The dose-response curves are
plots of physiological response against O 3 absorption. When O 3 uptake (Mansfield
and Free-Smith, 1984; Reich, 1987; Winner et al., 1991) was used as the basis for
comparison, responses in all plant species became comparable despite the differences
in antioxidant status. This finding suggests a common biochemical mechanism
for O 3 phytotoxicity.
Since O 3 penetrates the leaves via stomata, the first target of O 3 or its reactive
species are the plasma membrane (Heath, 1980; Heath, 1987; Heath, 1994b; Heath
and Taylor, 1997) of the mesophyll cells, which are severely damaged (changes in
permeability, fluidity, and ionic and metabolic disturbances) if the detoxifying system
of the apoplast is overcharged.
4. FATE OF OZONE
The fate of O 3 upon entry into the leaf is not well known. In order to study the
reactions that may occur within the leaf it is necessary to know the chemical reactions
of O 3 in vitro. However, a chemical reaction that participates in aqueous
solutions does not necessarily take place in living organisms. It has been reported
that, in aqueous solutions, O 3 decomposes to form reactive oxygen species (ROS),
including OH· and O 2 · – radicals (Grimes et al., 1983; Byvoet et al., 1995) but the
question remains whether production rates are important in the leaves (Runeckles,
1992; Heath and Taylor, 1997). In addition, O 3 reacts with various solutes of the
apoplast fluid, at reaction rate orders of a greater magnitude than with water (Lyons
et al., 1999).
It has been reported that O 3 concentrations in intracellular spaces are close to zero
(Laisk et al., 1989). This means that O 3 is absorbed and rapidly decomposed in
the cell walls and plasmalemma (Figure 1). It does not penetrate into deeper
layers of cells. O 3 decomposition at the cell wall and plasma membrane indicates
detoxification, but the O 3-generated reactive oxygen species can be implicated
in phytotoxic responses. Toxic oxyradicals such as superoxide anion (O 2 · – ) have
been suggested as being responsible for injury caused by O 3 (Alscher and Hess,
1993). The evidence for the role of O 2 · – in O 3 phytotoxicity is indirect and
controversial. Increased SOD activity in bean as being a result of O 3 exposure
has been reported (Lee and Bennett, 1982). Electron paramagnetic resonance
spectroscopy can be used to observe the spectra of free radicals directly. Free
radicals measurements have been attempted by Mhelhorn et al. (1990), although
identification of the specific radicals has not been provided. Direct observations
of radical signals have revealed the appearance of a signal with the characteristics
of the isotropic superoxide anion signal during exposure to low levels of O 3
(Runeckles and Vaarnou, 1997). Thus, initial reaction of O 3 could result in the