282
U. Heber et al.
constituents (Elstner 1990). For instance, S02 reacts with water to form
sulfurous acid. Since acidification must be avoided, bases need to be produced
for neutralization. Anions formed slightly above neutrality are HS0 3 -
(bisulfite) and sol- (sulfite). In the presence of oxygen radicals, which are
normal products of photosynthetic oxygen reduction in the Mehler reaction,
an electron may be stripped of sulfite. This results in the highly reactive
sulfur trioxide anion radical which, together with oxygen radicals, initiates a
radical chain reaction which is capable of oxidizing sulfite to sulfate at
extremely high rates (Asada and Takahashi 1987). Within the chain reaction,
and actually terminating those reaction chains, cellular materials are oxidized.
Other reactions of sulfite, for instance the reaction with carbonyl and
disulfide groups, lead to the formation of addition compounds. Sulfitolysis of
oxidized thioredoxin interferes with regulation and activity of Calvin cycle
enzymes (Wiirfel et al. 1990). There is abundant literature on other S02
effects (e.g., Ziegler 1975; Hiillgren 1978; Rennenberg 1984).
N02 has by itself a radical structure. Together with the nitrite which is
formed during its hydration, it is an oxidant (Elstner 1984). However, much
stronger oxidants, actually some of the strongest oxidants known, are ozone
and the hydroxyl radical which is formed when ozone reacts with water.
They attack oxidizable cell constituents. In a specific reaction, ozone splits
the double bonds of fatty acid residues in biomembranes forming ozonides
which are then subject to degradation. All these reactions are potentially
damaging. There is growing evidence that ambient levels of ozone decrease
productivity in tree and crop species (Reich and Amundson 1985; Matyssek
et al. 1992).
14.5 Detoxification
To be effective, detoxifying reactions must intercept reactive air pollutants
or their toxic products before cell damage can occur. Cells possess mechanisms to scavenge radicals and to cope with oxidative stress. Ascorbate and
glutathione are not only important radical scavengers, they are also easily
oxidized by less aggressive oxidants. In contrast to glutathione, ascorbate is
not only localized in the cytoplasm and the vacuole of cells, but, at lower
concentrations than in the cytoplasm, also outside the plasmalemma in the
apoplasm (Polle et al. 1991; Takahama et al. 1993; Luwe et al. 1993).
Apoplasmic phenolics also act to intercept ozone or hydroxyl radicals, or to
reduce hydrogen peroxide, which can be formed from ozone (Takahama et
al. 1993). Particularly well protected against oxidative stress are chloroplasts
(Halliwell and Foyer 1978; Halliwell 1978; Asada and Takahashi 1987).
They contain superoxide dis mutase (SOD) for the effective detoxification of
oxygen radicals which are formed by isolated thylakoid membranes in the
U. Heber et al.
constituents (Elstner 1990). For instance, S02 reacts with water to form
sulfurous acid. Since acidification must be avoided, bases need to be produced
for neutralization. Anions formed slightly above neutrality are HS0 3 -
(bisulfite) and sol- (sulfite). In the presence of oxygen radicals, which are
normal products of photosynthetic oxygen reduction in the Mehler reaction,
an electron may be stripped of sulfite. This results in the highly reactive
sulfur trioxide anion radical which, together with oxygen radicals, initiates a
radical chain reaction which is capable of oxidizing sulfite to sulfate at
extremely high rates (Asada and Takahashi 1987). Within the chain reaction,
and actually terminating those reaction chains, cellular materials are oxidized.
Other reactions of sulfite, for instance the reaction with carbonyl and
disulfide groups, lead to the formation of addition compounds. Sulfitolysis of
oxidized thioredoxin interferes with regulation and activity of Calvin cycle
enzymes (Wiirfel et al. 1990). There is abundant literature on other S02
effects (e.g., Ziegler 1975; Hiillgren 1978; Rennenberg 1984).
N02 has by itself a radical structure. Together with the nitrite which is
formed during its hydration, it is an oxidant (Elstner 1984). However, much
stronger oxidants, actually some of the strongest oxidants known, are ozone
and the hydroxyl radical which is formed when ozone reacts with water.
They attack oxidizable cell constituents. In a specific reaction, ozone splits
the double bonds of fatty acid residues in biomembranes forming ozonides
which are then subject to degradation. All these reactions are potentially
damaging. There is growing evidence that ambient levels of ozone decrease
productivity in tree and crop species (Reich and Amundson 1985; Matyssek
et al. 1992).
14.5 Detoxification
To be effective, detoxifying reactions must intercept reactive air pollutants
or their toxic products before cell damage can occur. Cells possess mechanisms to scavenge radicals and to cope with oxidative stress. Ascorbate and
glutathione are not only important radical scavengers, they are also easily
oxidized by less aggressive oxidants. In contrast to glutathione, ascorbate is
not only localized in the cytoplasm and the vacuole of cells, but, at lower
concentrations than in the cytoplasm, also outside the plasmalemma in the
apoplasm (Polle et al. 1991; Takahama et al. 1993; Luwe et al. 1993).
Apoplasmic phenolics also act to intercept ozone or hydroxyl radicals, or to
reduce hydrogen peroxide, which can be formed from ozone (Takahama et
al. 1993). Particularly well protected against oxidative stress are chloroplasts
(Halliwell and Foyer 1978; Halliwell 1978; Asada and Takahashi 1987).
They contain superoxide dis mutase (SOD) for the effective detoxification of
oxygen radicals which are formed by isolated thylakoid membranes in the