Air Pollution, Photosynthesis and Forest Decline: Interactions and Consequences
283
Mehler reaction at rates exceeding 3011mol(mg chlorophyll)-lh- 1 , i.e., far
more than 150 times the rate of S02 or ozone influx into leaves, when the
external pollutant concentration is 100ppb. The H20 2 produced by disproportion at ion of the oxygen radicals is destroyed by chloroplastic ascorbate
peroxidase. Ascorbate is oxidized when H20 2 is reduced. It is regenerated
from the resultant monodehydroascorbate and de hydro ascorbate by specific
reductases. Electron donors for the regeneration of ascorbate are glutathione
and NADPH. Oxidized glutathione is itself reduced by NADPH.
In spite of the potentially high rate of oxidant formation in photosynthesis,
the activities of enzymes and the concentrations of substrates involved
in detoxification of the reactive oxygen species are so high that oxidant
production in the chloroplasts in the light does not interfere with photosynthesis although enzymes of the chloroplast stroma are highly sensitive to
oxidants (Kaiser 1976). In this situation, oxidants derived from incoming air
pollutants will be effectively reduced together with endogenously produced
oxidants as long as pollutant concentrations are not excessive.
As in the chloroplasts, ascorbate and glutathione act as antioxidants and
radical scavengers also in other cytoplasmic compartments. Catalase, which
destroys H 2 0 2 , may be concentrated in the peroxisomes to an extent that
crystallization occurs. In the vacuoles, peroxidases use H20 2 as an electron
acceptor to oxidize phenols and other substrates (Takahama 1993).
Ascorbate is also a vacuolar solute. In biomembranes, tocopherol is an
effective radical scavenger and antioxidant (Elstner 1990).
It appears that the primary function of these and other antioxidative
systems consists in the detoxification of radicals and oxidants produced
during the normal life span of a cell by respiration, photosynthesis, and
by other cellular activities. Nevertheless, they also help to cope with the
oxidative stresses produced by anthropogenic air pollutants.
14.5.1 The Path of Air Pollutants
The first cellular barrier that air pollutants have to overcome once they have
passed through the stomata is the aqueous phase of the apoplasm, which
includes the cell wall. The apoplasm contains ascorbate at a concentration
approximately one-tenth or less of the cytoplasmic concentration (Luwe
et al. 1993). Antioxidants of the apoplasm include phenolic substances.
Apoplasmic peroxidases oxidize phenolics using hydrogen peroxide as electron acceptor (Pfanz et al. 1990; Takahama et al. 1992). Phenolic radicals
produced by the peroxidase reaction are either intermediates of lignin
formation or are reduced by ascorbate. Air pollutants not intercepted in the
apoplasm have to cross the plasmalemma before they can interact with
cytoplasmic constituents. While it is very unlikely that ozone will ever reach
the central vacuole of fully differentiated leaf cells owing to its extremely
high reactivity (Urbach et al. 1989; Lange et al. 1989a), the vacuole is a site
283
Mehler reaction at rates exceeding 3011mol(mg chlorophyll)-lh- 1 , i.e., far
more than 150 times the rate of S02 or ozone influx into leaves, when the
external pollutant concentration is 100ppb. The H20 2 produced by disproportion at ion of the oxygen radicals is destroyed by chloroplastic ascorbate
peroxidase. Ascorbate is oxidized when H20 2 is reduced. It is regenerated
from the resultant monodehydroascorbate and de hydro ascorbate by specific
reductases. Electron donors for the regeneration of ascorbate are glutathione
and NADPH. Oxidized glutathione is itself reduced by NADPH.
In spite of the potentially high rate of oxidant formation in photosynthesis,
the activities of enzymes and the concentrations of substrates involved
in detoxification of the reactive oxygen species are so high that oxidant
production in the chloroplasts in the light does not interfere with photosynthesis although enzymes of the chloroplast stroma are highly sensitive to
oxidants (Kaiser 1976). In this situation, oxidants derived from incoming air
pollutants will be effectively reduced together with endogenously produced
oxidants as long as pollutant concentrations are not excessive.
As in the chloroplasts, ascorbate and glutathione act as antioxidants and
radical scavengers also in other cytoplasmic compartments. Catalase, which
destroys H 2 0 2 , may be concentrated in the peroxisomes to an extent that
crystallization occurs. In the vacuoles, peroxidases use H20 2 as an electron
acceptor to oxidize phenols and other substrates (Takahama 1993).
Ascorbate is also a vacuolar solute. In biomembranes, tocopherol is an
effective radical scavenger and antioxidant (Elstner 1990).
It appears that the primary function of these and other antioxidative
systems consists in the detoxification of radicals and oxidants produced
during the normal life span of a cell by respiration, photosynthesis, and
by other cellular activities. Nevertheless, they also help to cope with the
oxidative stresses produced by anthropogenic air pollutants.
14.5.1 The Path of Air Pollutants
The first cellular barrier that air pollutants have to overcome once they have
passed through the stomata is the aqueous phase of the apoplasm, which
includes the cell wall. The apoplasm contains ascorbate at a concentration
approximately one-tenth or less of the cytoplasmic concentration (Luwe
et al. 1993). Antioxidants of the apoplasm include phenolic substances.
Apoplasmic peroxidases oxidize phenolics using hydrogen peroxide as electron acceptor (Pfanz et al. 1990; Takahama et al. 1992). Phenolic radicals
produced by the peroxidase reaction are either intermediates of lignin
formation or are reduced by ascorbate. Air pollutants not intercepted in the
apoplasm have to cross the plasmalemma before they can interact with
cytoplasmic constituents. While it is very unlikely that ozone will ever reach
the central vacuole of fully differentiated leaf cells owing to its extremely
high reactivity (Urbach et al. 1989; Lange et al. 1989a), the vacuole is a site