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U. Heber et al.
at high concentrations, there is good reason to doubt that photosynthesis of
leaves at high concentrations, there is good reason to doubt that photosynthesis is a primary target at ambient concentrations. For this reason, our
discussion does not focus on the direct effects of air pollutants on photosynthesis which can often be observed only at concentrations far above
ambient concentrations. Rather, it considers the role of chloroplasts in
detoxification of air pollutants within a broader context.
14.2 Sites of Interaction of Air Pollutants with Plants
Leaves are highly effective gas exchange systems when stomata are open.
When they are closed, gas exchange is severely curtailed. Epidermal tissues
are protected, to different degrees, by thick cell walls incrusted with cutinlike substances or suberin, or by a cuticle which may be covered by waxes.
These layers act to slow diffusion and to intercept reactive agents, preventing
them from reaching living tissues at concentrations which are toxic (Lendzian
1984; Lendzian and Kerstiens 1988, 1991). In consequence, all gaseous air
pollutants act mainly on internal tissues in direct contact with the intercellular
air space of leaves and needles.
Since the leaf area index in forests, i.e., the ratio of total projected leaf
area to the underlying ground area, is usually high, interaction of gaseous
air pollutants with the soil is negligible compared with interactions in the
canopy. With rain or fog, air pollutants dissolve in droplets according to
Henry's distribution law, thereby effectively cleaning the air. This alters the
site of impact on plants. S02 and N02 are hydrated in water to form sulfurous, nitrous, and nitric acid. Ozone decomposes in water, finally yielding
H20 2 and H20. Sulfurous and nitrous acid are oxidized, producing sulfuric
and nitric acid. Since the surface tension of water revents aqueous solutions
from entering leaves through stomata (Ziegler 1988), rain water, after wetting
those plant surfaces which are not water-repellent (thereby exchanging
protons of the acids with available cations; d. Mitterhuber et al. 1989)
reaches the soil, where acids interact with soil particles which may be
considered as complex cation and anion exchangers. According to the
Hofmeister lyotropic power series, which defines relative exchange strength
and groups cations in this order
AI3+ > H+ > Ca 2 + > Mg2+ > K+ ~ NH4+ > Na+,
protons of the acids exchange with cations of the soil piut.icles. Liberated
cations are, together with their counter anions, primarily nitrate, chloride,
and sulfate, subject to leaching, which depletes the soil of essential nutrients
(Ulrich 1980). It is doubtful whether direct effects of acidification on root
growth are generally important (Kreutzer and Gottlein 1991), but, as will be
shown, decreased cation availability is an important factor in the toxicity of
gaseous air pollutants which enter plants via leaves or· needles.
U. Heber et al.
at high concentrations, there is good reason to doubt that photosynthesis of
leaves at high concentrations, there is good reason to doubt that photosynthesis is a primary target at ambient concentrations. For this reason, our
discussion does not focus on the direct effects of air pollutants on photosynthesis which can often be observed only at concentrations far above
ambient concentrations. Rather, it considers the role of chloroplasts in
detoxification of air pollutants within a broader context.
14.2 Sites of Interaction of Air Pollutants with Plants
Leaves are highly effective gas exchange systems when stomata are open.
When they are closed, gas exchange is severely curtailed. Epidermal tissues
are protected, to different degrees, by thick cell walls incrusted with cutinlike substances or suberin, or by a cuticle which may be covered by waxes.
These layers act to slow diffusion and to intercept reactive agents, preventing
them from reaching living tissues at concentrations which are toxic (Lendzian
1984; Lendzian and Kerstiens 1988, 1991). In consequence, all gaseous air
pollutants act mainly on internal tissues in direct contact with the intercellular
air space of leaves and needles.
Since the leaf area index in forests, i.e., the ratio of total projected leaf
area to the underlying ground area, is usually high, interaction of gaseous
air pollutants with the soil is negligible compared with interactions in the
canopy. With rain or fog, air pollutants dissolve in droplets according to
Henry's distribution law, thereby effectively cleaning the air. This alters the
site of impact on plants. S02 and N02 are hydrated in water to form sulfurous, nitrous, and nitric acid. Ozone decomposes in water, finally yielding
H20 2 and H20. Sulfurous and nitrous acid are oxidized, producing sulfuric
and nitric acid. Since the surface tension of water revents aqueous solutions
from entering leaves through stomata (Ziegler 1988), rain water, after wetting
those plant surfaces which are not water-repellent (thereby exchanging
protons of the acids with available cations; d. Mitterhuber et al. 1989)
reaches the soil, where acids interact with soil particles which may be
considered as complex cation and anion exchangers. According to the
Hofmeister lyotropic power series, which defines relative exchange strength
and groups cations in this order
AI3+ > H+ > Ca 2 + > Mg2+ > K+ ~ NH4+ > Na+,
protons of the acids exchange with cations of the soil piut.icles. Liberated
cations are, together with their counter anions, primarily nitrate, chloride,
and sulfate, subject to leaching, which depletes the soil of essential nutrients
(Ulrich 1980). It is doubtful whether direct effects of acidification on root
growth are generally important (Kreutzer and Gottlein 1991), but, as will be
shown, decreased cation availability is an important factor in the toxicity of
gaseous air pollutants which enter plants via leaves or· needles.