Air Pollution, Photosynthesis and Forest Decline: Interactions and Consequences
289
by oxidation to sulfuric acid, protons need to be exported. The ATPase
functions in this export. On the other hand, cation import is required to
satisfy the need for counterions of sulfate. Proton export solves the problem
of cellular acidification at the level of the leaf, but constitutes a burden for
other tissues. It functions on the basis of proton/cation exchange. A final
solution to the problem of cellular or tissue acidification is proton export
into the soil at the root/soil interface (Kaupenjohann et al. 1988; Thomas
and Runge 1992; Kaiser et al. 1993a). This requires proton transport from
the leaves to the roots. In exchange for exported protons, cations must be
imported. They are taken up from the soil and transported from the roots
into leaves via the transpiration stream. In this way, protons produced as a
consequence of hydration of S02 in the leaves finally reach the soil in
exchange for cations which are needed as counterions of sulfate (Slovik et
al. 1992a,b; Kaiser et al. 1993a,b). Roots have long been known to excrete
protons. ATPase-mediated proton export is required for energy-dependent
ion uptake by roots. This import is necessary to satisfy the requirements of
growth for nutrient ions.
In the presence of S02 in the atmosphere, cations are needed not only for
growth, but also for the neutralization of the sulfuric acid which is formed in
the leaves. The mechanisms of cellular pH stabilization compete successfully
with growth for cations. In consequence, growth is retarded. It has long
been known that growth is reduced in areas where the atmosphere is polluted
by S02. Moreover, so-called novel forest decline ("neuartige Waldschaden")
is often characterized by symptoms of cation deficiency. Different nutrient
cations (e.g., K +, Ca 2 +, Mg2+) may be immobilized in vacuoles as counterions of airborne sulfate at different ratios depending on cation availability
in the soil. The nutrient cation which is closest to the minimum supply in the
soil will be most sensitive to vacuolar immobilization. Particularly well
known is the expression of magnesium deficiency in Norway spruce by the
extensive yellowing of needles (Lange et al. 1989a; Schulze et al. 1989). On
soils poor in magnesium, S02 will enhance or promote magnesium deficiency. This situation is particularly likely to arise on soils exposed to acid
leaching. It has been mentioned above that potentially acidic air pollutants
are washed out of the atmosphere when it rains. In acid rain, protons
exchange for nutrient cations of the soil according to the Hofmeister lyotropic power series. This decreases cation availability for roots. It is often
not recognized that cation deficiency may be a consequence of acid stress
which is exerted both above and below the soil surface, i.e., in leaves and at
the level of the root system (Slovik et al. 1992a,b).
14.5.6 Interactions Between Different Air Pollutants
Perhaps it is not surprising that in fumigation experiments with elevated
concentrations of different air pollutants simultaneously, more damage is
produced than is caused by the individual components alone. However, the
289
by oxidation to sulfuric acid, protons need to be exported. The ATPase
functions in this export. On the other hand, cation import is required to
satisfy the need for counterions of sulfate. Proton export solves the problem
of cellular acidification at the level of the leaf, but constitutes a burden for
other tissues. It functions on the basis of proton/cation exchange. A final
solution to the problem of cellular or tissue acidification is proton export
into the soil at the root/soil interface (Kaupenjohann et al. 1988; Thomas
and Runge 1992; Kaiser et al. 1993a). This requires proton transport from
the leaves to the roots. In exchange for exported protons, cations must be
imported. They are taken up from the soil and transported from the roots
into leaves via the transpiration stream. In this way, protons produced as a
consequence of hydration of S02 in the leaves finally reach the soil in
exchange for cations which are needed as counterions of sulfate (Slovik et
al. 1992a,b; Kaiser et al. 1993a,b). Roots have long been known to excrete
protons. ATPase-mediated proton export is required for energy-dependent
ion uptake by roots. This import is necessary to satisfy the requirements of
growth for nutrient ions.
In the presence of S02 in the atmosphere, cations are needed not only for
growth, but also for the neutralization of the sulfuric acid which is formed in
the leaves. The mechanisms of cellular pH stabilization compete successfully
with growth for cations. In consequence, growth is retarded. It has long
been known that growth is reduced in areas where the atmosphere is polluted
by S02. Moreover, so-called novel forest decline ("neuartige Waldschaden")
is often characterized by symptoms of cation deficiency. Different nutrient
cations (e.g., K +, Ca 2 +, Mg2+) may be immobilized in vacuoles as counterions of airborne sulfate at different ratios depending on cation availability
in the soil. The nutrient cation which is closest to the minimum supply in the
soil will be most sensitive to vacuolar immobilization. Particularly well
known is the expression of magnesium deficiency in Norway spruce by the
extensive yellowing of needles (Lange et al. 1989a; Schulze et al. 1989). On
soils poor in magnesium, S02 will enhance or promote magnesium deficiency. This situation is particularly likely to arise on soils exposed to acid
leaching. It has been mentioned above that potentially acidic air pollutants
are washed out of the atmosphere when it rains. In acid rain, protons
exchange for nutrient cations of the soil according to the Hofmeister lyotropic power series. This decreases cation availability for roots. It is often
not recognized that cation deficiency may be a consequence of acid stress
which is exerted both above and below the soil surface, i.e., in leaves and at
the level of the root system (Slovik et al. 1992a,b).
14.5.6 Interactions Between Different Air Pollutants
Perhaps it is not surprising that in fumigation experiments with elevated
concentrations of different air pollutants simultaneously, more damage is
produced than is caused by the individual components alone. However, the