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U. Heber et al.
acidification of needles was observed only in exposure chambers during
fumigation of young spruce trees for several weeks with concentrations of
sulfur dioxide which were much higher than those that spruce has to endure
in areas exposed to polluted air (Kaiser et al. 1991, 1993a).
14.5.5 Acid-Dependent Cation Requirements
There are several mechanisms by which a cell can cope with imported acid
which cannot be degraded (for a detailed discussion, see Slovik et al.
1992a,b):
1. A high proton buffering capacity might prevent a deleterious decrease in
pH (Pfanz and Heber 1986). However, buffering does not prevent acidification. It only decreases it. For cells to remain viable during prolonged
influx of S02, cytoplasmic pH values must be maintained under strict
control.
2. Degradation of endogenous organic anions and reduction of nitrate yields
hydroxyl ions which can neutralize airborne sulfuric acid (Heber et al.
1987). Such reactions are the chemical basis of cellular pH-stat mechanisms (Raven 1986). However, it appears that the chemical pH-stat is
overtaxed in the long term, when sulfate accumulates to high levels in
cells.
3. Airborne acid may be sequestered intracellularly, e.g., into vacuoles.
4. Airborne acid is exported from the cells or bases are imported.
All of these possibilities occur. Of particular importance is sequestration
and/or export when acid stress is chronic. The tonoplast membrane which
separates the cytosol from the vacuole contains two proton-translocating
enzymes, an ATPase, and a pyrophosphatase in addition to anion transporters (Walker and Leigh 1981; Sze 1985). Sulfate is transported into the
vacuoles of leaf cells, where it is sequestered (Kaiser et al. 1989). This
transport is energy-dependent. In principle, not only sulfate but also the
protons of sulfuric acid could be stored inside the vacuoles. However, it
seems that 'proton storage in the vacuoles as an effective means of cytoplasmic pH regulation can be observed only temporarily under acute acid
stress (Heber et al. 1987). If it were effective in the long term, homogenates
of needles from polluted areas with high sulfate contents should be more
acidic than homogenates of needles from unpolluted areas. This is not
the case (Pfanz and Beyschlag 1993; d. Slovik et al. 1992a,b). Also, the
vacuolar acidification observed when leaves are fumigated with S02 is
reversed after termination of fumigation (Heber et al. 1989a). There is the
question of how vacuolar sulfuric acid is neutralized. Obviously, bases can
be mobilized, but there is a limit to the extent of base mobilization in leaves.
However, not only the tonoplast, but also the plasmalemma possesses a
proton-translocating ATPase. Its function is to power energy-dependent ion
uptake into the cells. After S02 has entered cells and has been detoxified
U. Heber et al.
acidification of needles was observed only in exposure chambers during
fumigation of young spruce trees for several weeks with concentrations of
sulfur dioxide which were much higher than those that spruce has to endure
in areas exposed to polluted air (Kaiser et al. 1991, 1993a).
14.5.5 Acid-Dependent Cation Requirements
There are several mechanisms by which a cell can cope with imported acid
which cannot be degraded (for a detailed discussion, see Slovik et al.
1992a,b):
1. A high proton buffering capacity might prevent a deleterious decrease in
pH (Pfanz and Heber 1986). However, buffering does not prevent acidification. It only decreases it. For cells to remain viable during prolonged
influx of S02, cytoplasmic pH values must be maintained under strict
control.
2. Degradation of endogenous organic anions and reduction of nitrate yields
hydroxyl ions which can neutralize airborne sulfuric acid (Heber et al.
1987). Such reactions are the chemical basis of cellular pH-stat mechanisms (Raven 1986). However, it appears that the chemical pH-stat is
overtaxed in the long term, when sulfate accumulates to high levels in
cells.
3. Airborne acid may be sequestered intracellularly, e.g., into vacuoles.
4. Airborne acid is exported from the cells or bases are imported.
All of these possibilities occur. Of particular importance is sequestration
and/or export when acid stress is chronic. The tonoplast membrane which
separates the cytosol from the vacuole contains two proton-translocating
enzymes, an ATPase, and a pyrophosphatase in addition to anion transporters (Walker and Leigh 1981; Sze 1985). Sulfate is transported into the
vacuoles of leaf cells, where it is sequestered (Kaiser et al. 1989). This
transport is energy-dependent. In principle, not only sulfate but also the
protons of sulfuric acid could be stored inside the vacuoles. However, it
seems that 'proton storage in the vacuoles as an effective means of cytoplasmic pH regulation can be observed only temporarily under acute acid
stress (Heber et al. 1987). If it were effective in the long term, homogenates
of needles from polluted areas with high sulfate contents should be more
acidic than homogenates of needles from unpolluted areas. This is not
the case (Pfanz and Beyschlag 1993; d. Slovik et al. 1992a,b). Also, the
vacuolar acidification observed when leaves are fumigated with S02 is
reversed after termination of fumigation (Heber et al. 1989a). There is the
question of how vacuolar sulfuric acid is neutralized. Obviously, bases can
be mobilized, but there is a limit to the extent of base mobilization in leaves.
However, not only the tonoplast, but also the plasmalemma possesses a
proton-translocating ATPase. Its function is to power energy-dependent ion
uptake into the cells. After S02 has entered cells and has been detoxified