Air Pollution, Photosynthesis and Forest Decline: Interactions and Consequences
287
sulfate which is observed in the presence of isolated thylakoids. Actually,
reductive and oxidative detoxification of S02 have a comparable maximum
capacity in spinach chloroplasts (Dittrich et al. 1992). At 20De, it is about
2 ~mol sulfite (mg chlorophyll)~1 h~1 which is either reduced or oxidized.
This must be compared to a S02 flux into leaves between 20 and 100 nmol
(mg chlorophyll)~1 h~\ when the S02 concentration in air is about 50ppb
and the sum of boundary layer and stomatal resistances is between 2 and
10 s cm ~ 1. Apparently, detoxification capacities of leaves for S02 are high,
although they do not approach the detoxification capacity for N02. However,
reductive detoxification of S02 appears to be effective only when sinks are
available for reduced sulfur. Proteins constitute a main sink. In trees, net
protein synthesis is a seasonal affair. During most of the year, protein
synthesis and protein degradation match each other and there is, in the
absence of growth, no additional demand for reduced sulfur. It is therefore
not surprising that in Norway spruce sulfate accumulation accounts for
practically all S02 which has been taken up (Kaiser et al. 1991). Herbaceous
plants have a much higher capacity for the reductive detoxification of S02
than woody plants. Grasses thrive where spruce has been killed by overexposure to S02 (Heber et al. 1987).
Very high detoxification capacities are actually required for a successful
competition with toxic reactions. Even very brief exposure of leaves to
5 ppm S02 produces considerable inhibition of photosynthesis. This concentration is more than about 25 times the peak concentrations of S02
observed in the field during inversion periods in the winter. If fumigation
with 5 ppm S02 is discontinued after little more than 5 min, recovery of
photosynthesis may be observed within 30 min or more, indicating the presence of repair mechanisms (Veljovic-Jovanovic et al. 1993). Effective repair
requires light, ambient air levels of oxygen, and summer temperatures. Very
little is known about acute toxicity and detoxification of S02 at winter
temperatures.
In contrast to reductive detoxification, oxidative detoxification, which
leads to sulfuric acid, burdens the organism with protons. Short-term pH
regulation is highly effective at 20 DC, but slower at 16 DC (Heber et al.
1989b). Normally, its capacity is far higher than needed to deal with the
acidification caused by S02 influx. However, problems arise in the long
term. In needles of spruce from the Ore Mountains (Erzgebirge), average
sulfate concentrations have been measured which approach or even exceed
100mM (Kaiser et al. 1991; Kaiser et al. 1993b; Pfanz and Beyschlag 1991).
Accordingly, 200 mEq I ~ 1 H+ would have burdened the cells, if all of this
sulfate had been airborne. In fact, comparisons with sulfate contents in
needles from unpolluted areas show that most of the sulfate must have been
derived from S02. Nevertheless, the pH of homogenates of needles from
spruce grown in polluted and unpolluted areas proved to be almost identical.
Apparently, needles usually manage to neutralize the sulfuric acid which
slowly accumulates during long-term exposure to SOz-polluted air. Persisting
287
sulfate which is observed in the presence of isolated thylakoids. Actually,
reductive and oxidative detoxification of S02 have a comparable maximum
capacity in spinach chloroplasts (Dittrich et al. 1992). At 20De, it is about
2 ~mol sulfite (mg chlorophyll)~1 h~1 which is either reduced or oxidized.
This must be compared to a S02 flux into leaves between 20 and 100 nmol
(mg chlorophyll)~1 h~\ when the S02 concentration in air is about 50ppb
and the sum of boundary layer and stomatal resistances is between 2 and
10 s cm ~ 1. Apparently, detoxification capacities of leaves for S02 are high,
although they do not approach the detoxification capacity for N02. However,
reductive detoxification of S02 appears to be effective only when sinks are
available for reduced sulfur. Proteins constitute a main sink. In trees, net
protein synthesis is a seasonal affair. During most of the year, protein
synthesis and protein degradation match each other and there is, in the
absence of growth, no additional demand for reduced sulfur. It is therefore
not surprising that in Norway spruce sulfate accumulation accounts for
practically all S02 which has been taken up (Kaiser et al. 1991). Herbaceous
plants have a much higher capacity for the reductive detoxification of S02
than woody plants. Grasses thrive where spruce has been killed by overexposure to S02 (Heber et al. 1987).
Very high detoxification capacities are actually required for a successful
competition with toxic reactions. Even very brief exposure of leaves to
5 ppm S02 produces considerable inhibition of photosynthesis. This concentration is more than about 25 times the peak concentrations of S02
observed in the field during inversion periods in the winter. If fumigation
with 5 ppm S02 is discontinued after little more than 5 min, recovery of
photosynthesis may be observed within 30 min or more, indicating the presence of repair mechanisms (Veljovic-Jovanovic et al. 1993). Effective repair
requires light, ambient air levels of oxygen, and summer temperatures. Very
little is known about acute toxicity and detoxification of S02 at winter
temperatures.
In contrast to reductive detoxification, oxidative detoxification, which
leads to sulfuric acid, burdens the organism with protons. Short-term pH
regulation is highly effective at 20 DC, but slower at 16 DC (Heber et al.
1989b). Normally, its capacity is far higher than needed to deal with the
acidification caused by S02 influx. However, problems arise in the long
term. In needles of spruce from the Ore Mountains (Erzgebirge), average
sulfate concentrations have been measured which approach or even exceed
100mM (Kaiser et al. 1991; Kaiser et al. 1993b; Pfanz and Beyschlag 1991).
Accordingly, 200 mEq I ~ 1 H+ would have burdened the cells, if all of this
sulfate had been airborne. In fact, comparisons with sulfate contents in
needles from unpolluted areas show that most of the sulfate must have been
derived from S02. Nevertheless, the pH of homogenates of needles from
spruce grown in polluted and unpolluted areas proved to be almost identical.
Apparently, needles usually manage to neutralize the sulfuric acid which
slowly accumulates during long-term exposure to SOz-polluted air. Persisting