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U. Heber et al.
of deposition of products of detoxification such as nitrate, in the case of
N02, or sulfate, in the case of S02 (Kaiser et al. 1989).
14.5.2 The Fate of Nitrogen Oxides
Owing to its lipid solubility, N02 can penetrate biomembranes. However,
before it reaches the plasmalemma of leaf cells, it reacts with water in the
apoplasm. The resultant nitrate and nitrite anions enter the cells. In the
cytosol, nitrate is either reduced to nitrite or transported into the vacuole,
where it may be stored. Nitrite is taken up into chloroplasts by a specific
nitrite carrier of the chloroplast envelope (Kramer et al. 1988).
In vitro, illuminated spinach chloroplasts may evolve oxygen during the
reduction of substrate concentrations of nitrite at rates close to 100 !lmol
(mg chlorophyll)-l h- 1 (Heber and Purczeld 1978). In contrast, fluxes of
N0 2 into leaves are distinctly less than 40nmol(mg chlorophyll)-lh- 1 at
commonly observed N02 concentrations in air (10 or 20ppb). In view of
the very high detoxification capacity of nitrite reductase, it is perhaps not
surprising that short exposure of leaves from herbaceous plants to 10 000 or
even 20000ppb N02 failed to produce persisting leaf damage (Yin 1990).
Photosynthesis was not even appreciably suppressed by such high N02
levels. The slight inhibition occasionally observed was reversed rapidly after
termination of fumigation. Instead of the expected acidification of the cytosol
by nitrous and nitric acid (cf. Wellburn 1990), transient alkalization was
observed in the presence of high concentrations of N02 in the air (Heber et
al. 1989a). This is attributed to the production of hydroxyl ions during
nitrite and nitrate reduction. The oxidative capabilities of N02 and nitrite
were efficiently neutralized by the antioxidative defenses of herbaceous leaf
cells. We conclude that oxidative interference of N02 or nitrite with leaf
metabolism is unlikely to cause acute leaf damage at commonly observed
N02 (or NO) concentrations. In herbaceous plants, N02 detoxification is
fast and highly effective. There, nitrite reduction proceeds even in the dark,
although at slower rates than in the light (Kaiser et al. 1992). Even in
spruce, which was fumigated for a period of 85 days, 10 h per day with
300ppb N02, neither nitrite nor nitrate accumulation was observed in the
needles (Kaiser et al. 1991). Apparently, fast reductive detoxification of
N0 2 is not restricted to herbaceous plants. Nevertheless, there are plants,
which accumulate nitrite in leaves after fumigation with N02 (Zeevaart
1976; Yoneyama et al. 1979). Wellburn (1990) proposes that in leaves of
plants which assimilate nitrogen mainly in roots, damage by N02 immisssion
may be possible because of insufficient nitrite detoxification.
The permanent NO radical is potentially more phytotoxic than N02 (cf.
Wellburn 1990), but its water solubility is low. For this reason, its diffusional
flux into leaves which is controlled by stomatal opening, is slow. In addition,
the NO concentration is usually about one order of magnitude smaller than
U. Heber et al.
of deposition of products of detoxification such as nitrate, in the case of
N02, or sulfate, in the case of S02 (Kaiser et al. 1989).
14.5.2 The Fate of Nitrogen Oxides
Owing to its lipid solubility, N02 can penetrate biomembranes. However,
before it reaches the plasmalemma of leaf cells, it reacts with water in the
apoplasm. The resultant nitrate and nitrite anions enter the cells. In the
cytosol, nitrate is either reduced to nitrite or transported into the vacuole,
where it may be stored. Nitrite is taken up into chloroplasts by a specific
nitrite carrier of the chloroplast envelope (Kramer et al. 1988).
In vitro, illuminated spinach chloroplasts may evolve oxygen during the
reduction of substrate concentrations of nitrite at rates close to 100 !lmol
(mg chlorophyll)-l h- 1 (Heber and Purczeld 1978). In contrast, fluxes of
N0 2 into leaves are distinctly less than 40nmol(mg chlorophyll)-lh- 1 at
commonly observed N02 concentrations in air (10 or 20ppb). In view of
the very high detoxification capacity of nitrite reductase, it is perhaps not
surprising that short exposure of leaves from herbaceous plants to 10 000 or
even 20000ppb N02 failed to produce persisting leaf damage (Yin 1990).
Photosynthesis was not even appreciably suppressed by such high N02
levels. The slight inhibition occasionally observed was reversed rapidly after
termination of fumigation. Instead of the expected acidification of the cytosol
by nitrous and nitric acid (cf. Wellburn 1990), transient alkalization was
observed in the presence of high concentrations of N02 in the air (Heber et
al. 1989a). This is attributed to the production of hydroxyl ions during
nitrite and nitrate reduction. The oxidative capabilities of N02 and nitrite
were efficiently neutralized by the antioxidative defenses of herbaceous leaf
cells. We conclude that oxidative interference of N02 or nitrite with leaf
metabolism is unlikely to cause acute leaf damage at commonly observed
N02 (or NO) concentrations. In herbaceous plants, N02 detoxification is
fast and highly effective. There, nitrite reduction proceeds even in the dark,
although at slower rates than in the light (Kaiser et al. 1992). Even in
spruce, which was fumigated for a period of 85 days, 10 h per day with
300ppb N02, neither nitrite nor nitrate accumulation was observed in the
needles (Kaiser et al. 1991). Apparently, fast reductive detoxification of
N0 2 is not restricted to herbaceous plants. Nevertheless, there are plants,
which accumulate nitrite in leaves after fumigation with N02 (Zeevaart
1976; Yoneyama et al. 1979). Wellburn (1990) proposes that in leaves of
plants which assimilate nitrogen mainly in roots, damage by N02 immisssion
may be possible because of insufficient nitrite detoxification.
The permanent NO radical is potentially more phytotoxic than N02 (cf.
Wellburn 1990), but its water solubility is low. For this reason, its diffusional
flux into leaves which is controlled by stomatal opening, is slow. In addition,
the NO concentration is usually about one order of magnitude smaller than