Excessive concentrations of sulfur dioxide in the air are toxic for plants
Sulfur dioxide in the air, which is formed in particularly high amounts during the smelting of sulfur containing ores, and also during the combustion of fossil fuel, can cover the total nutritional sulfur requirement of a plant. In higher concentrations, however, it leads to dramatic damage in plants. Gaseous SO2 is taken up via the stomata into the leaves, where it is converted to sulfite:
SO2 + OH- → SO32- + H+
Plants possess protective mechanisms for removing the sulfite which has been formed in the leaves, e.g., sulfite is converted by the sulfite reduct-ase to hydrogen sulfide and then further into cysteine. When cysteine is formed in increasing amounts it can be converted to glutath-ione. Therefore, in SO2-polluted plants an accumulation of glutathione is observed in the leaves. Excessive hydrogen sulfide can leak out of the leaves through the stomata, although only in small amounts. Alternatively, sulfite can be oxidized, possibly by peroxidases in the leaf, to sulfate. Since this sulfate cannot be removed by transport from the leaves, it is finally depos-ited in the vacuoles of the leaf cells as K+ or Mg++ -sulfate. When the deposit site is full, the leaves are abscised. This explains in part the toxic effect of SO2 on pine trees: the early loss of the pine needles of SO2-polluted trees is due to a large extent to the fact that the capacity of the vacuoles for the final deposition of sulfate is exhausted. In cation-deficient soils, the high cation demand for the final deposition of sulfate can lead to a serious K+ or Mg++ deficiency in leaves or pine needles. The bleaching of pine needles, often observed during SO2 pollution, is partly attributed to a decreased availability of Mg++ ions resulting in a reduced chlorophyll content.