Sulfur-dioxide fluxes into different cellular compartments of leaves photosynthesizing in a polluted atmosphere

A computer model is used to analyze fluxes of SO₂ from polluted air into leaves and the intracellular distribution of sulfur species derived from SO₂. The analysis considers only effects of acidification and of anion accumulation. (i) The SO₂ flux into leaves is practically exclusively controlled by the boundary-layer resistance of leaves to gas diffusion and by stomatal opening. At constant stomatal opening, flux is proportional to the concentration of SO₂ in air. (ii) The sink capacity of cellular compartments for SO₂ depends on intracellular pH and the intracellular localization of reactions capable of oxidizing or reducing SO₂. In the mesophyll of illuminated leaves, the chloroplasts possess the highest trapping potential for SO₂. (iii) If intracellular ion transport were insignificant, and if bisulfite and sulfite could not be oxidized or reduced, leaves with opened stomata would rapidly be killed both by the accumulation of sulfites and by acidification of chloroplasts and cytosol even if SO₂ levels in air did not exceed concentrations thought to be permissible. Acidification and sulfite accumulation would remain confined largely to the chloroplasts and to the cytosol under these conditions. (iv) Transport of bisulfite and protons produced by hydration of SO₂ into the vacuole cannot solve the problem of cytoplasmic accumulation of bisulfite and sulfite and of cytoplasmic acidification, because SO₂ generated in the acidic vacuole from the bisulfite anion would diffuse back into the cytoplasm. (v) Oxidation to sulfate which is known to occur mainly in the chloroplasts can solve the problem of cytoplasmic sulfite and bisulfite accumulation, but aggravates the problem of chloroplastic and cytosolic acidification. (vi) A temporary solution to the problem of acidification requires the transfer of H⁺ and sulfate into the vacuole. This transport needs to be energized. The storage capacity of the vacuole for protons and sulfate defines the extent to which SO₂ can be detoxified by oxidation and removal of the resulting protons and sulfate anions from the cytoplasm. Calculations show that even at atmospheric levels of SO₂ thought to be tolerable, known vacuolar buffer capacities are insufficient to cope with proton production during oxidation of SO₂ to sulfate within a vegetation period. (vii) A permanent solution to the problem of acidification is the removal of protons. Protons are consumed during the reduction of sulfate to sulfide. Proteins and peptides contain sulfur at the level of sulfide. During photosynthesis in the presence of the permissible concentration of 0.05l·l^-1 SO₂, sulfur may be deposited in plants at a ratio not far from 1/500 in relation to carbon. The content of reduced sulfur to carbon is similar to that ratio only in fast-growing, protein-rich plants. Such plants may experience little difficulty in detoxifying SO₂. In contrast, many trees may contain reduced sulfur at a ratio as low as 1/10 000 in relation to carbon. Excess sulfur deposited in such trees during photosynthesis in polluted air gives rise to sulfate and protons. If detoxification of SO₂ by reduction is inadequate, and if the storage capacity of the vacuoles for protons and sulfate is exhausted, damage is unavoidable. Calculations indicate that trees with a low ratio of reduced S to C cannot tolerate long-term exposure to concentrations of SO₂ as low as 0.02 or 0.03 l·l^-1 which so far have been considered to be non-toxic to sensitive plant species.