Flux of SO(2) into Leaf Cells and Cellular Acidification by SO(2)

A comparison of fluxes of SO₂ from the atmosphere into leaves with fluxes across biomembranes revealed that, apart from the cuticle, the main barrier to SO₂ entry into leaves are the stomates. SO₂ fluxes into leaves can be calculated with an accuracy sufficient for many purposes on the assumption that the intracellular SO₂ concentration is zero. SO₂ entering green leaf cells is trapped in the cytoplasm. In the light, the products formed in its reaction with water are processed particularly in the chloroplasts. Flux of SO₂ to the acidic central vacuole of leaf cells is insignificant. Intracellular acidification of barley mesophyll protoplasts by SO₂ was measured by the uptake of ¹⁴C-labeled 5,5-dimethyl-oxazolidine-2,4-dione. The measured acidification was similar to the acidification calculated from known buffer capacities and the rate of SO₂ influx when the H⁺/SO₂ ratio was assumed to be 2. A comparison of photosynthesis inhibition by SO₂ with calculated acidification revealed different mechanisms of inhibition at low and at high concentrations of SO₂. At very low concentrations, inhibition by SO₂ was even smaller than expected from calculated acidification. The data suggest that, if acidification cannot be compensated by pH-stabilizing cellular mechanisms, it is a main factor of SO₂ toxicity at low SO₂ levels. At high levels of SO₂, anion toxicity and/or radical formation during oxidation of SO₂ to sulfate may play a large role in inhibition.