Flux of SO2 into Leaf Cells and Cellular Acidification by SO2

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Abstract
A comparison of fluxes of SO2 from the atmosphere into [barley] 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 cencentration 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.