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

A comparison of fluxes of SO(2) from the atmosphere into leaves with fluxes across biomembranes revealed that, apart from the cuticle, the main barrier to SO(2) entry into leaves are the stomates. SO(2) fluxes into leaves can be calculated with an accuracy sufficient for many purposes on the assumption that the intracellular SO(2) concentration is zero. SO(2) 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 SO(2) to the acidic central vacuole of leaf cells is insignificant. Intracellular acidification of barley mesophyll protoplasts by SO(2) was measured by the uptake of (14)C-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 SO(2) influx when the H(+)/SO(2) ratio was assumed to be 2. A comparison of photosynthesis inhibition by SO(2) with calculated acidification revealed different mechanisms of inhibition at low and at high concentrations of SO(2). At very low concentrations, inhibition by SO(2) 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 SO(2) toxicity at low SO(2) levels. At high levels of SO(2), anion toxicity and/or radical formation during oxidation of SO(2) to sulfate may play a large role in inhibition.