Mesophyll Resistances to SO2 Fluxes into Leaves 1

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Uptake of label from solutions containing 35SO2, H35SO3− and 35SO32− into mesophyll protoplasts, vacuoles, and chloroplasts isolated from young barley leaves was measured at different pH values. Uptake was fast at low pH, when the concentration of SO2 was high, and low at high pH, when the concentration of SO2 was low. When the resistance (R) of plasmalemma, tonoplast, and chloroplast envelope to the penetration of SO2 was calculated from rates of uptake of label, comparable values were obtained for the different biomembranes at low pH values. R was close to 8000 seconds per meter and permeability coefficients were close to 1.25 × 10−4 meters per second. Under these conditions R may describe resistance to SO2 diffusion across a lipid bilayer. At higher pH values, R decreased. As R was calculated on the assumption that SO2 is the only penetrating molecular species, the data suggest that carrier-mediated anion transport contributes to the uptake of sulfur at physiological pH values thereby decreasing apparent RSO2. The contribution of anion transport appeared to be smaller for transfer across the plasmalemma than for transfer across the tonoplast. It was large for transfer across the chloroplast envelope. The phosphate translocator of the chloroplast envelope catalyzed uptake of SO32− into chloroplasts at neutral pH. Uptake was decreased in the presence of high levels of phosphate or sulfate and by pyridoxal phosphate. SO2 transfer into cells leads to the intracellular liberation of one or two protons, depending on pH and oxidizing conditions. When the divalent sulfite anion is exchanged across the chloroplast envelope, bisulfite formation results in proton uptake in the chloroplast stroma, whereas SO2 uptake into chloroplasts lowers the stroma pH.

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