Inhibition of Photosynthesis by Sulfite in Mesophyll Protoplasts Isolated from Vicia faba L. in Relation to Intracellular Sulfite Accumulation

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Abstract
Inhibition of photosynthesis by Na2SO3 in mesophyll protoplasts isolated from Vicia faba leaves and uptake of sulfite by the protoplasts were examined at various pH values of the incubation medium containing Na2SO3. As the pH of the incubation medium lowered, the rate of photosynthesis in the protoplasts decreased and the amount of sulfite taken up by the protoplasts increased. Most of sulfite accumulated in the protoplasts was not metabolized during the dark incubation, as measured with an ion chromatograph. Photosynthetic O2 evolution by the chloroplasts isolated from Vicia mesophyll protoplasts was inhibited by exogenously-applied Na2SO3 over pH region examined (7.4–9.0). The sulfite concentration required for a half inhibition of photosynthesis by the isolated chloroplasts was similar to the intracellular sulfite level required for that by the protoplasts. These results indicate that the intracellular sulfite accumulated in the protoplasts in an unmetabolized state is responsible for the inhibition of protoplast photosynthesis.