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

Inhibition of photosynthesis by Na₂SO₃ in mesophyll protoplastsisolated from Vicia faba leaves and uptake of sulfite by theprotoplasts were examined at various pH values of the incubationmedium containing Na₂SO₃. As the pH of the incubation mediumlowered, the rate of photosynthesis in the protoplasts decreasedand the amount of sulfite taken up by the protoplasts increased.Most of sulfite accumulated in the protoplasts was not metabolizedduring the dark incubation, as measured with an ion chromatograph.Photosynthetic O₂ evolution by the chloroplasts isolated fromVicia mesophyll protoplasts was inhibited by exogenously-appliedNa₂SO₃ over pH region examined (7.4–9.0). The sulfiteconcentration required for a half inhibition of photosynthesisby the isolated chloroplasts was similar to the intracellularsulfite level required for that by the protoplasts. These resultsindicate that the intracellular sulfite accumulated in the protoplastsin an unmetabolized state is responsible for the inhibitionof protoplast photosynthesis. (Received January 24, 1985; Accepted May 29, 1985)