Photosynthetic oxygen evolutioni by isolated spinach (Spinacia oleracea L.) chloroplasts approached complete inhibition in the presence of a 5 mM concentration of sulfur dioxide. A similar inhibition was observed in the presence of equimolar concentrations of bisulfite ions, suggesting a parallel mode of action. In contrast, an equimolar concentration of sulfite ions was markedly less inhibitory and sulfate ions caused negligible inhibition of apparent photosynthesis. The mode of action of sulfur dioxide and related sulfur anions in inhibiting photosynthesis was found to be essentially independent of direct hydrogen-ion effects.

Sulfur dioxide is a major atmospheric contaminant and phytotoxicant resulting from the combustion of sulfur-containing fossil fuels. The phyto-toxic behavior of the gas has received considerable attention (6,8,18). Foliar absorption studies have revealed that SO2 is extensively absorbed by leaves when stomates are open (3,6), suggesting that vegetation may act as an important "sink" for atmospheric pollutants (7).Cormis (5) demonstrated that plants exposed to SO2 released measurable quantities of H2S into the surrounding atmosphere. The phenomenon was shown to result from a photoreductive process since no H2S was detected during fumigations with SO2 in darkness. The present investigation examines certain aspects of the foliar absorption of SO2, its cytoplasmic distribution, and a possible mechanism allowing formation of hydrogen sulfide. MATERIALS AND METHODSChamber Fumigations. Six-to eight-week-old spinach plants (Spinacea oleracea L.), grown in a controlled-environment cabinet, were exposed to 35 02 in a glass fumigation chamber (17). The chamber consisted of an 18-liter bell jar, the basal flange of which was clamped to form an airtight seal with an inert-surfaced supporting platform. A squirrel-cage fan blade located within the chamber was used to circulate air within-the closed system.Baffles situated in order to direct the air flow pattern also served as a supporting platform for a single pot of plants.All procedures were performed within the confines of a laboratory hood. Sulfur dioxide containing 35SO2 was generated from the reaction of H235S04 (International Chemical and Nuclear Corp., Irving, Calif.) with thin copper "turnings." The pressure generated by externally applied heat and the reaction carried the evolved gas and confined air through an impinger containing concentrated H2SO4 to remove water vapor and then into a 30-liter Mylar gas bag (Calibrated Instruments Co., Ardsley, N.Y.) contained in a second bell jar. When the SO2 generation was completed, this supply system was isolated by turning a threeway valve attached to the inlet of the Mylar gas bag which served as a gas reservoir. A measured quantity of laboratory air was then added to the bag to dilute the gaseous mixture to the desired SO2 concentration for introduction into the chamber. Withdrawal of a measured volume of the radioactive gas mixture was accomplished by a 50-ml gastight syringe attached to the gas line leading from the reservoir to the fumigation chamber. Measured aliquots could either be expelled into the fumigation chamber, or collected in midget impingers containing 0.03 N H2O2 solutions for analysis (10).Illumination during the fumigations was provided by a bank of five 150-w reflector flood lamps yielding a light intensity of 1200 ft-c at the position of the leaves within the chamber. Plants were illuminated in position for 20 min before lowering the bell jar over them in preparation for exposure to labeled SO2. The chamber temperature during fumigations was 29 + 2 C.Immediately after clamping the chamber in place, a me...