The photoreduction of K3[Fe(CN)6] by isolated and sonicated spinach chloroplasts is increased by SO 3 (--) (concentrations tested: 0.25-5 mM). This stimulation increases with SO 3 (--) concentrations from 0.25-3 mM. The ferricyanide-reduction with SO 3 (--) is inhibited by DCMU (10(-6)M) to about 90%. Inhibition of the photoreduction by pretreatment of the chloroplasts with Tris-buffer is compensated by increasing concentrations of SO 3 (--) (tested up to 3 mM). 2. The photoreduction of NADP in isolated chloroplasts is also enhanced by SO 3 (--) (concentrations tested: 0.25-3.0 mM). It is completely inhibited by DCMU (10(-6)M). In contrast to the results with ferricyanide as electron acceptor, SO 3 (--) does not overcome the inhibition of NADP reduction caused by pretreatment of the chloroplasts with Tris-buffer. 3. In illuminated isolated chloroplasts SO 3 (--) concentrations <1 mM do not alter the ATP-concentration, concentrations>1 mM decrease it. 4. The photosynthetic fixation of (14)CO2 by isolated chloroplasts is increased by SO 3 (--) concentrations <1 mM, but decreased by concentrations>1 mM. In total (14)CO2 fixed, at stimulating concentrations (0.25 and 0.5 mM SO 3 (--) ), the relative amount of sugar-monophosphates is increased, whereas that of sugardiphosphates and of PGA is decreased. 5. It is proposed that these specific effects on photosynthetic processes help to explain the well known fact that photosynthetic gas exchange and the yield of plants are stimulated by low doses of SO2.
