Figure 1.

Effects of sorbitol and a water-channel blocker on the photosynthetic electron-transport activities of PSII and PSI in intact cells. Cells were incubated in the presence of 1.0 m sorbitol and 100 μm p-chloromercuriphenyl-sulfonic acid (the water-channel blocker) or in their absence. At designated times, a portion of each cell suspension was withdrawn and the activities of PSII and PSI were determined. A, The oxygen-evolving activity of PSII was examined after the addition of 1.0 mm BQ to the suspension. The activity that corresponded to 100% was 533 ± 38 μmol O₂ mg⁻¹ Chl h⁻¹. B, The electron-transport activity of PSI was determined by monitoring the uptake of oxygen at 32°C after the addition of 15 μm DCMU, 0.1 mm DCIP, 5 mm ascorbate, and 0.1 mm MV to the suspension. The rate of oxygen uptake that corresponded to 100% was 353 ± 52 μmol O₂ mg⁻¹ Chl h⁻¹ in the absence of sorbitol and p-chloromercuriphenyl-sulfonic acid (○); in the presence of 1.0 m sorbitol (●); and in the presence of 1.0 m sorbitol and 100 μm p-chloromercuriphenyl-sulfonic acid (▴). Each point represents the average with se of results from four independent experiments.