PAO (25 μM) prevents stomatal closure induced by four closing signals. The closing signal was applied immediately after the first aperture measurement, and changes in aperture followed with time. Each point shows the mean and SEM of 2–4 strips, from 24 to 88 pores. In each curve, white and black symbols show measurements in the absence and presence of 25 μM PAO, respectively, in 30 mM KCl. (a) 10 μM ABA. (b) Addition of 10 mM Ca2+ externally. (c) Addition of 0.2 mM H2O2. (d) Transfer to dark.