Effect of PAO on the pH dependence of the PM H+-ATPase activity. PM was isolated from radish seedlings (A) and cultured Arabidopsis cells (B) untreated (○, C), pretreated with 100 μm PAO for 180 min (●, PAO), pretreated with 5 μm FC for 120 min (▵, FC), or pretreated with 100 μm PAO for 60 min followed by a treatment with 5 μm FC for 120 min (▴, PAO,FC). PM H+-ATPase activity is expressed as a percentage of that measured at pH 6.2 for radish seedlings (C = 0.31; PAO = 0.21; FC = 0.28; PAO,FC = 0.21 μmol min−1 mg−1 protein) and at pH 6.0 for Arabidopsis (C = 0.56; PAO = 0.46; FC = 0.46; PAO,FC = 0.27 μmol min−1 mg−1 protein).