For means of in planta recording of leaf apoplastic pH values, 7.5 μg/ml of the fluorescent pH indicator Pt-GFP or 25 μM of the pH-sensitive dye Oregon Green 488-dextran (Invitrogen GmbH, Darmstadt, Germany) were loaded into the leaf apoplast of intact plants following the step-by-step instructions that were given elsewhere [34 (link)]. Measurements were started 2 hours after loading.
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