English: Ratiometric real-time quantitation of leaf apoplastic pH in response to the given illumination regime used for inverse microscopy image acquisition. pH as recorded at the adaxial face of Vicia faba leaves is plotted over time. Leaf apoplastic pH was discriminated within three apoplastic components, viz. in the stomatal cavity (n = 6 ROI; green kinetic; mean ± SE of ROIs), in the epidermal apoplast (n = 20 ROI; red kinetic; mean ± SE of ROIs) and in the apoplast surrounding the palisade mesophyll (n = 20 ROI; blue kinetic; mean ± SE of ROIs). No effects on apoplastic pH were detected that were attributable to the illumination of the specimen with the excitation or emission wavelengths used for imaging. Representative kinetics of four equivalent recordings of plants gained from independent experiments.