Light-induced pH and K+ changes in the apoplast of intact leaves

The K+-sensitive fluorescent dye benzofuran isophthalate (PBFI) and the pH-sensitive fluorescein isothiocyanate dextran (FITC-Dextran) were used to investigate the influence of light/dark transitions on apoplastic pH and K+ concentration in intact leaves of Vicia faba L. with fluorescence ratio imaging microscopy. Illumination by red light led to an acidification in the leaf apoplast due to light-induced H+ extrusion. Similar apoplastic pH responses were found on adaxial and abaxial sides of leaves after light/dark transition. Stomatal opening resulted only in a slight pH decrease (0.2 units) in the leaf apoplast. Gradients of apoplastic pH exist in the leaf apoplast, being about 0.5-1.0 units lower in the center of the xylem veins as compared with surrounding cells. The apoplastic K+ concentration in intact leaves declined during the light period. A steeper light-induced decrease in apoplastic K+, possibly caused by higher apoplastic K+, was found on the abaxial side of leaves concentration. Simultaneous measurements of apoplastic pH and K+ demonstrated that a light-induced decline in apoplastic K+ concentration indicative of net K+ uptake into leaf cells occurs independent of apoplastic pH changes. It is suggested that the driving force that is generated by H+ extrusion into the leaf apoplast due to H+-ATPase activity is sufficient for passive K+ influx into the leaf cells.