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 in vestigate the influence of light/dark transitions on apoplastic pH and K+ c oncentration in intact leaves of Vicia faba L. with fluorescence ratio imag ing microscopy. Illumination by red light led to an acidification in the le af apoplast due to light-induced H+ extrusion. Similar apoplastic pH respon ses were found on adaxial and abaxial sides of leaves after light/dark tran sition. 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 compa red with surrounding cells. The apoplastic K+ concentration in intact leave s declined during the light period. A steeper light-induced decrease in apo plastic K+, possibly caused by higher apoplastic K+, was found on the abaxi al side of leaves concentration. Simultaneous measurements of apoplastic pH and K+ demonstrated that a light-induced decline in apoplastic K+ concentr ation indicative of net K+ uptake into leaf cells occurs independent of apo plastic pH changes. It is suggested that the driving force that is generate d by H+ extrusion into the leaf apoplast due to H+-ATPase activity is suffi cient for passive K+ influx into the leaf cells.