Determination of apoplastic K+ in intact leaves by ratio imaging of PBFI fluorescence

The tetraammonium salt of the K+ binding fluorescent dye benzofuran isophthalate (PBFI) was used to investigate the influence of potassium nutrition (0.1-2.1 mol m(-3)) on apoplastic K+ in Vicia faba leaves by means of ratio imaging. As a reference the infiltration-centrifugation method was used. Both methods reflected the influence of K+ supply on apoplastic K+ concentration. The abaxial leaf side revealed significantly higher K+ concentrations (20-25 mol m(-3)) than the adaxial side (5-8 mol m(-3)). Application of CCCP led to an immediate increase in apoplastic K+ demonstrating the reliability of the PBFI method.