I imply the measurement of the free phosphate-ion concentration with time resolution of seconds (or even shorter perods) in microenvironments, such as xylem, root or micromycetal (mycorrhyzal fungi) cells as well as water phase in the space, which is adjacent to root surface. I anticipate the device for real-time measurement of soluble phosphate-ion may include something like micro-electrode (micropippette electrode), similar to electrodes, used in pH-meters.