Long term measurement of the trans-root electric potential in a persimmon tree in the field

A convenient system via a perfect liquid junction was developed for measuri ng the trans-root electric potential (TRP) of a tree in the field. This sys tem enabled continuous measurement of the TRP of an old kaki tree (Diospyro s kaki,Japanese persimmon), which showed clear diurnal oscillation througho ut two successive years. The level of the TRP on the average for every ten days had two minima (ca. -70 mV) in mid winter and mid summer, and two maxi ma (ca. -30 mV) in October and May. A regular seasonal change in the amplit ude of the oscillation was observed; it was at its maximum (ca. 70 mV) in e arly spring just before the sprouting of new leaves, and at its minimum (ca . 10 mV) during summer and early autumn when leaves were fully expanded. Th e relationship between the TRP, water transport and rainfall is discussed.