The electrogenic proton pumping from parenchyma symplast into xylem—direct demonstration by xylem perfusion

Abstract We have devised an experimental system for simultaneous measurement of the activity of the xylem electrogenic ion pump, which is located on the inner cell membrane between the parenchyma symplast (p) and the xylem (x). and pH of the xylem exudate of a hypocotyl segment of Vigna unguiculata under pressurized xylem perfusion. Anoxia caused immediate depolarization of the inner cell membrane followed by alkalization of the xylem exudate several minutes later. Activity of the xylem pump was recovered by reaeration and acidification of the xylem exudate took place. These results indicate that the xylem pump is the respiration-dependent electrogenic proton-pump extruding proton from the parenchyma symplast into the xylem.