A reduced xylem pressure altered the electric and growth responses in cucumber hypocotyls

This study measured the electric and growth responses in excised cucumber hypocotyls and compared them with those in intact seedlings. Root excision (first severing cut) eliminated most of the positive xylem pressure (P(x)) in the hypocotyl, caused a rapid, transient drop in the hypocotyl growth rate (GR) and some small, local depolarization near the cut site. Although accompanied by a smaller decrease in P(x), a second, severing cut in the basal hypocotyl caused a decrease in GR which was no longer transient and a depolarization which was increased in both size and extent. These changes were not wound effects because they could he simulated by root incubation in mannitol. The reduced GR recovery occurred also in the absence of electric changes after a second increase in the mannitol concentration incubating the root of intact seedlings. Increased electric sensitivity and altered growth response therefore appear to be two independent examples of physiological changes resulting from a lowered P(x).