Voltage changes along geranium petioles after leaf blade excision

Voltage changes were measured along petioles of geranium (Pelargonium hortorum) plants after leaf blade excision. These voltages were detected using a non-invasive, non-polarizable, Ag/AgCl measuring electrode, which was located between 20 and 50 mm from the excision site. The initial onset of these voltage responses was so rapid that it occurred while the excision was still in progress. After a few seconds, voltage changes typically attained maximum values ranging from 1 to 20 mV, and then returned slowly toward zero. In most cases the predominant voltage change was negative, but in some cases the change was predominantly positive. In addition to the voltage response to the first cut, a voltage change also occurred in the same petiole after a second cut. This second cut response was usually much smaller than the first Experiments using the Scholander pressure bomb indicated that geranium xylem vessel fluid is under a tensile stress ranging from 100 to 300 kPa. Thus, whenever a leaf was excised, the forces acting on this xylem fluid were immediately unbalanced. This imbalance would have caused the fluid to be accelerated away from the cut until a new balance was achieved. The resulting fluid motion appears to be responsible for the petiole voltage change which occurred immediately after each excision.