Slow Wave Potentials –a Propagating Electrical Signal Unique to Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 296

Fig. 20.2. The application of negative (vacuum) and positive pressure steps of equal size
(60 kPa) to the submersed basal cut of an etiolated pea epicotyl shows that only positive
pressurestepsleadtoincreasedgrowthrate( GR ) and water uptake as well as the generation
ofaSWPdepolarization,heremeasuredattheepicotylsurfaceatadistanceof60mm
will vary in amplitude and lag in appearance only if the xylem pressure in
the measured stem section increases between 30 and 80 kPa. For pressure
increases larger than 80 kPa, depolarizations will be identical, i.e., they
will appear immediately and with similar, i.e., maximal, amplitudes. Thus,
a SWP will appear immediately and simultaneously along entire stem or
leaf sections as long as their change in xylem pressure exceeds 80 kPa (e.g.,
in monocot leaves after flame stimulation; Malone and Stankovic 1991;
Malone 1992).
Unlike ideal tubes, xylem conduits leak water in a centrifugal direction
and they do so preferably upon an increase in xylem pressure (Canny 1995).
When a chamber enforced a constant pressure increase of 50 kPa to the vas-
culature of the submersed basal end of a pea stem segment, it was found
this pressure is not transmitted in full amplitude to the apical end of the
segment (Fig. 20.4). The transmitted pressure steps decline with increasing
length of the measured segments and completely disappear when the epi-
cotyl length exceeds 12 cm. While Fig. 20.4 shows a linear drop of the xylem
pressure from the base to the tip of the pea epicotyl, Fig. 20.3 predicts that
an axial pressure gradient in the range 30−80 kPa should create a series of
depolarizations with increasing lag and decreasing amplitudes. Figures 20.3
and 20.4 therefore provide the basis for understanding SWPs. The appar-
ent, decremented apical “movement” of a SWP is not due to genuine axial
propagation but to delayed electrical responses to increasingly smaller hy-
draulic signals (Stahlberg and Cosgrove 1997c). One cannot avoid being
impressed by the accuracy with which plant stems translate pressure steps
into electrical signals, distances into increasing lags of the depolarization
produced, and by the reliability with which these computations create an
always perfect illusion of an electrical signal propagating along the surface.