Slow Wave Potentials –a Propagating Electrical Signal Unique to Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 303

transient increases in the cell input resistance during pressure-induced
SWPs (Stahlberg and Cosgrove 1997c).
Why is the repolarization of SWPs slower than in APs? Repolarization of
plant APs is believed to involve the combined action of voltage-dependent
closures of the depolarizing ion channels, the voltage-dependent activation
of repolarizing K + currents and an increased activity of P-type H + pumps.
Never going far beyond the Nernst potential for K + ions, SWP depolar-
izations are not likely to be compensated by large outward K + currents.
Another cause for the delayed repolarization of SWPs is the elimination
oftheroleP-typeH + pumps play in the repolarization efforts of plant
cells. If the second stage of AP repolarization is mediated by a P-type H +
pumping ATPase (as suggested by Orpitov et al. 2002 for cucurbit cells)
a turgor-inhibited pump would explain the slower repolarization of SWPs.
SWPs share one important feature with APs and WPs; a refractory period
during which the plant cells are unable to repeat the voltage signal when
subjected to the same stimulus (Zawadzki et al. 1991). When a sequence of
pressurestepswasappliedwith10minintervalsinbetween,allstepscaused
a transient increase in growth rate but only the first pressure application
generated a SWP (Stahlberg and Cosgrove 1996). Systematic studies of
refractory periods of SWPs in green plants are completely wanting.
20.6
The Effects of SWPs: Targeted Organs
SWPs trail hydraulic signals and very few studies differentiate whether an
effect is caused by either the hydraulic or the electric component. One at-
tempt was the comparison of the growth behavior of cucumber and pea
seedlings before, during and after the passage of the electrical SWP signal
into the growth zone (Stahlberg and Cosgrove 1997c). Application of small,
sustained pressure steps to the stem base rapidly and transiently increased
the growth rate due to a hydraulically mediated increase in apoplastic (and
turgor) pressure. The delayed appearance of the electrical signal in the
apical growth zone coincided with an unexpected, drastic drop in growth
rate. The sustained slow wave depolarization in cucumbers paralleled a sus-
tained growth inhibition of their hypocotyls, while a transient slow wave
depolarization in peas had a transient effect on the epicotyl growth. Related
results from sunflowers show a sustained shrinking of the upper stem after
the passage of a flame-stimulated SWP (Stankovic et al. 1998).
Mobility of SWPs in both directions of the plant axis suggests two po-
tential targets: the growing shoots with young canopy leaves and the root.
In addition to stem growth, dramatic responses have been reported for
leaves known to undergo particularly large, amplified SW depolarizations