Slow Wave Potentials –a Propagating Electrical Signal Unique to Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 293

Fig. 20.1. Extracellular and intracellular recordings of the three major types of propagating
depolarization signals (action potentials, APs , slow wave potentials, SWPs , and wound
potentials, WPs ). They show in a an AP recorded from the abaxial surface of a Dionea
muscipula leaf after touching a trigger hair located on the opposite lobe at a distance of
15 mm, in b an AP recorded from the epicotyl of a 3-week-old, intact sunflower plant after
reduction of bright illumination from 150 to 10 µmol m −2 s −1 white incandescent light, in c
a SWP from the epicotyl of an intact sunflower plant after one puncture of the hypocotyl in
air 17 cm below the recording site, in d a SWP from the epicotyl of an intact sunflower plant
after one puncture of the hypocotyl under water and 20 cm below the recording site, in e
aSWPfromtheepicotylofanintactsunflowerplantaftera2-mm-deepcutinthesubmersed
surface of the hypocotyl 20 cm below the recording site, in f a SWP from the epicotyl of an
intact sunflower plant after a 1-s-long torching of the tip of a canopy leaf 15 cm above the
recording site, in g a combined signal of AP and SWP in the petiole of Mimosa pudica after
burning of the leaflet tip (redrawn from Houwinck 1935; Sibaoka 1953; Roblin 1985), and
in h a WP from the epicotyl of an intact sunflower plant after one puncture directly at the
recording site. Although APs, SWPs and WPs differ in rates of repolarization, this difference
can be too small for reliable identification
(SWPs, also called variation potentials) and wound potentials (WPs). All
three signals represent a transient depolarization. However, only SWPs
and APs use the vascular bundles to cover longer distances and potentially
spread through the entire plant. Examples a and b in Fig. 20.1 show an
AP from a flytrap leaf (a) that is of a very short duration and an AP from
a sunflower stem (b) that - like most APs in higher plants - lasts for a much
longer time of 30−50 s. Within this range APs are characterized by a rapid
depolarization and a rapid repolarization. When the depolarization passes
a certain threshold, excitable plant cells are able to amplify the signal to
a full AP spike by strictly implementing the all-or-nothing rule. Therefore,
within the same cell types, the propagation of such APs will proceed in the
form of nondecrementing signals with constant amplitude.
SWPs were first discovered by recognition that their kinetic appearance
was different from that of APs (Houwinck 1935; Sibaoka 1969, 1991; Umrath