Characteristics and Functions of Phloem-Transmitted Electrical Signals in Higher Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 325

22.4
Characteristics of Phloem-Transmitted Action Potentials
Almost all living plant cells have a resting potential with a net negative
charge localized on the inner side of the plasma membrane, caused by un-
equal distribution of positive and negative ions via the membrane. Within
the phloem, the membrane potential of companion cells is slightly more
negative than that of sieve elements (van der Schoot and van Bel 1989).
When a cell is stimulated, its electrical characteristics change significantly.
Theconformationofionchannelsintheplasmamembranechangein
response to heat, touch, light or other environmental cues, leading to an in-
crease or a decrease of the pore of the channels. This in turn controls the rate
at which ions can migrate across the membrane and the size of the electrical
potential difference. The latter changes from a negative to a more positive
value (depolarization) during an action potential. Subsequent to depolar-
ization the original resting potential is re-established (repolarization).
The ion transport processes which create the conditions necessary for the
generation of an action potential were investigated intensively in members
of the green algal family Characaea (Tazawa et al. 1987), indicating that
calcium influx as well as chloride and potassium efflux are involved (Beilby
and Coster 1979; Lunevsky et al. 1983; Spyropoulos et al. 1961; Oda 1976).
The previously described ion displacements during an action potential
were confirmed in trees by a method involving inhibitors of ionic channels
as well as energy-dispersive X-ray microanalysis (Fromm and Spanswick
1993). Results from these studies lead to the conclusion that calcium influx
as well as potassium and chloride efflux are involved in the propagation of
action potentials (Fig. 22.1). In willow roots, an apparent efflux of anions
and cations of 200−700 pmol cm −2 per action potential was calculated, us-
ing a vibrating probe in combination with the microelectrode technique
(Fromm et al. 1997). Electrical stimulation of willow trees showed that the
stimulus required to trigger an action potential depends on both its inten-
sity and its duration (Fromm and Spanswick 1993). Increasing stimulus
strength changes neither the amplitude nor the shape of the action poten-
tial, indicating that it conforms to the all-or-none principle. Stimulus sensi-
tivity seems to depend on the individual plant because the excitability var-
ied considerably although the trees were grown under identical conditions.
Since the degree of excitability may also change in the course of a day and the
season, it is likely that excitability depends on diurnal and seasonal rhythms
(Zawadzki et al. 1991). As regards refractory periods, they were found to
be much longer in plants than in animals, in the range between 2 min and
5 h (Zawadzki et al. 1991; Fromm and Spanswick 1993), while transmission
velocities are between 2 and 10 cm s −1 (Fromm 1991; Fromm and Bauer
1994; Mancuso 1999), and in soybean up to 30 ms −1 (Volkov et al. 2000).