References

324 J. Fromm, S. Lautner
cells (Samejima and Sibaoka 1983). Further studies showed that the sieve
tubes also play a role in conducting action potentials. This assumption
wasconfirmedinstudieswhichusedtheaphidstylettechnique,asfirst
described by Wright and Fisher (1981) for Salix exigua and found to be
suitable for studies on other species such as Mimosa (Fromm and Eschrich
1988b). Cooling the apical end of a Mimosa petiole generates a rapidly
moving action potential which propagates basipetally through the sieve
tubes.Asforanimalcells,anactionpotentialisdefinedasapropagating,
transient change in voltage with an all-or-none response, dependent
on voltage-gated ion channels and capable of travelling through any living
cells sharing common membranes. In contrast to action potentials,
wounding, e.g. by flame-stimulation, causes the appearance of an irregular,
so-called variation potential (Sibaoka 1966, 1969; Malone 1996). There is
widespread agreement in the literature that the variation potential is not
a self-propagating signal, but a local electrical response to the passage of
chemical substances released from the wound site and propagated through
the xylem by hydraulic dispersal. In contrast to action potentials, variation
potentialsareabletopassthroughazoneofkilledplanttissue.Inthis
context, the idea of a hydraulic conductance of excitation was re-examined
in Mimosa, using a combination of electric and interferometric recordings
(Tinz-Füchtmeier and Gradmann 1990). Since no significant correlation
was detected between flame-stimulated electrical excitation and turgor
changes, the results render a hydraulic conductance of excitation unlikely;
but rather confirm the primary role of electrical events in rapid conductance
of excitation in plants.
In the last 2 decades strong evidence has been accumulated that electrical
transmission in sieve tubes also takes place in species that do not display
readily visible reactions. In maize leaves both electrical stimulus and coldshock
trigger action potentials with amplitudes exceeding 50 mV which are
transmitted without diminution in sieve tubes at velocities of 3−5 cm s −1
(Fromm and Bauer 1994). Sieve tubes also serve as a pathway for electrical
signalling in root-to-shoot communication of water-stressed maize plants
(Fromm and Fei 1998). Watering the root system after a drought period of
4 days induced a rapidly transmitted action potential. By contrast, waterstressing
of roots through the addition of osmolyte to the root medium
caused a different electrical signal to be measured in sieve tubes, indicating
that the form of the generated signals depends on the type of stimulation.
Not only in maize, but in the wounded tomato plant too, the pathway for
systemic electrical signal transduction may be associated with the phloem
(Rhodes et al. 1996).