Long-Distance Signal Transmission in Trees - Communication in Plants: Neuronal Aspects of Plant Life - page 341

A strong interaction between APs and hormones has been shown in
willow roots by Fromm and Eschrich (1993), with a physiological role in
the gas exchange of leaves. So, APs may be evoked by plant hormones. The
application of indole acetic acid (IAA) or isopentenyladenine (IPA) in the
root medium triggered a propagating AP with an amplitude of more than
80 mV (IAA) or 50 mV (IPA): 3 min later the CO 2 uptake increased and
the transpiration rate first slightly decreased and then increased (IAA) or
definitely decreased (IPA). In contrast, ABA treatment resulted in a hyper-
polarization on the membrane potential, explained by Fromm et al. (1997)
assuming that K + leaves the cortex cells. Consequently, CO 2 uptake and
transpiration decreased sharply after 3 min following stimulation. From
these results, ABA-induced stomatal closure seems not to be based on
the hormone itself, but on the ABA-induced electrical signal, which by
membrane processes causes the stomata to close. Accordingly, Fromm and
Eschrich (1993) proposed that information on the soil water content was
electrically transmitted to the leaves. With the use of inhibitors of ion chan-
nels and energy-dispersive X-ray microanalysis it was demonstrated that
influx of Ca 2+ and efflux of Cl − and K + are responsible for the current
flowing during APs in willow roots (Fromm and Spanswick 1993). Efflux
of negative ions would reduce the endogenous outward current at the basal
elongation zone and enhance the endogenous inward current at the apical
elongation zone.
Grapevine ( Vitis vinifera ) plants exhibit different forms of rapid commu-
nication after a stimulus. Following perception of environmental stimuli,
hydraulic and electrical signals, travelling for long distances in the plant,
are early events in the coordination of the whole plant or some of its organs.
The velocity of propagation of the front of the main negative-going signal
(VP) was 2.7 mm s −1 , while an AP propagated along the shoot with a veloc-
ity of about 100 mm s −1 (Mancuso 1999). Koziolek et al. (2004) showed that
wound-induced electrical signals propagate with a velocity of 4−8 mm s −1
within different pinnae of a M. pudica leaf. Another type of signal that could
be involved in the regulation of photosynthesis after wounding is a chem-
ical signal spreading from the stimulation site through the phloem. The
transport velocities in the phloem typically proved to be 50−100 cm h −1
(Canny 1975), which is much too slow to account for observed modifica-
tions in gas exchange. Also, the possibility of a chemical transport in the
xylem can be ruled out because the stimulus was applied upstream within
the leaf. Transport through the symplasm might be another pathway for
a chemical signal, but the speed of this process (up to 15 µM m s −1 in
higher plants; Tyree 1970) is even slower than the transport velocity in the
phloem.