References

26 Signals and Signalling Pathways in Plant Wound Responses 397
spondingly were not active importers via the phloem. These small wounds
were not accompanied by electrical activity (Table 26.1). Steam girdling
the petiole of leaf 1 prevented the systemic induction of PI activity by
a small mechanical wound to that leaf (Table 26.1). These results are consistent
with the report (Nelson et al. 1983) from Ryan’s group that systemic
signalling was prevented by prior hot air (80 ◦ C) treatment of the petiole
of the leaf that was to be wounded; the wounding protocol used by that
group is, in our view, similar to wounding method 3, i.e. a small mechanical
wound. We conclude that for small mechanical wounds the systemic
signal is a chemical elicitor that is exported from the wounded leaf in the
phloem.
Electrical events are associated only with severe wounds. Malone (1996)
has provided evidence, on several grounds, that such electrical events are
likely to be associated with hydraulic dispersal. On this basis, and from
our evidence presented earlier for the similarities of the patterns of hydraulic
dispersal (Fig. 26.1) and electrical activity, we conclude that the
electrical events could be responses to chemicals transported in the xylem
by hydraulic dispersal from the wound site, rather than action potentials
propagated from that site. This is contrary to the conclusion in an earlier
paper reporting work from our laboratory in which we used only severe
wounds (Wildon et al. 1992); see also Davies (2004).
The conclusions here raise interesting questions about the results reported
in our previous paper (Rhodes et al. 1996), which was the most
detailed study to date of electrical events at the cell level in the wounded
plant. In that work we used intracellular recording from all the cell types in
the petiole of unwounded leaf 1 to further characterise electrical events following
a severe wound (heat, wound-type 5) to cotyledon 1. The recording
setup is shown in Fig. 26.2.
Fig.26.2. Schematic diagram of the arrangement of electrodes on the petiole of leaf 1 of
a tomato seedling, used to record wound-induced electrical events. Each plant was wounded,
using heat produced by passing an electric current for 30 s through a small wire element
held over, but not touching, cotyledon 1 (Reproduced from Rhodes et al. 1996, with the
permission of Springer-Verlag)