References

310 E.Davies,B.Stankovic
It is assumed that they are a means of communication (i.e., intercellular
signals) that can be generated and transmitted much more rapidly than
can conventional chemical (hormonal) signals.
The two major types of putative signal are APs and VPs (Fig. 21.1). APs
are defined as all-or-nothing, self-perpetuating signals, which are transmitted
with essentially constant velocity and magnitude and are driven by
voltage-gated channels (Zawadzki et al. 1991). Plant APs are very unlike
neuronal APs, but are very similar to APs in other animal tissues (heart,
epithelium). The neuronal AP can be thought of as the biological equivalent
of a telephone, being designed for maximum rate of information transmission
(velocity) and minimum amount of information leakage (security).
This is done by having an ion-impermeable sheath over much of the neu-
Fig.21.1. Typical action potentials (APs) andvariationpotentials(VPs) measured in sunflower.
a The plant was stimulated electrically (5 V for 1 s) at a point about 5 cm below the
lowest petiole (±) or heat-wounded with a gentle flame applied to the tip of a leaf (W).
Measuring electrodes (inserted silver wires) were placed along the stem, and a reference
electrode was placed in the pot. The electrical responses to electrical stimulus are action
potentials that are shown in b. The electrical responses to the heat-wounding stimulus are
variation potentials that are shown in c. Note that electrical stimulation evoked a pure AP,
while flame-wounding evoked a combined AP/VP, with the former “traveling” faster than
the latter (Reproduced from Davies et al. 1991, with permission)