|
Unlike action potentials (APs) slow wave potentials (SWPs, sometimes called
variation potentials) are a particular creation of higher plants with no parallel occurrence in animals. The
generation of SWPs occurs under different conditions than that of action potentials (APs). APs are generated
and can be studied in excised stems and leaves. SWPs depend on the pressure difference between the atmosphere
and an intact plant interior. Excision of leaves, anti-transpirants, air of high humidity any other condition
that compromises the tension of the plant interior will severely reduce the range of SWP signals or suppress
them altogether. SWPs and APs both involve transient depolarizations of plant cells that spread from their
place of generation to distant parts of the plants. However, when compared in the same plant the
depolarization of APs is always shorter than that of an SWP. The available data suggest that this feature
reflects different ionic mechanisms. While opening anion channels mediate most of the depolarization in APs,
the depolarization of SWPs probably involves both opening of calcium channels and a transient shutdown of
stretch-sensitive H+-ATPases at the plasma membrane. A third defining particularity of SWPs
consists in their mode of propagation. SWPs appear to involve a sequence of local depolarizations that arise
in the wake of a rapid wave of positive pressure spreading through the xylem and finally resulting in rapid
water uptake and turgor increase by peripheral cells. Since poisoned or killed stem sections do not interrupt
SWP propagation, it follows a hydraulic rather than electric mechanism. Studying Mimosa responses,
Haberlandt suggested as early as 1890 (when only APs were known) to look for a hydraulically propagating
signal. It took a long time to realize that such a signal exists in the form of SWPs. |
