Agriculture Reference
In-Depth Information
19.1.4
Physiological Implication of Plant Excitation
APs, like other signals, besides input(s) and the ways of transmission have
to posses output(s), i.e., their passage has to “inform” distant cells about
locally acting stimuli and let them respond appropriately. Before beginning
a study of the physiological implication of APs, it is necessary to elaborate
detailed electrophysiological characteristics of the plant examined. One has
to find out if the signal is a real AP, which stimuli can evoke the AP, and how
often they can be applied. It is also important to know what the velocity
of AP transmission is, and which plant organs and tissues are excitable. In
experiments aiming at checking the consequences of APs it is also desirable
to break the AP transmission between the site of stimulus application and
AP destination.
Trap closures of carnivorous
D. muscipula
and
A. vesiculosa
are among
the best-documented consequences of excitation in plants. In
Dionaea
,
bending of one of the trigger hairs protruding from the upper part of
the trap leads to generation of an AP which spreads over the trap with
a velocity of approximately 10 cm s
−1
. What is important is that the trap
does not visibly move after the first stimulation. It is necessary that the
second bending of any of the trigger hairs occurs no later than 40 s after
the first, to make the trap close. The second stimulus is accompanied by
the second AP, whose velocity is much higher than that of the first one (up
25 cm s
−1
) (Sibaoka 1969). Such a double-excitation-triggered trap closure
protects the plant against an accidental stimulation. Trap reopening is
an energetically wasteful process. The response to the second AP can be
regarded as plant memory. Following the second AP the leaf closes quickly.
Theclosureis,however,notcomplete.Smallpraycanleavethetrap.When
the victim is too large to escape and strong enough to bend trigger hairs
many times in its struggle, the trap closes tightly and digestive glands
begin to release enzymes that decompose its body. In spite of more than
100 years of AP investigation in
Dionaea
adetailedmechanismofitstrap
closure is not known. Hodick and Sievers (1989) demonstrated that all
cells within the trap are excitable. There is also no special motor zone.
Such cells are separated neither anatomically nor electrophysiologically.
Thereisadelicatebalancebetweentissuetensionsinthetrapwhichis
shifted towards the closure after the second AP. Trap movement consists of
relatively slow followed by very fast phases. The other phase is attributed
to mechanical properties of the trap (Forterre et al. 2005). The question
is why only the second AP is able to release the tension. Probably the
second AP, when repeated soon enough after the first one, causes sufficient
accumulation of Ca
2+
in the cytosol and the release of Cl
−
and K
+
that
changes turgor, cell wall extensibility or makes the cytoskeleton rebuild.
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