References

19 Electrical Signals in Long-Distance Communication in Plants 281
E/mV
-50
-100
-150
-200
pCa
6.5
7.0
7.5
membrane potential
2+
Ca activity
5 min
Fig.19.2. Membrane potential changes (upper traces)andcytoplasmicCa 2+ activity (lower
traces)measuredinConocephalum conicum by ordinary and ion-selective microelectrodes,
respectively. Arrows indicate electrical stimulation. White and black bars denote light and
darkness, respectively (After Trebacz et al. 1994)
observed after application of Ca 2+ or anion channel inhibitors (Trebacz et
al. 1989). These findings were confirmed after employing ion-selective microelectrodes.
It was found that during the AP [Ca 2+ ]cyt increases from
resting 231 to 477 nM (Fig. 19.2) while Cl − and K + activities in the cytosol
decrease (Trebacz et al. 1994).
Application of A-9-C, an anion channel inhibitor, together with tetraethylammonium
(TEA) discloses voltage transients (VTs) evoked by light or
cold stimuli, which no longer have all-or-none character (Fig. 19.3). Their
amplitude depends on the stimulus strength and the period preceding
stimulation. There is evidence that VTs represent a calcium component of
APs (Trebacz et al. 1997; Krol and Trebacz 1999; Krol at al. 2003). In the
absence of A-9-C and TEA, the VT is short-circuited by a high membrane
conductance caused by the opening of anion and potassium channels during
the AP. A VT is a suitable tool to investigate the sources of [Ca 2+ ]cyt
increase. Reduction of the VT amplitude by impermeable lanthanides and
its dependence on external Ca 2+ concentration points to the apoplast as
asourceofCa 2+ . On the other hand, suppression of the VT by neomycin, an
inhibitor of phospholipase C catalyzing IP3 production, and magnification
of the VT by Sr 2+ ,whichisknowntoliberateCa 2+ from internal stores, is
evidence that those Ca 2+ sources play a role in the ion mechanism of the
AP (Krol et al. 2003).