Agriculture Reference
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into the leaves and vice versa, whereas APs (at least in sunflowers) cannot
do so (Dziubinska et al. 2001; Stahlberg et al. 2005b). A third defining par-
ticularity of hydraulic propagation is that it enables SWPs - but not APs -
to pass through stem sections where the living cells were killed or poisoned
(Stahlberg and Cosgrove 1992). Fourth, SWPs depend on the pressure gra-
dient between the atmosphere and an intact plant interior being under
tension. Means to specifically suppress SWP propagation include working
withexcisedsectionsorintactplantsunderwateroranatmosphereof
100% humidity (Mancuso 1998; Stahlberg et al. 2005b).
20.5
The Ionic Mechanism of SWPs
A highly negative membrane potential in plant cells characterizes an active
state that involves the participation of the P-type H
+
pump and generates
large potential gradients for ions such H
+
,K
+
,Ca
2+
and Cl
−
across the
plasma membrane. Three major ion fluxes can generate large depolariza-
tions in such cells: (1) an increased inward flow of Ca
2+
ions from the cell
walls into the cytoplasm, (2) an increased outward flow of Cl
−
and other
anions and (3) a rapid stop in the outward pumping of H
+
ions by the
P-type ATPase.
WhileplantAPshavebeenshowntoinvolvetheopeningofionchannels
(Sibaoka 1969, 1991; Fromm and Bauer 1994); SWPs are thought to reflect
atransientshutdownoftheP-typeprotonpumpattheplasmamembrane
(Table 20.1). Evidence for this mechanism is that (1) SWPs depolarize cells
by a maximal amount of 90-100 mV, leaving the membrane potential at
a negative voltage near the Nernst potential for K
+
ions, (2) amplitudes of
the SWP depolarization change continuously with the applied pressure size,
(3) SW depolarizations are reduced or suppressed by the use of metabolic
inhibitors (Julien et al. 1991; Stahlberg and Cosgrove 1992), (4) no mea-
surable change in the cell-input resistance accompanied the large SWP
depolarization of pea epicotyl cells (Stahlberg and Cosgrove 1992, 1996,
1997c), (5) SWP and the pH increase in the apoplast showed matching ki-
netics when measured with the fluorescent indicator DM-NERF (Stahlberg
and Cosgrove 1996), and that the growth rate of apical stems drops with
the arrival of the SWP signal (Stahlberg and Cosgrove 1997c; Stankovic et
al. 1998). Being the fastest effect on this mechanism known (see Palmgren
1998) the hydraulic or stretch-activated inhibition of the P-type H
+
pump
deserves more investigation. One cannot yet exclude that the ionic mecha-
nism of SWPs could be more complex and in some species involve the par-
ticipation of turgor-controlled or stretch-activated/inactivated ion chan-
nels. Unlike pea epicotyls, epidermal cells of cucumber hypocotyls show
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