References

328 J. Fromm, S. Lautner
hastobedoneontheresponsivenessofthevarioustypesofmoleculesthat
are involved in electron transport during electrical signalling.
Apart from Mimosa,consequencesofphloem-transmittedelectricalsignals
were also reported for tomato plants. They respond to wounding by the
induction of proteinase inhibitor (PI) activity in parts of the shoot distant
from the wound (Ryan 1990; Bowles 1990). The signal linking the wound
site with the induction of PI activity was suggested to be either chemical,
hydraulic or electrical. Wildon et al. (1992) provided evidence that the
systemic wound signal is a propagated electrical signal. Their hypothesis
is that the electrical signal passes from the wound site to the site of PI
induction through electrically excitable cells that are coupled via plasmodesmata.
Rhodes et al. (1996) studied the pathway of the electrical signal
in tomato and were able to show that the phloem is involved. In addition,
Stankovic and Davies (1997) found a definite relationship to exist between
action potentials that were stimulated electrically and large, rapid increases
in pin transcript.
Since long-distance transmission of electrical signals is associated with
the phloem pathway it is obvious that action potentials have an effect on
assimilate transport in the phloem. In maize leaves cold-shock as well
as electric stimulation evoke action potentials which propagate via the
sieve tubes away from the site of stimulation (Fromm and Bauer 1994).
Simultaneously, phloem transport in distant leaf parts is sharply reduced,
madevisiblebyautoradiographyatadistanceofover15cmfromthe
stimulation site. Evidence of a link between electrical signalling and the
interruption of phloem translocation was found through a decrease in
symplasmic K + and Cl − concentrations during action potentials. According
to Shiina and Tazawa (1986), who studied the effects of action potentials
ontheelongationgrowthinthestemofLuffa cylindrica,K + and Cl − efflux
from stimulated cells into the apoplast may reduce cell turgor and cause
growth retardation. In maize, ion efflux during action potentials may also
reduce the turgor of sieve tubes and trigger water efflux. Since water is
the transport medium for assimilates, a reduction in water content will
decrease phloem translocation (Fromm and Bauer 1994). However, the
interruption of phloem translocation may also be attributed to reduced
phloem loading because this process depends on the membrane potential
and the K + concentration in sieve tubes. Therefore, repeated irrigations of
the membrane potential during electrical signalling may have an effect on
apoplastic phloem loading.
Further studies on the consequences of phloem-transmitted electrical
signalling showed that action potentials are involved in the regulation of
gas exchange in maize (Fromm and Fei 1998). In these plants, the CO2
uptake and transpiration rate decreased strongly in drying soil. Subsequently,
plants were watered and increases in CO2 and H2Oexchangewere