References

374 E. Wagner et al.
metabolism at the apex (Albrechtová and Wagner 2004) and in the shape
of the apical meristem (Albrechtová et al. 2004), which led us to hypothesise
on the possible involvement of a change in water status during an early
phase of floral transition. Indeed, expression of a putative aquaporin CrAQP
increases during flower induction at the apex and in leaves of C. rubrum
(Albrechtová and Wagner 2004). Similar results were obtained in Pharbitis
nil (A. Tretyn, personal communication). Furthermore, an application of
an inhibitor of aquaporin activity to the apex delayed and partially inhibited
flowering in C. rubrum (Albrechtová and Wagner 2004). On the
basis of these observations, we proposed that changes in the water status
at the apical meristem might play an important role in the initial phase
of floral transition, leading to local changes in tissue tension caused by
increased turgor, which in turn might activate mechano-transductive ion
channels (Lang and Waldegger 1997). Local changes in tissue tension could
directly influence organogenesis by affecting local properties of cell walls
as proposed by Green (1994). Indeed, in C. rubrum the optical properties
of cell walls at the surface of the apex change locally in the early phases of
photoperiodic flower induction (Albrechtová et al. 2004).
On the basis of the observed early changes at the apex and of our measurements
of changes in electrical activity during flower induction, we
suggestthatthesignalsforfloweringmightbetransmittedfromleaves
to the apical meristem via hydraulic–electrochemical impulses. A similar
mode of action was shown in systemic wound reactions (Wildon et al. 1992).
A diurnal (circadian) rhythm in the resting potential of the plasma
membrane (Fig. 25.3) possibly reflects the daily change in photophile and
skotophile phases (Bünning 1977). This rhythm has its origin in a circadian
rhythm in energy metabolism and is probably the basis for circadianrhythmic
changes in sensitivity to signal perception, signal generation (e.g.
action potentials) and signal transduction. It is proposed that the communication
between plant organs (leaves, shoot apex, root) involves frequency-
2h
Time [h]
300 mV
Fig.25.3. Changes in resting surface membrane potential. Kinetics of the electric surface
membrane potential in a light–dark cycle of 8 h : 16 h. Platinum surface electrodes were
attached to a leaf petiole and the basipetal internode of a flowering plant of C. murale (see
Fig. 25.1; Wagner et al. 1998)