References

372 E. Wagner et al.
Exudation [µl/10 min]
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 12 24 36 48 60 72 84 96
Time [h]
Fig.25.2. Circadian rhythm of root exudation. Isolated root stock of a 5-week-old
Chenopodium rubrum (ecotyp 184) under constant conditions (25 ◦ C; 40% Hoagland’s
solution, dim white light) (Wagner et al. 1997)
offthemembranepotentialofcellstoallowrepairofadamagedcellmembrane
without losing too many ions from a localised injury. The generation
of action potentials in plant cells would depend on the sensitivity of cells
so that permeability of ions is increased by turgor-mediated mechanical
deformation. Such changes depend on the metabolic activity of the cells
leading to a change in turgor with subsequent changes in the activity of
mechano-transductive ion channels (Kloda and Martinac 2002; Lang and
Waldegger 1997).
Observed volume changes at the apex upon flower induction (Albrechtová
et al. 2004) and the rhythmic changes in root exudation (Fig. 25.2)
might be the basis for changes in the electrical activity at the root and
shoot apical meristems. The observed diurnal rhythm in the resting membrane
potential with significant changes for light-to-dark and dark-to-light
transitions possibly reflects a change between a photosynthesis-driven
(Fridlyand and Scheibe 1999; Karpinski et al. 1999) and a respirationdriven
energy metabolism. Such a rhythmic change in energy metabolism
as displayed in C. rubrum might be the essence of a circadian oscillator not
only in higher plants but also in cyanobacteria (Ivleva et al. 2005).
This working hypothesis will be developed in more detail (Sect. 25.4).
Redox and phosphorylation states are considered to be gating parameters
of energy metabolism to adapt the development of living systems in daily
light–dark cycles to avoid, for example, oxidative damage during light
conditions.