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coded (electric) signals (Wagner et al. 1998). Rhythmic integration over the
whole plant possibly involves modulation of turgor pressure via stretch-
activated ion channels and aquaporins, with concomitant changes in mem-
brane potential (Fig. 25.3).
The perception of a flower-inducing dark period might lead to a change
inelectrochemicalsignallingbetweenleavesandthestemandthuscould
represent “florigen”. The involvement of action and variation potentials
for integration of the whole plant was anticipated (Wagner et al. 1998).
Signal arrival at the apex might trigger cytoplasmic changes in pH and
Ca
2+
concentration as secondary messengers in photoperiodic control of
development (Love et al. 2004). Finally, the switch from the vegetative to
the flowering state is a threshold response, systemic in nature and involving
not only the apical meristem but also the axillary buds.
25.4
Evolution of Circadian Frequencies -
Timing of Metabolic Controls
Considering metabolic control of timing in photoperiodism (Wagner and
Cumming 1970; Wagner et al. 1975), it has to be kept in mind that evolution
from prokaryotic to eukaryotic organisms was paralleled by a correspond-
ing evolution in energy metabolism. From primary fermentation, energy
conservation progressed to anaerobic photosynthesis and then to carbon
dioxide fixation with acceptance of electrons by water and evolution of
oxygen (Bekker et al. 2004). In a progressively oxygenic biosphere respi-
ration developed with oxygen as the terminal electron acceptor. Evolving
life was paralleled by the corresponding evolution of tropospheric O
2
/CO
2
composition and feedback of oxygen on life processes via reactive oxy-
gen and reactive nitrogen species, which as signalling molecules became
crucial for control of development of prokaryotic and eukaryotic living
systems. Adaptation to the seasonal variation in day length resulted in
photoperiodic control of development with a circadian rhythm in energy
conservation and transformation to optimise energy-harvesting by pho-
tosynthesis (Foyer and Noctor 2003; Wagner et al. 1975). Photosynthesis,
on the other hand, acts as a metabolic regulator via redox signals (Oh and
Kaplan 2000; Pfannschmidt 2003; Pfannschmidt et al. 2001; Sherameti et
al. 2002; Zeilstra-Ryalls et al. 1998) in addition to specific photoreceptor
systems like phytochromes and cryptochromes. Finally, redox control inte-
grates rhythmic gene expression in prokaryotes (Ditty et al. 2003; Dvornyk
et al. 2003; Rutter et al. 2001; Tomita et al. 2004), as well as in chloroplasts,
mitochondria and the nucleus of eukaryotes (Forsberg et al. 2001; Tron et
al. 2002).
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