References

266 S. Shabala
tions in net H + efflux and K + uptake into the cell may be a useful strategy
to balance cell wall loosening and turgor-driven cell expansion, given the
observed phase shift between these two oscillatory cycles (Shabala 2003).
Another plant system exhibiting pronounced growth oscillations is pollen
tubes. A pulsating growth of pollen tubes has been reported elsewhere
(Feijo et al. 2001; Holdaway-Clarke and Hepler 2003). Both periodical
changes in cell wall strength (Holdaway-Clarke and Hepler 2003) and
rhythmical changes in cell turgor pressure (Messerli and Robinson 2003)
are likely to control such oscillatory growth. The direct link and the specific
mechanisms of growth oscillations and those in Ca 2+ ,H + ,K + and Cl − fluxes
from growing pollen tubes (Holdaway-Clarke and Hepler 2003) remain to
be revealed.
18.2.2.6
Cell Differentiation and Morphogenesis
Another important function of ultradian cellular oscillations may be their
involvement in cell differentiation and morphogenesis. There is no shortage
of models suggesting that pattern formation in developing organisms
may be the result of oscillatory dynamics (Jaeger and Goodwin 2001). Orientation
of cell division and other cambial effects in trees was suggested
to be a result of superposition of waves resulting from interacting cellular
oscillators (Hejnowicz 1975). The idea that the ultradian clocks may
control the cell division process is well established in microbiology (Lloyd
and Stupfel 1991). It has been shown that the ultradian clock exerts control
over energy-yielding processes, protein turnover, motility and the timing
of cell-division processes in a large number of unicellular organisms
(Lloyd and Kippert 1993). A key role of cyclic-AMP-dependent oscillations
in the cellular differentiation processes of Dictyostelium was demonstrated
elsewhere (Goldbeter et al. 1990).
For higher plants, direct experimental evidence is still lacking. Earlier we
showed the evidence for large-amplitude ultradian Ca 2+ flux oscillations
in the meristematic region of corn roots (Shabala and Newman 1997).
Unlike those in the elongation zone, these oscillations did not correlate with
root nutational movement. It was suggested that such oscillations serve as
a synchronising factor for cell division in the root meristem (Shabala and
Newman 1997). More direct evidence is needed to verify this hypothesis.
18.2.2.7
Photosynthesis
In the natural environment, light is probably the most widely and rapidly
fluctuating factor. During the course of a day, forest understorey plants
are exposed to brief periods of high light superimposed on a low-light