References

18 Oscillations in Plants 267
background with a period ranging from a few seconds to 10 min (Pearcy
1990). On a larger scale, light fluctuations caused by cloud movements
influence all plants growing in a particular area (Cardon et al. 1994), resulting
in light fluctuations in the minute range of periods. It is not surprising,
therefore, that leaf photosynthetic machinery is adjusted to such a “fluctuating”
regime.
Oscillations in photosynthesis are well-reported phenomena (Kocks and
Ross 1995; Lüttge and Hütt 2004) and have been found at various levels of
organisation. Rhythmical changes in chlorophyll a fluorescence, phosphorylation,
oxygen evolution and CO2 assimilation are all examples of such
oscillatory behaviour. The period range for such oscillations is usually
around several minutes (Siebke et al. 1992). In most cases, such oscillations
are strongly damped and, being induced by sudden changes in one
of the environmental variables (light, CO2, etc.), disappear after several cycles
(Siebke et al. 1992; Kocks and Ross 1995). However, non-damped (for
several hours) oscillations have also been reported (Siebke and Weis 1995).
18.2.2.8
Osmotic Adjustment
As discussed before, both leaf and axial organ movements are mediated
by turgor and volume changes in the epidermal (in the case of nutations)
or pulvini (in the case of leaf movement) cells. In addition, oscillatory
ion transport mechanisms were shown to operate in planktonic diatoms
for adjustment of buoyancy by appropriate uptake and release of ions
(Gradmann and Boyd 1995). In higher plants, rapid (1−2 min period) cycles
of K + uptake and release in osmotically stressed leaf mesophyll cells were
reported (Shabala et al. 2000). All these facts point out the possibility of the
“fine-tuning” mechanisms of osmotic adjustment being realised through
oscillatory ion uptake across the plasma membrane.
18.2.2.9
Ultradian Rhythms in Time-Keeping
Molecular and genetics aspects of circadian rhythms have been the subject
of recent reviews (Webb 2003). Transcriptional/translational genetics
models are favoured (Dunlap 1998). Several clock genes have been identified
(Webb 2003). However, it appears that circadian systems will almost
certainly be made up of more than one interconnected feedback loop, and
there are many discomforting facts for the current genetics model. Dunlap
(1998) himself called it as a “pleasing caricature of reality”.
One of the striking features of oscillations in plants is the coexistence
of ultradian and circadian modulations of the same physiological process.
Can ultradian rhythms be a part of the circadian clock mechanism?