Agriculture Reference
In-Depth Information
The idea of the circadian clock being composed of a population of
strongly coupled ultradian oscillators is rather old (Pavlidis 1971). Accord-
ing to this scenario, a 24 h rhythmicity can be achieved as a result of the
frequency reduction (“beating”) in an ensemble of interacting oscillators
whose periods are much faster (in a minute range). The major drawback for
all coupling models is the scale invariance. This problem was successfully
resolved by suggesting the hierarchical coupling model (Barrio et al. 1996).
If ultradian oscillators are assumed to be membrane-based, the tempera-
ture compensation problem may be achieved by temperature adaptation
of membrane lipids as suggested in recent experiments on Neurospora
(Lakin-Thomas 1998).
18.2.2.10
Oscillations as a Part of an Encoding Mechanism
The concept of frequency-encoded environmental information in plant
cells has become rather popular among physiologists in relation to os-
cillatory Ca 2+ spikes in guard cells (McAinsh et al. 1995; McAinsh and
Hetherington 1998); however, it appears that this is also true for many
other rhythmical processes in plants. Different environmental factors such
as low temperatures, N and Fe starvation, mechanical stress, UV and salin-
ity are known to modify characteristics of plant rhythms (Erdei et al.
1998). Cytosolic Ca 2+ spikes in response to the nod factor were found in
wild-type alfalfa plants, but not in non-nodulating mutant (Ehrhardt et al.
1996). Oscillations in root ion fluxes showed a clear dependence on solu-
tion pH, osmolality and nutrient availability (Shabala 2003). Answering the
question about decoding mechanisms remains a great challenge for future
research.
18.3
Advantages and Principles of Oscillatory Control
18.3.1
Feedback Control, Damping and Self-Sustained Oscillations
Why do plants (or plant components) exhibit oscillatory behaviour? The
answer is rather simple: such behaviour is an intrinsic feature of every
feedback-controlled system.
Virtually every aspect of plant metabolism is controlled by a large num-
ber of positive and negative feedback systems. Thus, it is expected that every
physiological parameter in plants will oscillate, with some characteristic
frequency (period), under certain conditions. What are these conditions?
 
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