Oscillations in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 263

Te m p oral h i e rarc hy implies that several oscillatory processes, with very
different periods, may occur at the same cellular compartment, but because
of the difference in the time scales in many cases one of these processes
can be considered as stationary. The most apparent example is coexistence
of circadian and ultradian oscillations for the same process. Oscillations in
stomatal conductance (Barrs 1971; Cowan 1972) or leaf movement (Engel-
mann and Antkowiak 1998) are a good example. Another illustration may
be net H + flux oscillations in the elongation zone of corn roots, with 7-min
and 1.5-h period components (Shabala et al. 1997a).
The period range of reported oscillations in plants is extremely broad
(Fig. 18.1), from a few milliseconds (oscillations in ion channel con-
ductance; conformational changes in macromolecules) to several years
(flowering cycles in some species) (Ashoff 1981; Shabala 1989, and refer-
ences within). Most reported oscillations are between these two extremes
(Fig. 18.1). These can be further divided into two major groups. One such
group is circadian oscillations, with free-running periods between 20 and
28 h (Aschoff 1981; Webb 2003). The other (larger) group is oscillations in
the minute range of periods. The latter group, the ultradian oscillations, is
amajorfocusofthispaper.
18.2.2
Functional Expression
Despite the ubiquitous presence of ultradian oscillations, their physiolog-
ical role in plants is not always clear. Most published reports are phe-
nomenological in nature, with no clear evidence for “functionality”. In this
section, I review some of the known evidence for ultradian oscillations in
plants, with a major emphasis on the physiological significance of observed
phenomena.
18.2.2.1
Leaf Movement
Oscillatory leaf movements, with periods from several minutes to 1−2 h
have been reported for many species (Antkowiak and Engelmann 1995;
Sharma et al. 2003). It is believed that reversible changes in leaf angle
in plants bearing pulvini are caused by changes in the size and turgor of
pulvini motor cells; these in turn are regulated by the rhythmical movement
of K + and Cl − into and out of such cells (Kim et al. 1993; Antkowiak and
Engelmann 1995; Engelmann and Antkowiak 1998).
The functional role of ultradian oscillations in leaf movement is yet to be
demonstrated in direct experiments. Several authors suggest that fast ul-
tradian leaf movements may serve to regulate the light intensity, optimise