References

18 Oscillations in Plants 265
Driessche 2000). Recent experiments in our laboratory have found a strong
(R >0. 9) correlation between root circumnutation patterns and rhythmic
changes in K + uptake in vertically grown maize roots (Shabala 2003), with
periodical (t ∼ 50 min) K + flux changes from influx to efflux at two opposite
sides in opposite phase.
Plant circumnutations show strong dependence on temperature (Q 10 ∼ 2;
Schuster and Engelmann 1997), light intensity (Anderson-Bernadas et al.
1997) and quality (Schuster and Engelmann 1997), mechanical stimulation
(Anderson-Bernadas et al. 1997) and chemical composition of the
medium (Buer et al. 2000). All these observations point out the possibility
of “encoding” environmental information by nutational patterns.
The traditional view of the physiological role of plant nutations is that
nutational growth of an organ maximises the penetration of the medium in
which it grows (Barlow et al. 1994). Recently, Inoue et al. (1999) showed that
in rice root circumnutation was important in creating a larger pushing force
of the seminal root without causing floating of the seedling on flooded and
very soft soil, thus leading to a higher seedling establishment percentage in
the field. More direct evidence is needed to fully reveal the role of nutations
in plant axial organs.
18.2.2.4
Root Nutrient Acquisition
Despite the widely reported substantial diurnal changes in the rates of nutrient
uptake by roots (reviewed by Shabala 2003), ultradian oscillations in
root nutrient acquisitions remains underexplored. Nonetheless, such oscillations
have been reported in several plant species (Knaritonashvili et al.
1997; Macduff and Dhanoa 1996; Shabala et al. 1997; Shabala and Knowles
2002; Shabala 2003), with periods ranging from 5 min to several hours.
Importantly, such oscillations appear to be extremely sensitive to environmental
conditions, with their periods showing a strong dependence on solution
pH, temperature and osmolality (Shabala 2003). This may be strong
evidence for a frequency-encoding mechanism operating in plant roots,
similar to one reported for guard cells (McAinsh et al. 1995; Blatt 2000).
18.2.2.5
Growth
There is increasing evidence that axial growth of many plant organs is also
rhythmically modulated. Ultradian oscillations in stem growth with periods
of several minutes were reported for stems, hypocotyls, roots and sporangiphores
(see Shabala 2003 for references). Oscillations in membranetransport
activity are likely to be the major driving force (Shabala et al.
1997a; Tyerman et al. 2001). It has been suggested that periodical fluctua-