Oscillations in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 264

water and nutrient supply and reduce leaflet temperature by increasing
transpiration (Engelmann and Antkowiak 1998; Sharma et al. 2003). The
latter authors also reported that the period of oscillations in leaflet move-
ment rhythm in Desmodium was strongly dependent on light intensity.
Thus, these oscillations could serve the purpose of optimising the amount of
light falling on the leaflet or/and facilitating transpiration of water through
stomata.
18.2.2.2
Stomatal Control
Stomatal oscillations originate from several feedback loops regulating leaf
water and gas status (Raschke 1975; Willmer 1988). This is undoubtedly the
best-known class of ultradian oscillators in plants which have been known
for a long time (Barrs 1971; Cowan 1972). These oscillations usually occur
within a 10−90 min range (Raschke 1975) and are caused by abrupt changes
of external parameters such as light, CO 2 ,O 2 ,H 2 O and temperature (Giersch
1994; Luttge and Hutt 2004). Stomatal oscillations are an intrinsic feature
of plant leaves, and the major reason these oscillations are not observed in
every experiment is the fact that various leaf parts tend to oscillate out of
phase (Siebke and Weis 1995). Ionic mechanisms of guard cell signalling
have been the subject of numerous and comprehensive reviews (McAinsh
and Hetherington 1998; Blatt 2000). The important question addressed here
is: what are physiological implications of such oscillations?
Stomatal “patchiness” is a widely reported phenomenon (Cardon et al.
1994; Mott and Buckley 1998). Importantly, such “patchiness” is usually
limited to a region bound by veins. The latter may be a result of a specific
ionic (particularly, K + ) “microenvironment” in such regions, as revealed
by correlating fine leaf anatomical structure and net K + and H + flux pro-
files (Shabala et al. 2002). Taken together, it may be suggested that such
“small-scale” oscillations reported in plants (Siebke and Weis 1995) are
required for “fine-tuning” of guard cell osmotic balance and optimising
leaf photosynthesis for a specific light and ionic environment.
18.2.2.3
Nutations
Nutations of plant axial organs, with periods ranging from a few minutes
toseveralhours,arefoundinbothrootsandshootsofalargenumber
of species (reviewed by Barlow et al. 1994). These nutations are a result
of differential growth, with membrane-transport processes being a likely
“controlling mechanism” (Hejnowicz and Sievers 1995; Barlow et al. 1994;
Shabala and Newman 1997). In nutating organs, cells on the convex side
display a turgor pressure 10% greater than those of the concave side (Vanden