Plants as well as animals can tell the time without the aid of wrist watches [53]. This fact is due to the universal phenomenon of endogenous clocks. Depending on the natural environmental periodicities organisms display a range of biological rhythms. A rhythm is defined as a selfsustaining repeating fluctuation pattern. The time required to complete one cycle is called period. According to the free-running periods (e.g. periods in constant conditions) different biological rhythms are classified: ultradian, circadian, infradian, circannual, etc. The entrainment by the naturally occurring 24 h light/dark cycle reveals circadian oscillations. All eukaryotic organisms appear to have evolved circadian rhythms, for example in enzyme activity, body temperature, locomotion, mitotic index, fragrance formation, conidia formation, 02 evolution, CO2 exchange, ion uptake, leaf movement, bioluminiscence and photosynthetic activity [reviewed in 7, 19, 53]. Plants, exhibiting rhythmic 'sleep' movements, stomatal opening and closure, stem elongations and so on, have been historically important to study circadian rhythms [ 53 ]. Recently, attention was drawn to a new parameter exhibiting diurnal and circadian rhythms in plants: oscillations of messenger RNA accumulations (Fig. 1A). A