Preface

Rhythmic processes, e.g. leaf movements, have attracted the curiosity of man since ancient times. Research into so-called sleep movements of leaves supplied the basic kinetic information for Bünning (1977), who developed the theory of the physiological clock. Here he characterised and described a circadian (∼24 h) temperature-compensated oscillation as the cellular time-measuring principle of eukaryotic and, as demonstrated more recently, of prokaryotic systems as well. Bünning's work served as the basis for the more complex research into circadian and rhythmic processes in modern times, and, in a way, revolutionised this fascinating field using molecular genetic tools.

Besides circadian oscillations, living systems display higher and lower frequency phenomena in metabolism and behavior, in signal perception and in communication. This special volume dedicated to plant chronobiology introduces the reader to ultradian phenomena, where cycles take only hours or even minutes, in plants and yeast (Lloyd this issue); metabolic ultradian cycling in yeast seems particularly relevant as it drives a temporal, genome-wide transcription and coordination of essential metabolic processes (Tu et al. 2005).

Biochemical and biophysical oscillations seem to be the hardware of biological cellular control networks in adaptation to environmental constraints like the change of seasons. The evolution of the circadian time-measuring principle—being the basis for the anticipation of seasonal change in environmental conditions—shows features of “learning” by adaptation, so creating a circadian rhythm software (Lüttge 2003). The metabolic control of timing by gating development and behaviour allows sophisticated adaptation of individual cells as shown for Chlamydomonas (Iliev et al. this issue), for whole plants like Arabidopsis (Schöning et al. this issue), or for populations of plants in their competition for survival as demonstrated by photoperiodic control of growth and differentiation via a combinatorial interaction of input and output variables to the living system (Jarillo & Piñero this issue; Albrechtová et al. this issue).

The demonstration of a diurnal rhythm in surface membrane compound action potentials and the correlation of these with rhythmic leaf movements and stem growth of Chenopodium rubrum (Wagner et al. 2006) prompted us to view the circadian oscillation as a hydro-electrochemical phenomenon at the cellular and organismic level. This view is supported by diurnal and circadian regulation of plasma membrane aquaporins and is discussed in detail in Moshelion et al. (2002). The hydro-electrochemical integration depends very much on the compartmentation and the formation of temporal patterning of calcium ions as key players (Malho et al. this issue). The basis for temperature-compensated time keeping in the metabolic control nets of transcription and translation, protein synthesis and turnover is attributed to the ratios of redox and phosphorylation potential, so-called macroparameters, which control the physiological clock via two-component signal-transduction systems (Albrechtová et al. this issue). These ratios react and adapt to changing environmental constraints, securing a stable and continuous, but flexible, rhythmicity of metabolic processes and gene expression. This in turn, is probably the key for successful living on a planet with a seasonal change in day length depending on latitude.