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Notes: Abstract The energy requirements of the various phases of the circadian clock in the laminar pulvini cells of primary leaves of Phaseolus coccineus L. were investigated using 4-h pulses of NaCN (5 mM) and NaN3 (1 mM). The induced phase shifts were calculated from the timing of the subjective night position during the third cycle after the treatment. Both inhibitors produce advances during phases which are correlated with the upward movement of the leaf (ca. 0–12 h after the maximum of the subjective night position) and during phases which are correlated with the downward movement of the leaf (ca. 20–28 h after the maximum of the subjective night position). Maximal advances are induced during the phase which is correlated with the maximum of the subjective night position (hour 0), whereas during phases which are correlated with the subjective day position (ca. 12–20 h after the maximum of the subjective night position) the inhibitors have no effect or induce only small advances. These results demonstrate that the part of the circadian cycle which, according to Bünning's tension-relaxation model of the circadian clock, is characterized by features of relaxation, represents a sequence of phases with decreasing energy requirement, whereas the tension part of the circadian cycle represents a sequence of phases with increasing energy requirement. The energy requirement for changing and maintaining the leaf positions was investigated by continuously offering NaCN, NaN3, and dinitrophenol (DNP) to leaves with intact and half (flexor cut away) pulvini. The substances inhibit in both pulvini the upward movement or induce a downward movement, depending on the leaf position, when the transfer to the inhibitor solution takes place. These results give evidence that the movement of intact pulvini reflects the turgor (volume) state of the extensor cells and that the increase of turgor (volume) and high turgor (volume) state requires more energy than the decrease of turgor (volume) or low turgor (small volume) state. Therefore, the time course of the energy requirements of the circadian clock and the clock-controlled turgor (volume states or leaf movement) is out of phase during a circadian cycle. Consequently the reaction of the clock-controlled leaf movement to the reduced energy supply can mask the clock behavior in pulse and step experiments. The phase response curves towards CN- and N 3 - reflect the time course of the CN--induced membrane depolarizations (the energy requirement of the electrogenic pump) in extensor cells of the pulvinus (Freudling et al. (1980), Plant Physiol. 65, 966–968), and both are out of phase with the time course of the energy requirement of the turgor. Consequently it is hypothesized that in Phaseolus advances are due to membrane depolarization and that at least in this organism electric properties of the plasmalemma are essentially involved in the mechanism of the circadian clock.