Diverse signals synchronise the circadian clock controlling the oscillations in chlorophyll content of etiolated Phaseolus vulgaris leaves

Abstract The endogenous circadian rhythm which controls the biogenesis of thylakoid components in Phaseolus vulgaris , as reported by the capacity of the etiolated leaf to form chlorophyll in the light, is set by imbibition (sowing) of seeds: leaves of 9-day-old etiolated seedlings sown different hours of the day and exposed concomitantly to continuous light, show a propensity for accumulating chlorophyll amounts which oscillates according to the time of day at sowing. In addition, harvest/cutting (removal of the primary leaves attached to the cotyledons from etiolated plants) sets a new rhythm and affects positively the amplitude of the oscillations. The rhythmic patterns, however, obtained from series harvested various hours apart during a day and normalised as to the phase of the pre-existing rhythm do not fall in phase. This indicates that harvest results in a variable phase shift dependent on the phase of the pre-existing rhythm set by sowing/imbibition. Similar observations are made when a light-pulse is used as the new synchroniser after harvest. In these cases, the ‘morning’ harvested series exhibit lower amplitude of the chlorophyll oscillations than the later harvested series. Along the same line, a second light-pulse, applied a few hours following the first light-pulse, sets a new rhythm with variable amplitude that depends on the phase of the rhythm set by the first light-signal. It can, therefore, be concluded that sowing/imbibition of seeds acts as the initial ‘Zeitgeber’ signal and that the pre-existing rhythm modulates the response of the plant to the new signal (harvest, light).