Periodical growth response of Lemna gibba G3 to light-break

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Brief exposure to light promotes frond multiplication in Lenma gibba G3 in darkness. Extent of promotion changes periodically with the time of the light-break. Response curves are interpreted in terms of a superposition of two modes of growth response to light-break, which are, respectively, under the control of different physiological timing devices; circadian oscillation and the hourglass-type clock. Circadian oscillation, which consists of a half-cycle of increasing photophily followed by another half-cycle of declining photophily, starts at a light-on signal and continues for a few days with rapid damping. The 24-hr period of oscillation is the same at temperatures ranging from 16 to 26℃. The hourglass is released by a light-off signal to 'accumulate sand' or to increase photophily in a sigmoidal way with time and is temperature-sensitive; the tempo of 'sand accumulation' being quicker at 21℃ than at 16 or 26℃. Oscillation is hastened to fade-out at 21℃, most likely due to the accelerated pace of the hourglass. Red and far-red reversrbility is disclosed in both modes of growth response.