Light-independent endogenous circadian rhythm in the capacity for chlorophyll formation

The level of Protochlorophyll(ide) accumulating in leaves of 10-12 day old etiolated Phaseolus vulgaris plants, which were sown every 4-6 h during 2-3 consecutive days, was found to follow circadian oscillations. Similar oscillations in the accumulation of PChl(ide) were observed in etiolated leaves of plants obtained from the same sowing, but harvested every 4-6 h for 2-3 consecutive days and kept thereafter in petri dishes in the dark prior to synchronous pigment extraction. Parallel circadian oscillations were observed in the capacity for Chi synthesis and accumulation, after synchronous exposure of these leaves to light for 4 h. Individual rhythms obtained from series of plants sown 6 h apart during a day, then harvested and kept in the dark prior to exposure to light, were found to fall in phase, in cases where leaves were exposed to light at the respective time of day at sowing. Otherwise, normalization as to the delay between sowing and exposure to light was necessary. The results suggest that the capacity of the etiolated plant to synthesize and accumulate Chl in the light, is under control of a light-independent endogenous oscillator. Germination offers the means for expression of the oscillator, while harvest of cotyledons ''freezes'' the hands of the clock.