An endogenous rhythm in the rate of carbon dioxide output of Bryophyllum. IV. Effect of intensity of illumination on entrainment of the rhythm by cycles of light & darkness

Abstract

The rhythm of CO2 emission of excised leaves of Bryophyllum fedtschenkoi was rapidly entrained by 16:16, 20:20, and 24:24 hour cycles at both high (3000 lux) and low (25 lux) intensities. No upper limit of entrainment was observed on increasing the period of the cycles. On the other hand, when the period of the light-dark cycles was less than 24 hours the limit of entrainment was a function of light intensity. At 1000 lux entrainment occurred to 8:8, 6:6 and 3:3 hour cycles, at 500 lux it occurred to 6:6 but not to 3:3 hour cycles while at 100 lux entrainment no longer occurred to 6:6 hour cycles. The limits of entrainment are not fixed characteristics of the basic oscillating system, and the term should not be used unless the level of radiation energy employed is stated. Entrainment of the rhythm of CO2 output observed in these experiments is due to entrainment of the unidentified basic oscillating system in the leaf cells and not merely to an effect of the cycles on the biochemical process being monitored. On transferring leaves from entraining cycles to prolonged darkness a circadian period reappears, and the next peak occurs a specific time after the onset of continuous darkness. Frequency demultiplication did not occur in those experiments in which the rhythm was not entrained by the cycles. The results are discussed in relation to previous studies.