An Endogenous Rhythm in the Rate of CO2Output ofBryophyllum

Abstract

SUMMARY The effects of light, darkness, and changes in light intensity on the phase and period of the endogenous rhythm in the rate of C02 output of excised leaves of Bryophyllum fedtschenkoi have been examined. The duration, intensity, and wavelength of a short light treatment, and the point in the cycle at which it is administered, determine the degree of phase shift induced in a rhythm persisting in darkness. When light treatments of 3 and 6 hours' duration, at an intensity of 3,000 lux, are applied between the peaks the phase is completely reset, the first post-treatment peak occurring 18-19 hours after the end of the treatment. The degree of phase shift is therefore determined not by the duration of the treatment but by the time at which the treatment ter minates. One hour's illumination has little or no effect. The phase is unaffected when light treatments of up to 5 hours' duration at an intensity of 3,000 lux are applied at the crest of a peak. Over the range 8-3,000 lux the intensity of light during a 6-hour treatment applied between the peaks does not affect the efficiency with which that treatment completely resets the phase. At an intensity of 2 lux, however, the phase delay is equal to the duration of the treatment. A 6-hour red-light treatment (850 ergs/cm.2/sec.) applied between the peaks completely resets the phase whereas blue light (10,860 ergs/cm.2/sec.) has no effect on the phase but induces a slight protraction of the period. Moreover, continuous red light inhibits the rhythm, which recommences in blue light. A rhythm is induced in illuminated leaves when the light intensity is either gradually or suddenly reduced by at least 80 per cent. Whether a given intensity of illumination inhibits or permits the persistence of a rhythm depends upon the light intensity by which it is immediately preceded. A rhythm will persist in illuminated leaves for approximately as long as in leaves in darkness and the phase shows no correlation with time of day. The period is unaffected by the intensity of white light (from 0-500 lux) to which the leaves are subjected. The duration of a short dark treatment, and the point in the cycle at which it is applied, determine the degree of phase shift induced in a rhythm in illuminated leaves. The phase is reset when 3-, 6-, and 9-hour dark treatments are applied at the crest of a peak, the amount of phase shift increasing to a maximum with 9 hours' darkness. The phase shift is not equal to the dura tion of the treatment. The phase is unaffected when 3- and 6-hour dark treat ments are applied between the peaks. The variation in the sensitivity of the phase of a rhythm persisting in darkness to short light treatments is in the opposite sense to that of a rhythm persisting in