An endogenous rhythm in the rate of carbon dioxide output of Bryophyllum V. The dependence of rhythmicity upon aerobic metabolism.

Excised leaves of the succulent plant Bryophyllum fedtschenkoi exhibit a circadian rhythm of carbon dioxide metabolism in prolonged darkness. The extent to which the basic oscillating system controlling this rhythm depends upon aerobic metabolism has been ascertained by depriving the leaves of oxygen for various times at different phases of the cycle. Scanning the cycle with a 6-hour period of anaerobic conditions revealed a permanent phase shift to be induced only when oxygen deprivation occurs between the peaks of carbon dioxide output when the oxygen dependent "dark" fixation of carbon dioxide is occurring at a maximum rate. In this part of the cycle the delay induced is approximately equal to the duration of the treatment. The delay of the first post-treatment peak is slightly greater than that of subsequent peaks. No delay is induced by depriving the leaves of oxygen for up to 6 hours near the apex of a peak of the rhythm of CO2 output when the rate of "dark" fixation of carbon dioxide is either very low or zero, but further prolongation of the treatment induces a delay which is less than the length of time for which the leaves are deprived of oxygen.These results are discussed in relation to the mechanism of the oscillating system in leaves of Bryophyllum.