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.

Excised leaves of the succulent plant Bryophyllum fedtschenkoi show an endogenous circadian rhythm in their rate of carbon dioxide emission (17,18). The rhythm occurs when leaves kept at a constant temperature and in an air stream initially free from carbon dioxide are transferred from light to prolonged darkness. The entrainment of the basic oscillating system in Bryophyllum leaf cells by various cycles of light and darkness and the extent to which entrainment depends upon the light intensity in the cycles are described in this paper. Exposure of an organism to cycles other than 24 hours in length can modify the period of its endogenous circadian rhythm. When this occurs the period usually becomes either equal to, or a multiple of, that of the stimulating cycles. The former phenomenon is termed "entrainment" and the latter is a special type of entrainment known as "frequency demultiplication". Both plant and animal rhythms can be entrained by cycles of light and darkness having a period other than 24 hours. In most organisms such entrainment occurs only when the amount by which the period differs from 24 hours lies within certain limits, and these appear to be different in different organisms. When the period of the light-dark cycles is less than 24 hours, clearly defined limits of entrainment have been observed in all organisms investigated. For example, the rhythm of leaf movement in Canavalia ensiformis entrains to 8: 8 but not to 6: 6 hour cycles of light and darkness (8), whereas that of petal movement in Kalanchoe blossfeldiana entrains to 6: 6 but not to 4: 4 hour cycles (5). On the other hand, when the period of the stimulating cycles exceeds 24 hours limits of entrainment have been observed in a few organisms (2,4, 12) but not in others (8,9, 10, 14, 15). Entrainment data for plant and animal rhythms have recently been tabulated and discussed by Bruce (1).