The role of the epidermis in the generation of the circadian rhythm of carbon dioxide fixation in leaves of Bryophyllum fedtschenkoi.

The role of the epidermis in the generation of the endogenous circadian rhythm of CO2 exchange in leaves of Bryophyllum fedtschenkoi has been examined. At 25° C the rhythm of CO2 output exhibited by whole leaves kept in continuous darkness and an initially CO2-free air stream also occurs in isolated pieces of mesophyll. The sensitivity to light of the rhythms in whole leaves and in isolated mesophyll appears to be identical. At 15° C, however, no rhythm is observed in isolated mesophyll tissue, despite there being a conspicuous rhythm in intact leaves. The rhythm of net CO2 assimilation in whole leaves kept in continuous light and a stream of normal air at either 25° C or at 15° C is abolished by removal of the epidermis, although at 15° C and under the higher of the two light levels used, there is an indication that rhythmicity may begin to reappear after the third day of the experiment. Thus, only under certain environmental conditions is the rhythm of CO2 exchange in Bryophyllum leaves independent of the epidermis. The results indicate that the rhythm of carbon dioxide fixation in continuous darkness and CO2-free air is generated primarily in the mesophyll cells, whereas the rhythm in continuous light and normal air is generated in the stomatal guard cells or in an interaction of these cells with the mesophyll cells.