A circadian rhythm in the level of carbon dioxide compensation in Bryophyllum fedtschenkoi with zero values during the transient

Abstract

Detached shoots of Bryophyllum fedtschenkoi maintained in a closed system in the light exhibited an endogenous circadian rhythm in CO2 compensation. The rhythm was sensitive to changes in light intensity and temperature. At 15° C it damped rapidly in light of 78 J m(-2) s(-1), but at 10° C a rhythm of considerable amplitude was evident at this same light intensity. During the transient (i.e. the temporary state of the rhythm before it acquired its steady state) low compensation values between 0 and 5 ppm CO2 were achieved. When the plants were maintained at a higher light intensity prior to the measurements, the period of low compensation during the transient was extended, and zero values were obtained under some conditions.Studies of gas exchange at opposite phases of the rhythm revealed: (i) that the rate of uptake of (14)CO2 differed, both in light and darkness (the epidermis was removed during these observations to avoid interference from stomatal rhythms); (ii) 'photorespiration', estimated by extrapolation of the graph relating photosynthetic rate and CO2 concentration, was highest during the peaks of the rhythm in CO2 compensation; (iii) estimates of the capacity for 'photorespiration' by the glycine-1-(14)C assay indicated highest values during the troughs of the rhythm. These findings are discussed in relation to the C4-acid metabolism of this species. Low CO2 compensation is probably due to the activity of phosphoenolpyruvate carboxylase and not to the absence of processes involving CO2 evolution.