Circadian rhythms inKalanchoë: the pathway of(14)CO 2 fixation during prolonged light.

(14)CO2 was applied repeatedly at 3- to 6-h intervals toKalanchoë daigremontiana leaves during continuous light of differing irradiances. The circadian rhythm in net CO2 uptake in gasexchange measurements and its disappearance at high irradiances was confirmed by oscillating rates of(14)CO2 incorporation. At 10-30 W m(-2) a markedly circadian oscillation in the(14)CO2-uptake rate was measured; with increasing energy fluence rate the oscillation levelled off at a constant high uptake rate. The labelling patterns obtained during the 10 min of(14)CO2 fixation indicated that the rhythm of CO2 exchange is the consequence of a rhythmic behaviour in the C4 pathway of CO2 fixation. During the mininum of(14)CO2 uptake no C4 products were labelled; however, substantial amounts of label were transferred to C4 products during the peaks of(14)CO2 uptake. Metabolism of C3 and C4 products was also studied in pulsechase experiments at different points of the circadian cycle. In bright light (100 W m(-2)), when the(14)CO2 uptake was constantly high, the transfer of label into C4 products (malic acid) was high in spite of the fact that the malate pool is known to be reduced to a permanently low level under these conditions. This led us to the conclusion that it is not the capacity of the phosphoenolpyruvatecarboxylase-mediated CO2 fixation but rather the storage of malic acid in the vacuole that is disturbed under bright-light conditions when the circadian oscillation levelled off.