Circadian rhythms in crassulacean acid metabolism: phase relationships between gas exchange, leaf water relations and malate metabolism in Kalanchoë daigremontiana

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Abstract
Gas exchange, leaf water relations, malate content and phosphoenolpyruvate (PEP) carboxylase activity in crude extracts were examined for circadian rhythmicity in the crassulacean acid metabolism plant Kalanchoë daigremontiana. At low irradiance (20 W m-2) the rhythm in CO2 uptake continued for several days with a period length of approx. 22 h, whereas the transpiration rhythm was no longer apparent after 24 h. This shows that the CO2 rhythm in continuous light (LL) is not under stomatal control. Circadian oscillations in malate content were detectable for up to 72 h in LL but were of much reduced amplitude. This was reflected in the changes in leaf water relations, which quickly damped after transfer to LL. The activity of PEP carboxylase assayed immediately after extraction showed a rhythmicity for at least 18 h, but after 36 h, values from different plants were scattered. We suggest that the CO2-uptake rhythm is primarily the result of endogenous changes in the activity of PEP carboxylase, which competes to varying degrees with ribulose-1,5-bisphosphate carboxylase for CO2.