Carbon Dioxide and Water Vapor Exchange in the Crassulacean Acid Metabolism Plant Kalanchoë pinnáta during a Prolonged Light Period

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Abstract
Net CO2 and water vapor exchange were studied in the Crassulacean acid metabolism plant Kalanchoë pinnáta during a normal 12-hour light/12-hour dark cycle and during a prolonged light period. Leaf temperature and leaf-air vapor pressure difference were kept constant at 20 C and 9 to 10 millibar. There was a 25% increase in the rate of CO2 fixation during the first 6 hours prolonged light without change in stomatal conductance. This was associated with a decrease in the intracellular partial pressure of CO2, a decrease in the stimulation of net CO2 uptake by 2% O2, and a decrease in the CO2 compensation point from 45 to 0 microbar. In the normal light period after deacidification, leaves showed a normal light dependence of CO2 uptake but, in prolonged light, CO2 uptake was scarcely light-dependent. The increase in titratable acidity in prolonged light was similar to that in the dark. The results suggest a change from C3 photosynthetic CO2 fixation in the second part of the 12-hour light period to a mixed metabolism in prolonged light with both ribulose bisphosphate carboxylase and phosphoenolpyruvate carboxylase as primary carboxylating enzymes.