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

METABOLIC AND STOMATAL CONTROL OF CARBON METABOLISM

Department of Environmental Biology, Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra City, A.C.T. 2601, Australia

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.

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				H. Griffiths, W. E. Robe, J. Girnus, and K. Maxwell Leaf succulence determines the interplay between carboxylase systems and light use during Crassulacean acid metabolism in Kalanchoe species J. Exp. Bot., May 1, 2008; 59(7): 1851 - 1861. [Abstract] [Full Text] [PDF]