Crassulacean Acid Metabolism

Ulrich Lüttge, Darmstadt University of Technology, Darmstadt, Germany

Published online: December 2008

DOI: 10.1002/9780470015902.a0001296.pub2

Crassulacean acid metabolism (CAM) is a carbon dioxide acquisition, carbon dioxide transient storage and carbon dioxide concentrating mechanism of plants based on organic acid synthesis. In this variant of photosynthesis carbon dioxide can be fixed nocturnally in the dark and is used during the day for assimilation in the light. This has arisen polyphyletically many times during evolution. It is an ecophysiological adaptation that allows carbon dioxide acquisition with exceptionally economic use of water. It is regulated in a natural night/day rhythm but can also oscillate freely under the control of circadian biological clocks.

Key concepts

  • Metabolic mechanisms concentrating carbon dioxide internally in plants are important for photosynthesis at the low external carbon dioxide concentration in the present atmosphere.
  • Rearranged internal management of available metabolic housekeeping functions can generate new metabolic options of adaptive value.
  • Generation of isoforms of housekeeping enzymes can facilitate polyphyletic evolution of new metabolic pathways.
  • Circadian oscillations can be controlled by different biological clocks, which are feedback related to each other and present in many copies.
  • Flexibility in the expression of metabolic pathway variants facilitates adaptation under variable stress situations.
  • Stress adaptation in plants is for survival and mostly not for high productivity.

Keywords: circadian rhythmicity; carbon dioxide acquisition; malate; phosphoenolpyruvate carboxylase; vacuolar H+ ATPase

Figure 1. Crassulacean acid metabolism (CAM) – a carbon dioxide acquisition, carbon dioxide transient storage and carbon dioxide-concentrating mechanism. Simplified metabolic scheme of carbon flow, metabolic pathway and compartmentation. Left part: dark period; right part: light period; C, cytosol; M, mitochondrion; P, plastid (chloroplast); V, vacuole. The following abbreviations are used: AcCoA, acetyl-coenzyme A; ADPG, adenosine diphosphate glucose; [CH2O]n, carbohydrate; Citr, citrate; CoASH, coenzyme A; Di-PGA, 2,3-diphosphoglyceric acid; KGA, -ketoglutaric acid; Mal, malate; OAA, oxaloacetate; Pi, inorganic phosphate; PPi, inorganic pyrophosphate; PPCox, oxidative pentose phosphate cycle; PPCred, reductive pentose phosphate cycle (Calvin cycle); PEP, phosphoenolpyruvate; Pyr, pyruvate; TCA-C, tricarboxylic acid cycle; UDP and UTP, uridine-di-phosphate and uridine-tri-phosphate. Numbers refer to key enzymes highlighted in the text: (1) PEP-carboxylase (PEPC); (2) H+-transporting V-ATPase at the tonoplast; (3) inward rectifying malate channel at the tonoplast; (4) NAD(P)-dependent malic enzymes; (5) isocitrate dehydrogenase; (6) pyruvate, Pi dikinase (PPDK) and (7) NAD:glyceraldehyde-3-phosphate dehydrogenase.
Figure 2. Diurnal course of net carbon dioxide exchange (JCO2), leaf-conductance for water vapour (gH2O) and malate levels (per gram fresh weight, gFW) in a leaf of the CAM-plant Kalanchoë daigremontiana, with nocturnal (dark bar on top of the graph) stomatal opening, carbon dioxide uptake and malate accumulation (phase I); an early morning peak of carbon dioxide uptake (phase II); daytime malate remobilization and stomatal closure (phase III); and afternoon stomatal opening, carbon dioxide uptake and assimilation in the Calvin cycle (phase IV).
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 Further Reading
    Adams P, Nelsen DE, Yamada S et al. (1998) Growth and development of Mesembryanthemum crystallinum (Aizoaceae). New Phytologist 138: 171–190.
    book Kluge M and Ting IP (1978) Crassulacean Acid Metabolism. Analysis of an Ecological Adaptation. Berlin: Springer.
    Lüttge U (1987) Carbon dioxide and water demand: crassulacean acid metabolism (CAM), a versatile ecological adaptation exemplifying the need for integration in ecophysiological work. New Phytologist 106: 593–629.
    book Lüttge U (ed.) (1989) Vascular Plants as Epiphytes. Evolution and Ecophysiology. Berlin: Springer.
    Lüttge U (1993) The role of crassulacean acid metabolism (CAM) in the adaptation of plants to salinity. New Phytologist 125: 59–71.
    Ting IP (1985) Crassulacean acid metabolism. Annual Review of Plant Physiology 36: 595–622.
    Wilkins MB (1992) Circadian rhythms: their origin and control. New Phytologist 121: 347–375.
    book Winter K and Smith JAC (eds) (1996) Crassulacean Acid Metabolism. Biochemistry, Ecophysiology and Evolution. Berlin: Springer.

Related Sub-Topics:

Plant Responses to the Environment | Adaptive Evolution | Diversification of Plants and Animals | Evolution of Novelty | Evolutionary Ecology | Physiological Ecology | Organisms and the Physical Environment | Plant Evolution | Photosynthesis
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Article Title: Crassulacean Acid Metabolism
 
How to Cite close
Lüttge, Ulrich(Dec 2008) Crassulacean Acid Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001296.pub2]