Crassulacean Acid Metabolism


Crassulacean acid metabolism (CAM) is a carbon dioxide acquisition, transient storage and concentrating mechanism of plants based on organic acid synthesis. Amongst 350 000 species of vascular plants, 21 000 species perform CAM. In this variant of photosynthesis, carbon dioxide can be fixed nocturnally in the dark and is stored in the form of organic acids from which it is remobilised during the day for assimilation in the light. This has arisen polyphyletically during evolution. It is an ecophysiological adaptation that allows carbon dioxide acquisition with exceptionally economic use of water. CAM allows acclimation to a variety of interacting stresses. It is an adaptation for survival and not for high productivity. Nevertheless, performance of CAM species on drought‐prone marginal lands makes them potential alternative crop plants and biofuel plants. CAM 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.
  • Massive nocturnal CO2 uptake, fixation and storage in the form of organic acids result in inorganic carbon acquisition with high water‐use efficiency (WUE).
  • 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.
  • Resistance to drought stress and high water‐use efficiency supports the quest for new crop plants including biofuel crops to be grown on marginal land.

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; αKGA, α‐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 courses 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 (Hamet et Perrier de la Bâthie). The dark grey bar on top of the graph indicates the night or dark period with nocturnal stomatal opening, that is, high gH2O, carbon dioxide uptake, that is, high JCO2 and malate accumulation (phase I). An early morning peak of carbon dioxide uptake and stomatal opening between 8 and 9 h represents phase II of CAM. This is followed by daytime malate remobilisation and stomatal closure with negligible JCO2 and gH2O (phase III). In the afternoon gH2O increases due to stomatal opening and there is carbon dioxide uptake and assimilation in the Calvin cycle (phase IV).


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Further Reading

Hartwell J (2006) The circadian clock in CAM plants. In: Hall A and McWatters H (eds) Endogenous Plant Rhythms, pp. 211–236. Oxford: Blackwell.

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.

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.

Lüttge U (2004) Ecophysiology of crassulacean acid metabolism (CAM). Annals of Botany 93: 629–652.

Lüttge U (2007) Clusia. A Woody Neotropical Genus of Remarkable Plasticity and Diversity. Ecological Studies, vol. 194. Berlin ‐ Heidelberg ‐ New York: Springer.

Lüttge U (2012) Ecophysiology of CAM photosynthesis. Chapter 6. In: Flexas J, Loreto F and Medrano H (eds) Terrestrial Photosynthesis in a Changing Environment. A Molecular, Physiological and Ecological Approach, pp. 71–84. Cambridge: Cambridge University Press.

Ting IP (1985) Crassulacean acid metabolism. Annual Review of Plant Physiology 36: 595–622.

Winter K and Smith JAC (eds) (1996) Crassulacean Acid Metabolism. Biochemistry, Ecophysiology and Evolution. Berlin: Springer.

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Lüttge, Ulrich(Nov 2015) Crassulacean Acid Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001296.pub3]