Crassulacean Acid Metabolism. 

Crassula argentea Crassulaceae
Kalanchoe tubiflora Crassulaceae
Agave americana Crassulaceae 


 
 Many plants living under arid or semi-arid conditions employ an addition to normal carbon dioxide fixation known as Crassulacean Acid Metabolism or CAM. These are mainly succulents (such as those shown here) but also some epiphytes such as Tillandsia usneoides and orchids which, although they may live in humid conditions, can still suffer water stress due to a reliance on aerial roots for water absorption.  Families which include CAM representatives are listed in Table 1.
Land 
Plants
Aquatic Plants
 
Major Families
Minor Families
 
 
Agavaceae 
Asphodelaceae 
Orchidaceae 
Bromeliaceae 
Aizoaceae 
Didiereaceae 
Cactaceae 
Clusiaceae 
Crassulaceae 
Euphorbiaceae 
Asclepiadaceae 
 
 
 
 
 

Polypodiaceae 
Vittariaceae 
Welwitschiaceae 
Dracaenaceae 
Commelinaceae 
Piperaceae 
Portulacaceae 
Cucurbitaceae 
Passifloraceae 
Vitaceae 
Oxalidaceae 
Geraniaceae 
Asteraceae 
Apocynaceae 
Rubiaceae 
Gesneriaceae 
Lamiaceae 
Isoetaceae 
Alismataceae 
Hydrocharitaceae 
Crassulaceae 
Plantaginaceae 
Apiaceae 
 
 
 
 
 
 
 
 
 
 

Table 1. Plant families containing species capable of crassulacean acid metabolism (CAM).  Major families are those containing either a high proportion of species showing CMA or a large number of CAM species.  Minor families are those with only a small number of CAM species or species showing only weak CAM activity.  Nomenclature for the family names follows Dahlgren et al (1985) for the monocotyledons and Cronquist (1981) for the dicotyledons.  Within each column, families are listed in the order suggested by the phylogeneitc analysis of Chase et al (1993)
 This phenomenon was first reported in 1815 by Heyne who noticed that leaves of Cotyledon calycina tasted bitter in the morning but were tasteless by noon. Why he spent his time tasting leaves of plants is not recorded!

We now know that this is due to an accumulation of malic acid during the night which is then broken down during the day. This malic acid is formed by carbon dioxide fixation during the night using the enzyme phosphoenolpyruvate carboxylase and when it is broken down within the cells during the day, the carbon dioxide which is released can be re-fixed by the normal Calvin cycle to produce sugars.

Figure 1. Simpliefied schematic outline of the CAM cycle showing temporal organization of the principal metabolic steps.

So how does this benefit the plant?

Carbon dioxide is taken in through stomata with the simultaneous loss of water vapour.  By only requiring the stomata to be open during the (cooler) night for carbon dioxide uptake and keeping them closed during the (hotter) day, the plants lose much less water.  In extremely arid conditions, some CAM plants can permanently close their stomata so preventing any water loss and re-cycle the carbon dioxide within the cells (known as CAM-idling).  However this does not allow any net growth but can maintain the healthy state of the cells.
 

Dr Tom Ford

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November 10th, 1997
David Green