Crassula argentea Crassulaceae
Kalanchoe tubiflora Crassulaceae
Agave americana Crassulaceae
|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)|
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