Plants with a crassulacean acid metabolism (CAM) pathway also use PEP carboxylase with its strong power of concentrating CO2. In contrast to C3 and C4 plants, though, they open their stomata and fix CO2 at night (as malic acid). During the daytime the stomata are closed and the CO2 is released within the leaf and fixed by Rubisco. However, because the CO2 is then at a high concentration within the leaf, photorespiration is prevented, just as it is in plants using the C4 pathway. Plants using the CAM photosynthetic pathway have obvious advantages when water is in short supply, because their stomata are closed during the daytime when evaporative forces are strongest. The system is now known in a wide variety of families, not just the Crassulaceae. This appears to be a highly effective means of water conservation, but CAM species have not come to inherit the earth. One cost to CAM plants is the problem of storing the malic acid that is formed at night: most CAM plants are succulents with extensive water-storage tissues that cope with this problem.
In general, CAM plants are found in arid environments where strict stomatal control of daytime water is vital for survival (desert succulents) and where CO2 is in short supply during the daytime, for example in submerged aquatic plants, and in photosynthetic organs that lack stomata (e.g. the aerial photosynthetic roots of orchids). In some CAM plants, such as Opuntia basilaris, the stomata remain closed both day and night during drought. The CAM process then simply allows the plant to 'idle' - photosynthesizing only the CO2 produced internally by respiration (Szarek et al., 1973).
A taxonomic and systematic survey of C3, C4 and CAM photo-synthetic systems is given by Ehleringer and Monson (1993). They describe the very strong evidence that the C3 pathway is evolutionarily primitive and, very surprisingly, that the C4 and CAM systems must have arisen repeatedly and independently during the evolution of the plant kingdom.
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