Succulent CAM Plants

It should be mentioned that critical values either of surface expansion or of the degree of succulence (S) have not been defined which categorize a plant as being succulent. 

the characterization of a plant as a succulent includes a certain degree of subjectivity. It is rather difficult to determine if all succulents show CAM, or to ascertain if some nonsucculents can perform the CAM carbon metabolism. Other difficulties arise from some recent discoveries (Winter and von Willert, 1972 see also Winter and Liittge, 1976), that CAM is in some cases a rather flexible system. Its occurrence and the intensity of its occurrence seems at least in some species highly dependent on environmental conditions (see Chap. 5 and 6). Thus, failure to demonstrate CAM does not necessarily mean the absence of CAM in principle, but could mean its absence under the environmental conditions prevailing during the time of observation. 

Those succulents which unify the sites of malic acid synthesis, storage (large vacuoles), and conversion (chloroplasts) all within the same cells, can be expected to have CAM (see Fig. 2.3). In contrast, those succulents where the potential malic acid stores (water cells) are spatially separated from the sites of photosynthesis will not or only very weakly perform CAM, probably because of the complexity of transport of malic acid. 

This interpretation of the relationship between the occurrence of CAM and plant succulence certainly requires more evidence. However, it agrees well with Haberlandt s (Haberlandt, 1918) concept, which interprets the evolution of anatomical structures in plants as a tendency to provide the shortest transport pathways. This principle is verified in the assimilating cells of CAM plants. 

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Ci=330- = 275ppm 2 Evergreen needles, conifers 1 Succulents. CAM plants... 

Fig. 1.25. An ornamental bed of Kleinia, a leaf-succulent CAM plant of the sunflower family...

Nordal and his associates have studied organic acids in a variety of succulent CAM plants. Phorbic acid, an 8-carbon tricarboxylic acid, was detected in Euphorbia sp., Cereus, Opuntia, Kleinia, and Portulaca (Nordal and Ogner, 1964 ... 

Lange, O.L., Zuber,M. Frerea indica, a stem succulent CAM plant with deciduous C3 leaves. Oecologia 31,67-72(1977)... 

A few species of plants are capable of reducing carbon via either the C3 or Cl, pathway and are known as Crassulacean acid metabolism or CAM plants. As a result of tiieir photosynthetic flexibility, djeir 6 values most commonly range from approximately -12 to -27 %o (1<>). CAM plants are succulents, such as the cacti, and are seldom abundant or of economic importance. 

Jahren et al, 2001). Today most C4 plants are tropical grasses, and most CAM plants are submerged aquatic plants and desert succulents. Most other kinds of plants use the C3 photosynthetic pathway. There is the potential to recognize these various metabolic pathways from the isotopic composition of organic carbon in paleosols and in fossil plants, and in the fossils of animals which ate the plants (Ceding et al, 1997 MacFadden et ah, 1999 Krull and Retallack, 2000). 

Crassulacean acid metabolism (CAM) is yet another adaptation to increase the efficiency of the Calvin cycle. Crassulacean acid metabolism, named after the genus Crassulacea (the succulents), is a response to drought as well as warm conditions. In CAM plants, the stomata of the leaves are closed in... 

Plants (e.g, cactus and other succulents) that, unlike the C3 and C4 plants, temporarily separate the processes of carbon dioxide uptake and fixation when grown under arid conditions. They take up gaseous carbon dioxide at night when the stomata are open and water loss is minimal. During the day when the stomata are closed, the stored C02 is released and chemically processed. When CAM plants are not under water stress, they then follow C3 photosynthesis. 

Stable carbon isotope analysis is particularly useful in New World dietary studies since maize is often the only C4 plant contributing significantly to human diets its contribution to bone collagen and to bone apatite may be estimated by interpolation. Some caution is warranted, however, if succulent plants were present, since they utilize the alternative CAM (crassulacean acid metabolism) photosynthetic pathway which results in carbon isotope ratios similar to those of C4 plants. Nevertheless, CAM plants are unlikely to have been major sources of dietary protein, whether consumed directly or indirectly through herbivorous faunal intermediaries. 

Fig. 1.10. Euphorbia grandidens, a cactus-like succulent of Africa in the Euphorbiaceae which has been extensively studied as a CAM plant...

Fig. 1.16. Tillandsia usneoides ( Spanish moss ), an epiphyte. This species represents a type of CAM plant which is not succulent in the usual sense...

In succulents, all basic organs of the plant can function as water reservoirs. Thus, leaf succulents, stem succulents, and very rarely root succulents can be distinguished. CAM plants are only known as members of the leaf and stem succulents. 

Succulence reflects a tendency for reducing the surface of a plant relative to its volume (see Figs. 1.6, 1.10). If we assume a perfect sphere (perhaps the ideal succulent form) with a total volume of 1000 cm, the diameter would be 12.4 cm with a total surface area of 484 cm. A disc-like leaf of 1000 cm volume and 1 mm thick would have a total surface area over 20,000 cm. Hence, the disc shape will have a surface area some 41 times greater than the spherical shape of the same volume. Thermal consequences of the spherical shape for CAM plants will be discussed later (Chap. 5.3.6). The surface volume ratio of a plant or a plant organ, as well as its water-storing capacity, is quantitatively described by an index called Oberflachenentwicklung (i.e., surface expansion see Walter, 1926 Larcher, 1973)— where... 

The first observation of the diurnal fluctuation of organic acids in succulent plants is rather difficult to assess. Osmond (1978) believes that M.Grew in 1682 was the first to realize acid accumulation in a succulent plant (Aloe). As early as 1804, De Saussure noted that darkened Opuntia did not liberate CO2, but did take up CO2. Hence, De Saussure was in fact the first who observed dark CO2 fixation in a CAM plant (see Chap. 5.1.1). The causal connection between dark CO2 fixation and acid synthesis, however, was not realized until 1948 (Thurlow and Bonner, 1948 Thomas, 1949). In 1815, Heyne noted remarkable taste differences in diurnal alterations of the leaf acidity in Bryophyllum calcycinum. He wrote ... 

In any case, CAM plants such as cacti in their natural environments respond to water by exogenous nocturnal CO2 exchange and active CAM metabolism. Many other succulent species respond to water stress by shifting from C3 photosynthesis to CAM, thus conserving water. 

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