Crassulacean acid metabolism

The Crassulacean acid metabolism ( CAM short of Crassulacean Acid Metabolism ) is a special metabolism of various plants. While most plants carry out the uptake and fixation of carbon dioxide a day, these operations are separated in CAM plants with one another in time. The carbon dioxide needed for photosynthesis is hereby incorporated in the night, and stored chemically in the form of malic acid in the vacuoles of the cell. The following day, the carbon dioxide from the malic acid is released again and supplied to the structure of carbohydrates in the Calvin cycle. Because of the daily assembly and disassembly of these dicarboxylic acid and the associated proton amount in the alternation of day and night of this metabolism is also known as diurnal acid rhythm (Latin diurnus = "daily" ).

The advantage of the CAM mechanism is that the plant can leave their stomata closed during the ( hot ) day hours, thereby significantly loses less water through transpiration and yet always has a sufficient amount of carbon dioxide in the Calvin cycle. In addition, CAM is a favorable response to low carbon dioxide concentrations, as it can be observed, for example, even when submerged freshwater plants. This type of metabolism has been detected only in chlorophyllhaltigem tissue.

Named the Crassulacean acid metabolism is by the family of the Crassulaceae ( Crassulaceae ), in which it was first discovered.

• 3.1 obligate or constitutive CAM
• 3.2 Optional or inducible CAM ( C3 -CAM)
• 3.3 CAM - idling ( idling )
• 3.4 CAM " change" ( cycling )

• 7.1 isotope discrimination
• 7.2 Comparison between C3, C4 and CAM plants

History

At the beginning of the 19th century, certain aspects of Crassulacean acid metabolism were noticed. So observed the Swiss naturalist de Saussure in 1804, that the branches of Opuntia also produce oxygen in the light when no carbon dioxide present in the air; This is not the case in C3 plants. He concluded that the plant consumes substances produced under CO2 release. 1813 Benjamin Heyne discovered the diurnal acid rhythm at the Goethe- plant through a self- test: He noticed that the leaves in the morning extremely sour tasted, but in the afternoon had an herbal flavor. Heyne reported this observation of the Linnean Society and later published these findings.

The term Crassulacean Acid Metabolism was first used in January 1947 by Meirion Thomas ( 1894-1977 ) in a lecture to the Society for Experimental Biology. 1949, the term was part of the fourth edition of his standard work Plant Physiology. Middle of the 20th century was the elucidation of the Crassulacean acid metabolism mainly by the work of Walter Daniel Bonner ( 1878-1956 ) and his son James Frederick Bonner ( 1910-1996 ), Meirion Thomas, Thomas Archibald Bennet -Clark ( 1903-1975 ) and other researchers pursued.

Mechanism

Distinct from the C3 and C4 plants

For the synthesis of carbohydrates operate every green plant photosynthesis. Here, in the " dark reaction " carbon dioxide ( CO2) is fixed and up to carbohydrates. Most plants ( C3 plant ) conduct a described as C3 metabolism mechanism is passive in which carbon dioxide passes through the stomata in the cells and fixed as a substrate during the day in the Calvin cycle. An adaptation of this mechanism can be found in C4 plants, including energy consumption increase the active and therefore the concentration of CO2 for fixation. Here is a spatial separation (two cell types, mesophyll and Leitbündelscheidenzellen ) for the pre-fixing and metabolism of carbon dioxide instead. This allows the plants to close their stomata, partially because they are, in contrast to C3 plants is not restricted by the simple diffusion of carbon dioxide in the cells. When the stomata are partially closed, this also reduces the outflow of water from the plant. Therefore, one finds C4 plants preferably in dry and sunny places. However, the CO2 fixation in the Calvin cycle corresponds to that of C3 plants.

In order to survive in arid regions have CAM plants have mechanisms to separate in time the steps of CO2 fixation from those of the Calvin cycle. This allows the stomata remain closed during the heat of the day to minimize water loss. In the cooler night they will be open to the CO2 uptake. While carbon dioxide is fixed in the night to malate and stored in the vacuole, it is released on the following day and implemented by RuBisCO, the key enzyme of the " dark reaction ", analogous to a C3 plant. These biochemical reactions take place in a cell.

Kohlenstoffdioxidfixierung at night

At night, the CO2 fixation on phosphoenolpyruvate ( PEP) is similar by a cytosolic PEP carboxylase ( PEPC, EC 4.1.1.31 ) as in C4 plants. The enzyme condensed hydrogen carbonate ( HCO3- ) at PEP. The required PEP is usually provided by the degradation of starch, other plant species used for soluble sugars such as sucrose and fructans. CAM plants must be able to provide large amounts of PEP - so much that one night ration can be fixed to carbon dioxide. To this end is filled up during the day of the starch - storage which is capable to provide night PEP.

In aqueous solutions, although CO2 is in equilibrium with HCO3-, but this reaction is slow. A carbonic anhydrase (CA, EC 4.2.1.1 ) accelerates the equilibrium in favor of HCO3-, which is then converted by the PEPC to oxaloacetate (OA ). OA formed during the reaction is reduced by also cytosolic NAD - malate dehydrogenase (MDH, EC 1.1.1.37 ) to L- malate. This NADH is oxidized to NAD .

The malate is transported into the vacuole and stored there. The vacuoles of CAM plants are compared with other types of very large and can build up, which contributes to an overall high storage capacity of a comparatively high concentration of malate ( up to 0.2 M). Landfilling of malate in the vacuoles also keeps the pH in the cytosol constant.

In order to transport malate against the concentration gradient in the vacuole, two protons per malate are brought into the vacuole under ATP consumption. This catalyzes a membrane-located at the tonoplast ATPase of type V. By a secondary active transport malate follows the protons by a Dicarboxylattransporter. Storage is therefore in the protonated form of malate, malic acid, because the malic acid is present mainly in undissociated form with the very acidic conditions. This also has the advantage of resulting lower osmotic value. In the vacuoles of the guard cells potassium malate is stored, for example in the vacuole, which causes a relatively higher osmotic pressure. While the pH of the vacuoles of many plants, C3 and C4 is 5.5, in the CAM whether the plant increased acid concentration can be reached at night pH values ​​of 3.0. Therefore, the leaves of this plant taste in the morning very sour.

Release of carbon dioxide a day

On the day this way is formally reversed and released the carbon dioxide again. The stored malate is transported probably by a carrier- mediated transport of the vacuoles into the cytosol, in which two protons are transported. Maybe malic acid can diffuse than uncharged particles directly through the membrane. During the daytime, therefore, the pH value increases in the vacuole again, it reaches values ​​of 7.5-8.0. This process is also known as acidification.

The degradation of malate and the release of carbon dioxide is the same as in C4 plants depending on the nature in different ways, reflecting the independent training of CAM in different taxa. The usual ways are the NADP-dependent malic enzyme (with cacti plants, Agavengewächsen ), the NAD-dependent malic enzyme ( in the Crassulaceae family ) and phosphoenolpyruvate carboxykinase (eg bromeliad plants, lily family, Asclepiadaceae ).

• In plants with the NADP-dependent malic enzyme ( NADP -ME, EC 1.1.1.40 ) is decarboxylated the transported out of the vacuole L- malate to pyruvate. These reactions take place in the chloroplast. This NADP is reduced to NADPH.
• Some plants have a mitochondrielles, NAD -dependent malic enzyme (NAD -ME ​​, EC 1.1.1.39 ), which uses NAD rather than NADP as a cofactor. The reactions are analogous to the above-described NADP -ME. However, pyruvate is not returned directly in the mitochondrion to the cytosol, but first transported via the intermediate of L- alanine in the chloroplasts. There it is built to PEP and brought into the cytosol later. The amination to alanine and its reverse reaction catalyzed alanine transaminase ( EC 2.6.1.2 ).
• CAM plants of PEP carboxykinase type have two cycles. In addition to a NAD -ME ​​- circuit these also have a cytosolic PEP carboxykinase ( PEPCK; EC 4.1.1.49 ). L- malate is first oxidized by an NAD this malate DH (MDH, EC 1.1.1.37 ) to oxaloacetate under consumption of NAD . Oxaloacetate is then decarboxylated directly to PEP by PEPCK, this happens with consumption of ATP.

The liberated by the above-mentioned reactions carbon dioxide is introduced into the Calvin cycle and up to 3 -phosphoglycerate, which results in the further formation of monosaccharides (D -glucose, D -fructose ) or polysaccharides (sucrose ) to itself. The resulting in the decarboxylation of pyruvate is converted by a pyruvate phosphate dikinase ( PPDK, EC 2.7.9.1 ) with ATP to PEP. PEP is finally built up in the course of gluconeogenesis to starch, sucrose or fructan.

Since the majority of the carbohydrates formed is used to fill in the night before starch consumed inventories, the income of the CAM photosynthesis is low. In addition, the amount of malate storage is limited by the size of the vacuoles. As a result, the increase in biomass is limited.

Through the setting up and dismantling of malate to strong changes in pH from pH 7.5 to pH 3 result on the day into the night. Because of this change of day and night we therefore speak of a diurnal acid rhythm. This rhythm was first identified in 1984 in the breeding Kalanchoe daigremontiana leaf.

Citric acid in ice plant and is stored in the vacuoles addition of malic acid.

Forms of CAM

CAM may be classified into different types. A distinction is made, when the stomata are open. In addition, you can consult the amount of malic acid formed during the night and the CO2 uptake to disposition.

Obligate or constitutive CAM

The metabolic mechanism obligate CAM plants was further shown in detail above. This open their stomata always during the night, when even 99% of the CO2 fixation takes place. Depending on the strength of the CO2 uptake and subsequent malic acid formation, a distinction is in strong and weak obligate CAM. Typical representatives of the obligate CAM can be found for example in the Crassulaceae family ( Kalanchoe daigremontiana ) and the cactus plants ( Opuntia basilar and Opuntia ficus-indica ). In contrast to the other types of CAM these plants remain always obligatory CAM mode, regardless of the type and strength of abiotic stress. However, it has been observed that very young seedlings of obligate CAM plants, like, still can run a C3 metabolism, for example, at the Goethe- plant, Opuntia ficus- indica and Kalanchoe daigremontiana.

CAM in said plants can be divided into four phases. In Phase I, at the beginning of the night, the stomata are opened and carbon dioxide by PEPC fixed (see above). At the end of Phase I close the stomata. Some plants then enter phase II, in which open into the early, not dark hours of the morning, the stomata again. As atmospheric carbon dioxide passes into the plant, which is in particular fixed by the PEPC and additionally by RuBisCO. Light and low cytosolic CO2 levels are a signal to re-open the stomata. Later, the CAM plant enters Phase III, the stomata remain closed (see above). This malate is decarboxylated by the routes described above, so that the cytosolic concentration of CO2 rises sharply. This is a signal to close the stoma. During the afternoon, malate is consumed, so that the cytosolic concentration of CO2 falls again. Since light is still present, the stomata are open analog in Phase II again. This is Phase IV In this stage, the photorespiration is highest. Finally, at nightfall reaches the plant again in Phase I.

Phase II and IV provide the CAM plant an additional CO2 uptake, which is associated with faster growth. However, this is dependent on the availability of water, so that many obligate CAM plants Phase I and III dominate.

Optional or inducible CAM ( C3 -CAM)

It has been observed that some plants can have both a CO2 fixation in the sense of a C3 plant as well as a CAM plant. Representatives are often found in Mesembryanthemum plants, Crassulaceae family ( sedum ), Portulakgewächsen ( Calandrinia species), grapevine plants and many species of the genus Clusia. The best-studied plant with facultative CAM is the ice plant. Additional representatives bacon tree or Agave deserti. These plants are found in semi-arid regions, on rocky ground, on tree branches, and generally in habitats where water is scarce.

In the uninduced state, the CO2 uptake is analogous to a normal C3 plant on the day with open stomata. There is no malic acid formed during the night. By induction, but there is a nocturnal CO2 fixation and malic acid formation analogous to a weak obligate CAM plant. The induction is determined by drought ( during droughts ), salinity, high light levels, nitrogen and phosphate deficiency triggered. Sometimes it also depends on the age of the plant. If some seedlings still insufficient have large vacuoles, photosynthesis is carried out in terms of a C3 plant. Only in advancing age, the plant switches to CAM. Photoperiodism in Kalanchoe blossfeldiana induced switching between C3 and CAM.

The change between CAM and C3 - CO2 fixation is quick and usually reversible. Thus, rising in one day at temperatures and poorer water supply are switched to CAM. Sometimes also operates a part of the plant CAM while - operates another C3 photosynthesis - at least under good water supply. This is for example the case with Frerea indica, in which the stem succulent CAM has, the leaves, however, a C3 metabolism. In tropical Clusia species, it may even happen that a sheet of C3 metabolism, an opposite but CAM performs. Some facultative CAM plants can also operate their lives a C3 metabolism, without ever having to switch to CAM. Thus, in Clusia cylindrica is the C3 metabolism the major route for CO2 fixation.

When ice plant, a year-long salt plant, the change from C3 to CAM is irreversible under natural conditions. The plant grows in the Mediterranean, among others, and changes in water scarcity and salt stress on CAM. The latter is a response to associated water shortages, less because of the increased number of ions. It has also been found that the change to the CAM takes place even without these two stress factors in older plants. Salt and water stress accelerate only a programmed development in ice plant, which inevitably leads to the CAM. Also, the light intensity and quality can induce CAM in Mesembryanthemum. Under controlled experimental conditions, however, a change from CAM back to C3 metabolism could be observed in younger plants.

In the family of Portulakgewächse also a change of the CO2 fixation of C4 to the CAM has been observed. To set portulaca, summer purslane, Portulaca Mundula, but also Peperomia camptotricha at water scarcity their metabolism to CAM in order. However, the C4 and CAM metabolism is not carried out in one and the same cell.

CAM - idling ( idling )

When suffering from extreme water scarcity plants the stomata are closed to keep the water loss as low as possible both during the day and at night. It is the reattached during respiration released carbon dioxide, so that the nocturnal malic acid formation is low. During this stressful situation photosystems remain intact, the plant is "waiting" for better conditions and is thus in a kind of " idle ". Yet even here a diurnal acid rhythm takes place. This was first observed in Opuntia basilar artery.

There is no net carbon uptake, so that plants do not grow in this state, but they can survive.

CAM " change" ( cycling )

This is also referred to as " almost C3" mode, the stomata remain closed during the night. The released through respiration carbon dioxide is fixed, thereby small amounts are stored in malic acid in the vacuole. On the day of the CO2 fixation is carried out in terms of C3 photosynthesis. In addition, the malate formed at night is decarboxylated. Therefore, the plants show despite the open stomata during the day a diurnal acid rhythm. One can find plants with such a CAM type at sites where facultative CAM plants grow. Representative of this type are CAM Peperomia camptotricha or Talinum calycinum latter grows on dry, rocky soil. In Isoetes howellii, an aquatic plant, CAM -cycling is particularly pronounced. The epiphyte Codonanthe crassifolia operates CAM -cycling with good water supply, but up to when drought in the CAM - idling.

Regulation

Fixation of carbon dioxide by the PEP carboxylase ( PEPC ) during the day is prevented by an allosteric inhibition of the enzyme: With the flowing of the vacuole malate and a low pH - conditions a day - the PEPC is effectively inhibited. This is to prevent carbon dioxide is used as a substrate for an unnecessarily reaction other than of RuBisCO. L- Aspartate inhibits PEPC, whereas glucose-6 -phosphate and Triosephosphate activate the enzyme. When ice plant is encoded by two genes for PEPC, Pepc1 and Pepc2. Whose expression is affected by drought, abscisic acid and high salinity.

The enzyme activity is, however - in addition regulated by reversible phosphorylation at a serine residue - as in C4 plants. Thus, the dephosphorylated form of the PEPC 10-fold more sensitive to malate as the phosphorylated and active form during the night. Phosphorylation is mediated by a kinase, the PEP carboxylase kinase ( PPCK1 ). This is subject to rigid, circadian rhythm of their gene expression and, due to a rapid assembly and dismantling, even to its overall activity. The expression reached a peak in the middle of the night and then falls during the day down to a minimum level. This is not dependent on light.

The responsible for the dephosphorylation phosphatase ( type 2A ), however, shows no such rhythm in their activity.

During the Phase II of the CAM (see above) is not phosphorylated and PEPC active as malate is present stored in the vacuole. During Phase IV malate is mostly consumed and PEPC can not inhibit.

Occurrence

The CAM mechanism was detected in 34 plant families and 343 genera. It is estimated that the metabolic pathway occurs in more than 16,000 species, which corresponds to 6-8 % of all vascular plants. In addition to the eponymous family of the Crassulaceae (such as the Goethe- plant) are more families with CAM species:

• Asphodelus, such as the Aloe vera
• Agave
• Bromeliads, such as the pineapple
• Clusiaceae
• Didiereaceae
• Some epiphytic ferns, as Pyrrosia piloselloides and Pyrrosia longifolia, are CAM plants.
• Dog poison plants
• Cactus, such as Echinocactus grusonii
• Composites
• Lily plants
• Mesembryanthemum plants, such as the living stones
• Orchids, such as vanilla, and Phalaenopsis
• Portulakgewächse
• Grape Family
• Spurge.

Well-known representatives are the pineapple, Kalanchoe, but also the Central European sedums and Sempervivum species ( Sedum and Sempervivum ).

Many species occur in periodically dry areas, especially in deserts and savannas of the tropics and subtropics. However, savannas are not typical habitats of these plants. There, CAM plants are often overgrown by C3 or C4 grasses. One finds CAM plants often in coastal regions or near salt lakes, although they usually avoid salt and do not really belong to the salt plant. CAM plants grow in Restingas. These are coastal subtropical or tropical moist rain forests with sandy and nutrient-poor soil, which can be found in Brazil. Water shortage is also indicative of the locations of epiphytes in rain forests. Therefore, many epiphytes CAM plants, estimated to be between 50 to 60 % of all epiphytic orchids and bromeliads. But CAM metabolism also occurs in some freshwater plants, so the water Crassula and the European Ling beach, and in some species of the genus Brachsenkräuter ( Isoetes ) as the Lake quillwort. Finally, one finds CAM plants on inselbergs and at high altitudes, such as in the Alps (Mountain Houseleek) or in the Andes ( Echeveria quitensis, Oroya peruviana ).

Morphology

There is no typical appearance of a CAM plant because they are morphologically very different. All CAM plants, however, are characterized by a more or less pronounced succulence and have particularly large vacuoles. The latter is also referred to as " succulence at the cellular level ." The vacuoles may have up to 98% of the cell. Thus, the storage of water, and in particular of the Malate CAM cycle is generally allows. The succulent a CAM plant, the stronger is the CO2 uptake at night. In terms of water savings have these plants also have a thick cuticle, smaller stomata and a small surface / volume ratio.

Morphotypes of CAM plants can be divided differently. There are leaf succulents such as in Kalanchoearten. Sometimes these have a non- green, water-storing tissues that order is below or right in the mesophyll. In addition to leaf succulents are also found stem succulents that are characteristic especially in arid regions and deserts. These have a central, water-storage tissue in the middle of stem. Epiphytes form a distinct physiognomy. Besides strangling and climbing plants, there are also epiphytic thornless cactus. Frequently CAM plants are rosette plants with specializations such as the phytotelmata some bromeliads.

A special feature is the genus Clusia, dicotyledonous tree -forming species can operate the CAM. These are currently the only known trees with CAM. Through the photosynthetic plasticity of the genus Clusia has established in various ecosystem. Clusia species themselves differ in the expression of diverse CAM ( see also section forms of CAM). Some Clusia species, such as Clusia multiflora and Clusia cretosa, whereas pure C3 plants.

The Joshua tree lily even forms a "tree", but this is an example of secondary growth in monocots and should not be confused with secondary growth in dicotyledons.

Importance of CAM

The ecological advantage of CAM is that the CO2 uptake and thus the opening of the stomata during the night is done while they remain closed during the day and thus the water loss is greatly reduced. It follows the day an extremely low water consumption per dry weight of 18-100 ml · g -1, in contrast to 450-950 ml · g -1 in C3 plants. So that the water requirement is only about 5 to 10% in comparison with the case C3 plant. CAM is thus a successful strategy to operate despite the big water shortage photosynthesis can. This is also expressed by the water use efficiency (water -use efficiency, WUE ). The higher the efficiency, the lower the gain is carbon (see table below). In particularly severe drying the stomata remain closed during the night and it will only the released carbon dioxide by breathing reattached (CAM - idling ). This allows the plants to survive even long periods of extreme drought. In addition to saving water loss during the day undergoes a CAM plant an additional water gain in the night. Through storage of malic acid creates a high osmotic pressure in the vacuole. As a result, water is thereby absorbed from the ambient air, especially after the Taufall at night.

The increased energy demand for Kohlenstoffdioxidassimilation compared to C3 plants is significant in view of the high solar radiation at most sites of CAM plants negligible. In addition, the light saturation of photosynthesis is usually only at higher light intensities achieved than in C3 plants. Three molecules of ATP and two molecules of NADPH are required ( without loss of photorespiration ) fixed per molecule of carbon dioxide in the C3 metabolism. When CAM these values ​​are 4.8 to 5.9 ATP, and 3.2 to 3.9 moles of NADPH. While light usually is never a limiting factor, however, this could be a problem with epiphytes. During the rainy period in Rule forests can be a problem due to the humidity and evaporation, the amount of incident light. Individuals of the bromeliad Bromelia humilis crop showed, for example, in the shadow of a lower growth than conspecific plants growing at comparatively brighter locations.

CAM plants have to protect the photosystem all mechanisms of C3 plants ( zeaxanthin, xanthophyll, D1 -protein turnover, possibly photorespiration ). CAM plants usually show little photorespiration, since the cytosolic concentration of CO2 is very high due to the release of carbon dioxide from stored malate. Compared to C3 plants respond CAM plants also lower oxidative stress caused by ozone (O3 ) and sulfur dioxide ( SO2) is caused.

Finally CAM is a CO2 - enrichment pump. This allows aquatic CAM plants a CO2 gain in the night. So that they can be compared with other non -CAM argue, aquatic plants, depleting the water during the day a lot of carbon dioxide. Sometimes, even without competition from other plants of the CO2 content in the water is low when the water has a low pH. For example, Isoetes has howellii in this environment benefits by CAM. Also bromeliads in extremely humid tropical cloud forests benefit from CO2 enrichment. During the rainy season or in heavy fog of gas exchange on leaf surfaces is severely limited. This have CAM bromeliads an advantage over C3 bromeliads, especially during the dry season turn the water-saving CAM mechanisms are advantageous.

Another advantage of the ATP-driven CO2 pump is the increased concentration of carbon dioxide during the day. Characterized RuBisCO less is required, while the enzyme accounts for about 50% of the total soluble protein content in the sheet in C3 plants. It was discussed that this CAM plants in nitrogen-poor soils are at an advantage over C3 plants because they have a better nitrogen use efficiency ( nitrogen -use efficiency, NUE ). Nevertheless, it must, however, keep you, that C3 plants have with nitrogen-poor soils through appropriate adaptations so that CAM plants are not automatically have the advantage.

CAM plants always show a slower growth rate than C4 and C3 plants because of the limited storage capability of the vacuoles. However, CAM plants grow in habitats where primary survival is more important than the gain in biomass.

Isotope discrimination

As with isotope discrimination of C4 plants, PEP carboxylase in CAM plants do not discriminate so strongly against 13C as RuBisCO. Therefore, it corresponds to isotope ratio ( δ 13C ) in CO2 dark fixation of the C4 plants, with light fixation of external CO2 but that of C3 plants. With increasing drought stress, the proportion of dark fixation increases, the CAM plant is rich in 13C. The δ 13C value of a CAM plant is therefore a good measure of drought stress on their growing area, as it provides an indication of the extent of CAM metabolism in relation to the normal C3 metabolism. With good water supply show facultative CAM plants and what with CAM -cycling δ 13C values ​​that match those of C3 plants.

Comparison between C3, C4 and CAM plants

Evolution

Presumably, the Crassulacean acid metabolism has established a million 200 years ago in the Mesozoic in response to lower CO2 levels in the atmosphere. Although CAM is a beneficial " response" to water scarcity; However, going evolutionary biologists believe that there mechanisms for water conservation and water storage existed before CAM, such as protection against evaporation and succulence. These adjustments, however, the CO2 uptake difficult ( closed stomata, little diffusion of CO2 in succulent tissues ). Therefore, CAM is considered primarily as a response to low CO2 level.

It is believed, therefore, that one of the first evolutionary steps towards CAM was the ability during the night to reattach the carbon dioxide from the breathing. This happens especially in succulent plants nowadays. A second important step is the occurrence of specific translocators in the vacuole. They prevent uncontrolled C4 acids diffuse into the cytosol and thus change the pH adversely. The enzymes required for the CAM metabolism occur also in C3 plants. This has made it possible that CAM plants have evolved in different, unrelated families in monocots and dicots of different habitats. Due to the selective advantages associated with CAM under drought, salinity and especially low CO2 concentrations, CAM plants (like C4 plants ) several times and independently "developed" from C3 precursor plants in the course of evolution.

Use

CAM plants are used for agriculture only to a small extent. They are cultivated primarily in areas in which it is not profitable due to the high evapotranspiration and the insufficient rainfall, the cultivation of C3 and C4 plants. The pineapple, Opuntia ficus- indica, the sisal agave and the Blue Agave are CAM plants, which have gained agricultural importance.

The best areas for growing pineapples are in the tropics, South Africa and Australia. About 86 tonnes of fruit are obtained in the cultivation of pineapples per year and hectare. 2003, the international trade value of pineapple amounted to 1.9 billion U.S. dollars. The prickly pear cactus has been used commercially mainly in the early 20th century. Nowadays, it is mainly grown as food and animal feed in many parts of the world. The yield per year estimated to 47-50 tons per hectare.

Agaves are cultivated worldwide, mostly to gain fibers or produce alcoholic beverages. So you win from the sisal agave, the eponymous sisal fibers. The production was highest in the 1960s. Then she dropped continuously since the sisal fiber was replaced more and more by synthetic fibers of polypropylene. 2006 246.000 tons were produced.

Another important CAM plant is the Blue Agave, harvested from the per hectare per year 50 tons of dry matter. After knocking off the leaves out of the tree (the " Cabeza " ) is fermented by boiling sugar syrup extracted and processed into tequila. And the other sugar syrup Agavearten is distilled, for example to Mezcal.

The high sugar content in the blue agave is also used for the production of bioethanol. In particular, Mexico and much of the Karoo in South East Africa have moved for an extension into close consideration, since there the use of other cereals would be inappropriate. The annual production of ethanol from the Blue Agave is 14,000 liters per hectare, possibly this can be increased to 34,000 liters of ethanol per hectare.