Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions.[1] The stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2). The CO2 is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.
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CAM was first suspected by De Saussure in 1804 in his Recherches Chimiques sur la Vegetation, confirmed and refined by Aubert, E. in 1892 in his Recherches physiologiques de plants, Les grasses and expounded upon by Richards, H. M. 1915 in Acidity and Gas Interchange in Cacti, Carnegie Institution. The term CAM may have been coined by Ranson and Thomas in 1940, but they were not the first to discover this cycle. It was observed by the botanists Ranson and Thomas, in the Crassulaceae family of succulents (which includes jade plants and Sedum).[2] Its name refers to acid metabolism in Crassulaceae, not the metabolism of Crassulacean acid.
CAM plants are adapted to life in arid conditions by conserving water.
During the night, the CAM plant's stomata are open, allowing CO2 to enter and be fixated as organic acids that are stored in vacuoles. During the day the stomata are closed (thus preventing water loss), and the carbon is released to the Calvin cycle so that photosynthesis may take place.
The carbon dioxide is fixed in the mesophyll cell's cytoplasm by a PEP reaction similar to that of C4 plants. But, unlike C4 plants, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the Calvin cycle. Of course, the latter cannot operate during night because the light reactions that provide it with ATP and NADPH cannot take place without light.
The carbon in the organic acids is freed from the mesophyll cell's vacuoles and enters the chloroplast's stroma and, thus, into the Calvin cycle.
The most important benefit to the plant is the ability to leave most leaf stomata closed during the day.[3] CAM plants are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing CAM plants to grow in environments that would otherwise be far too dry. C3 plants, for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by CAM plants.[4]
The C4 pathway bears resemblance to CAM; both act to concentrate CO2 around RuBisCO, thereby increasing its efficiency. CAM concentrates it in time, providing CO2 during the day, and not at night, when respiration is the dominant reaction. C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO reaction centre in a "bundle sheath cell" being inundated with CO2. Due to the inactivity required by the CAM mechanism, C4 carbon fixation has a greater efficiency in terms of PGA synthesis.
CAM can be considered an adaptation to arid conditions. CAM plants often display other xerophytic characters, such as thick, reduced leaves with a low surface-area-to-volume ratio; thick cuticle; and stomata sunken into pits. Some shed their leaves during the dry season; others (the succulents) store water in vacuoles.
CAM plants not only are good at retaining water but also use nitrogen very efficiently. However, because their stomata are closed by day, they are less efficient at CO2 absorption. This limits the amount of carbon they have available for growth.
CAM plants can also be recognized as plants whose leaves have an increasing sour taste during the night yet become sweeter-tasting during the day. This is due to malic acid stored in the vacuoles of the plants' cells during the night and then used up during the day.[5]
Plants with CAM must control storage of CO2 and its reduction to branched carbohydrates in space and time.
At low temperatures (frequently at night), CAM plants open their guard cells, CO2 molecules diffuse into the spongy mesophyll's intracellular spaces and then into the cytoplasm. Here, they can meet phosphoenolpyruvate (PEP), which is a phosphorylated triose. During this time, CAM plants are synthesizing a protein called PEP carboxylase kinase (PEP-C kinase), whose expression can be inhibited by high temperatures (frequently at daylight) and the presence of malate. PEP-C kinase phosphorylates its target enzyme PEP carboxylase (PEP-C). Phosphorylation dramatically enhances the enzyme's capability to catalyze the formation of oxalacetate, which can be subsequently transformed into malate by NAD+ malate dehydrogenase. Malate is then transported via malate shuttles into the vacuole, where it is converted into the storage form malic acid. In contrast to PEP-C kinase, PEP-C is synthesized all the time but almost inhibited at daylight either by dephosphorylation via PEP-C phosphatase or directly by binding malate. The latter is not possible at low temperatures, since malate is efficiently transported into the vacuole, whereas PEP-C kinase readily inverts dephosphorylation.
At daylight, CAM plants close their guard cells and discharge malate that is subsequently transported into chloroplasts. There, depending on plant species, it is cleaved into pyruvate and CO2 either by malic enzyme or by PEP carboxykinase. CO2 is then introduced into the Calvin cycle, a coupled and self-recovering enzyme system, which is used to build branched carbohydrates. The by-product pyruvate can be further degraded in the mitochondrial citric acid cycle, thereby providing additional CO2 molecules for the Calvin Cycle. Pyruvate can also be used to recover PEP via pyruvate phosphate dikinase, a high-energy step, which requires ATP and an additional phosphate. During the following cool night, PEP is finally exported into the cytoplasm, where it is involved in fixing carbon dioxide via malate.
The majority of plants possessing CAM are either epiphytes (e.g., orchids, bromeliads) or succulent xerophytes (e.g., cacti, cactoid Euphorbias), but CAM is also found in hemiepiphytes (e.g., Clusia); lithophytes (e.g., Sedum, Sempervivum); terrestrial bromeliads; hydrophytes (e.g., Isoetes, Crassula (Tillaea); and in one halophyte, Mesembryanthemum crystallinum; one non-succulent terrestrial plant, (Dodonaea viscosa) and one mangrove associate (Sesuvium portulacastrum).
Some plants are able to switch between different methods of carbon fixation. Portulacaria afra, better known as Dwarf Jade Plant, normally uses C3 fixation but can use CAM if it is drought-stressed, whereas Portulaca oleracea, better known as Purslane, normally uses C4 fixation but is also able to switch to CAM when drought-stressed.[6][7]
CAM has evolved convergently many times.[8] It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families, but this is thought to be a considerable underestimate.[9] It is found in quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of CAM plants are angiosperms (flowering plants).
The following list summarizes the taxonomic distribution of CAM plants.
| Division | Class/Angiosperm group | Order | Family | Plant Type | Clade involved |
|---|---|---|---|---|---|
| Lycopodiophyta | Isoetopsida | Isoetales | Isoetaceae | hydrophyte | Isoetes[10] (the sole genus of class Isoetopsida) - I. howellii (seasonally submerged), I. macrospora, I. bolanderi, I. engelmanni, I. lacustris, I. sinensis, I. storkii, I. kirkii |
| Pteridophyta | Polypodiopsida | Polypodiales | Polypodiaceae | epiphyte, lithophyte | CAM is recorded from Microsorium, Platycerium and Polypodium,[11] Pyrrosia and Drymoglossum[12] and Microgramma |
| Pteridopsida | Pteridales | Vittariaceae[13] | epiphyte | Vittaria[14]
Anetium citrifolium[15] |
|
| Cycadophyta | Cycadopsida | Cycadales | Zamiaceae | Dioon edule[16] | |
| Pinophyta | Gnetopsida | Welwitschiales | Welwitschiaceae | xerophyte | Welwitschia mirabilis[17] (the sole species of the order Welwitschiales) |
| Magnoliophyta | magnoliids | Magnoliales | Piperaceae | epiphyte | Peperomia camptotricha[18] |
| eudicots | Caryophyllales | Plantaginaceae | hydrophyte | Littorella uniflora[10] | |
| Aizoaceae | xerophyte | widespread in the family; Mesembryanthemum crystallinum is a rare instance of an halophyte that displays CAM[19] | |||
| Cactaceae | xerophyte | all cacti have obligate Crassulacean Acid Metabolism in their stems; those few cacti with leaves have C3 Metabolism in those leaves; seedlings have C3 Metabolism. | |||
| Portulacaceae | xerophyte | recorded in approximately half of the genera (note: Portulacaceae is paraphyletic with respect to Cactaceae and Didieraceae)[20] | |||
| Didiereaceae | xerophyte | ||||
| Saxifragales | Crassulaceae | hydrophyte, xerophyte, lithophyte | CAM is widespread in the family | ||
| eudicots (rosids) | Vitales | Vitaceae[21] | Cissus,[22] Cyphostemma | ||
| Malpighiales | Clusiaceae | hemiepiphyte | Clusia[22][23] | ||
| Euphorbiaceae[21] | CAM is found is some species of Euphorbia[22][24] including some formerly placed in the sunk genera Monadenium,[22] Pedilanthus[24] and Synadenium. C4 photosynthesis is also found in Euphorbia (subgenus Chamaesyce). | ||||
| Passifloraceae[13] | xerophyte | Adenia | |||
| Geraniales | Geraniaceae | CAM is found in some succulent species of Pelargonium,[25] and is also reported from Geranium pratense | |||
| Cucurbitales | Cucurbitaceae | Xerosicyos danguyi,[26] Dendrosicyos socotrana, Momordica | |||
| Celastrales | Celastraceae | ||||
| Oxalidales | Oxalidaceae | ||||
| Brassicales | Moringaceae | Moringa | |||
| Sapindales | Sapindaceae | Dodonaea viscosa | |||
| Zygophyllaceae | Zygophyllum | ||||
| eudicots (asterids) | Ericales | Ebenaceae | |||
| Solanales | Convolvulaceae | Ipomaea | |||
| Gentianales | Rubiaceae | epiphyte | Hydnophytum and Myrmecodia | ||
| Apocynaceae | CAM is found in subfamily Asclepidioideae (Hoya,[22] Dischidia, Ceropegia, Stapelia,[24] Caralluma negevensis, Frerea indica,[27] Adenium, Huernia), and also in Carissa and Akocanthera | ||||
| Lamiales | Gesneriaceae | epiphyte | CAM was found Codonanthe crassifolia, but not in 3 other genera[28] | ||
| Lamiaceae | Plectranthus marrubioides, Coleus | ||||
| Apiales | Apiaceae | hydrophyte | Lilaeopsis lacustris | ||
| Asterales | Asteraceae[21] | some species of Senecio[29] | |||
| Magnoliophyta | monocots | Alismatales | Hydrocharitaceae | hydrophyte | Hydrilla,[21] Vallisneria |
| Alismataceae | hydrophyte | Sagittaria | |||
| Araceae | Zamioculcas zamiifolia is the only CAM plant in Araceae, and the only non-aquatic CAM plant in Alismatales[30] | ||||
| Poales | Bromeliaceae | epiphyte | Bromelioideae (91%), Puya (24%), Dyckia and related genera (all), Hechtia (all), Tillandsia (many)[31] | ||
| Cyperaceae | hydrophyte | Scirpus,[21] Eleocharis | |||
| Asparagales | Orchidaceae | epiphyte | |||
| Agavaceae[23] | xerophyte | Agave,[22] Hesperaloe, Yucca | |||
| Asphodelaceae[21] | xerophyte | Aloe,[22] Gasteria[22] and Haworthia | |||
| Ruscaceae[21] | Sansevieria,[22] Dracaena | ||||
| Commelinales | Commelinaceae | Callisia,[22] Tradescantia, Tripogandra |