CAM plant: Crassulacean acid metabolism (CAM) is a type of photosynthesis that allows some chlorophyllous land plants to fix carbon. This metabolism concerns succulent plants such as cacti or euphorbias, as well as many epiphytes living in environments that can be periodically very poor in water, such as deserts. The name of the metabolism comes from the family of the plant where it was observed for the first time, that is to say in Kalanchoe of the family Crassulaceae.
CAM plant anatomy: As CAM metabolism involves making large reserves of malic acid inside plant cells, plants with this metabolism have large cells with a low surface area/volume ratio. These large cells can then contain large vacuoles filled with organic acids (especially malate) and also they can retain large amounts of water. CAM plants also have modified leaves and stems, which do not have many stomata and have a thick cuticle in order to minimize the gas exchange of water vapor with the outside. It is mainly in their fleshy leaves that CAM plants make their reserves of malic acid, because it is the leaf cells that have large vacuoles.
Mechanisms: Unlike C4 plants, which perform a spatial separation between the initial fixation of CO2 and its use by the Calvin cycle, CAM plants achieve a temporal separation of the two pathways, since one takes place at night and the other the day. On the other hand, CAM photosynthetic metabolism involves a double carboxylation system just like C4 plants, since CO2 is fixed twice in the plant cell, once with phosphoenolpyruvate carboxylase in the cytosol, then a second time with the rubisco in the chloroplast.
At night: The stomata of CAM plants open only at night, because at this time the temperature is lower than during the day and the humidity is higher231. This causes low evapotranspiration and therefore low water loss. This is why CAM plants have specialized in fixing CO2 during the night1.
This attachment is carried out by PEP carboxylase on phosphoenolpyruvate (PEP), which comes from the degradation of starch and the sucrose produced in the chloroplast during the day. This fixation makes it possible to form oxaloacetate, which will be immediately reduced to malate, then stored in a vacuole in the form of malic acid, hence the name plant with acid metabolism1. In other words, during the night, the CAM plant "fills up" with CO2 in the form of malic acid, but it cannot immediately transform it into sugar because of the darkness. Indeed, like all other plants, CAM plants need daylight energy to complete the Calvin cycle and thus complete photosynthesis.
By day: As CAM plants live in arid environments, there is a large deficit in water vapor pressure between these plants and the environment that surrounds them during the day. Thus, this is why CAM plants are obliged to close their stomata during the day, because otherwise the water would evaporate by osmosis from these structures (by evapotranspiration), and this would cause significant water loss. .
Fortunately, the CAM plant solved this problem by fixing CO2 at night, then storing it as malic acid in a vacuole for later use during the day when the stomata are closed1. At the end of the night, malic acid is transformed back into malate, which is transferred out of the vacuole to be decarboxylated into CO2 and pyruvate in the cytosol. This decarboxylation takes place thanks to a NAD+ (or NADP+) dependent malic enzyme or a PEP carboxykinase3. After this step, the CO2 is fixed again, but this time thanks to the rubisco, to then enter the Calvin cycle1 in the chloroplast2. It is following this cycle that sugars such as starch and sucrose are synthesized1. The pyruvate will also be used to replenish the stock of starch and will be converted into sugars by reverse glycolysis.
The Calvin cycle must take place during the day, as it requires NADPH and ATP produced during the light phase of photosynthesis. As this clear phase requires light energy, it must occur during the day. Unlike C3 and C4 plant types, this step occurs in the absence of gas exchange with the atmosphere in CAM plants, because at this time their stomata are closed.
Remember that the only source of CO2 available to carry out the Calvin cycle during the day is the reserve of malic acid produced during the night. The amount of CO2 available therefore decreases rapidly during the day, until the reserve vacuole is empty1. This makes photosynthesis less efficient in CAM plants, because CO2 is a very limiting factor.
Physiological interests: One of the advantages of CAM plants is that their metabolism is mainly controlled by environmental factors, rather than a genetic basis like C4 plants. Indeed, Crassulacean metabolism can be constitutive or inducible. For an inducible species like Mesembryanthemum cristallinum, the metabolism adopted therefore depends on the quantity of water available to the plant. For example, in the spring, when the soil is waterlogged, CAM plants may exhibit C3 type metabolism during the day. On the other hand, in summer, the soils are drier and the lack of water, as well as the rise in the salt content of the soils cause rapid changes in the expression of their genes in CAM plants and therefore their enzymes, which allows them to set up the Crassulacean metabolism. We can therefore say that the carbon fixation mechanism of CAM plants is flexible and highly adapted to environmental variations1.
Another major advantage of this metabolism is to limit water loss through the closure of stomata during the day5, which is highly important for CAM plants that live in conditions of high light intensity and water stress. Water losses per fixed CO2 are 3 to 6 times less in CAM plants than in C4 plants and 4 to 10 times less than in C3 plants. Thus, CAM plants fix 10 to 40 kg of CO2 per kilo of transpired water, whereas C4 plants only fix 2 to 5 kg and C3 plants 1 to 3 kg.
The metabolism of CAM plants also helps avoid photorespiration which occurs particularly in C3 plants. Indeed, as CO2 is fixed in the first place by PEP carboxylase rather than by rubisco, there is practically no longer possible photorespiration due to the absence of affinity between PEP carboxylase and oxygen. PEP carboxylase has a much better affinity with CO2 than rubisco, so it binds more without wasting energy by binding oxygen unnecessarily5. On the other hand, CAM plants also contain rubisco (which has a great affinity with oxygen) to make the Calvin cycle, but remember that this cycle takes place during the day, when the stomata are closed. Rubisco is then saturated with CO2 from the storage vacuole and there is very little oxygen present, which is why rubisco does not photorespiration and why it is so effective in CAM and C45 plants. CAM plants are then said to have a high nitrogen use efficiency because rubisco is high in nitrogen.
Also, as CO2 is immediately converted into oxaloacetate as soon as it is fixed by PEP carboxylase, the system is never saturated with CO2 and it can fix more5. Thus, the enzyme PEP carboxylase works at full speed in the presence of CO2, which is why C4 and CAM plants are more efficient at fixing carbon than C3.
Physiological disadvantages: Crassulacean metabolism has many benefits, but like everything, it also comes with costs: Just like with C4s, the metabolism of CAM plants is more energy intensive than that of C3 plants. As it requires double enzymatic machinery for double carboxylation (i.e. more structures, more enzymes, etc.), this causes more costs for CAM plants, which produce these machinery from their energy reserves5.
On the other hand, CAM plants are the only ones able to store CO2 reserves in the form of malic acid, which turns out to be an advantage as much as a disadvantage, because these reserves inside a vacuole take up a lot of space in the plant cell. This requires larger cells and therefore other energy costs.
Finally, the overall photosynthetic activity of CAM plants is rather modest because, as mentioned, they are limited by their CO2 reserve. Thus, when the reserve vacuole is empty, there is no longer any photosynthesis possible for the plants. This severely limits their growth9, because their enzymes and all their photosynthetic machinery do not work as much as they can, since they do not have permanent access to CO2, unlike C4 and C3 which have their stomata open to gas exchange during the day. This is why CAM plants grow so slowly, like cacti for example.
Economic/agronomic interest: The interest of CAM plants is rather limited at the agronomic level, except with regard to pineapple, vanilla or agave. Note, however, that these plants can be grown in non-arid environments.
Distribution: CAM plants are generally found in regions that are extremely arid or salt-rich and very hot, where light is not limiting for growth, but where it is relatively cool at night3. These are the only conditions where CAM plants can outperform C3 and C4 in efficiency. Indeed, outside of these conditions, CAM plants have too slow growth and too high an energy cost to be advantageous. Their distribution is therefore relatively limited to these places.
CAM plants are also found as epiphytes in the crowns of trees in certain tropical regions, where there is sufficient atmospheric humidity to support their metabolism9, despite a limited root system.
Submerged CAM plants have even been found in some lakes where the majority of plants present use dissolved CO2 during the day for photosynthesis, but CAM plants use dissolved CO2 in the water at night. This benefits them, because there is more CO2 available at night than during the day, since during the day there are more plants competing for it.
List of CAM plants: This metabolism is generally found in plants living under arid conditions, for example the cactus or the pineapple. CAM plants represent approximately 10% of vascular plants1. More specifically, 23 families of angiosperms are considered to be CAM plants, among which we find:
– The Aizoaceae
– Cactaceae
– The Crassulaceae
– Pineapple
– Hoya carnosa
– A sansevière
- The mousse from Spain: Tillandsia usneoides
– Many species of orchids
– Agavaceae and Bromeliaceae.