CAM-Crassulacean Acid Metabolism

In a complete CAM plant, the stomata in the leaves close during the day to limit evapotranspiration, but open at night to absorb carbon dioxide (CO2) and enable it to permeate into the mesophyll cells. At night, CO2 is stored in vacuoles as the four-carbon acid malic acid, and during the day, the malate is transferred to chloroplasts and transformed back to CO2, which is then utilised during photosynthesis.

Observations concerning CAM were made for the first time by de Saussure which was published in the year 1804. Bryophyllum leaves were found to be acidic in the morning and tasteless by the afternoon, according to Benjamin Heyne, who discovered them in India in 1812. It was discovered by botanists Ranson and Thomas in the succulent family Crassulaceae, and they named it after them (which includes jade plants and Sedum).  Its name refers to acid metabolism in the Crassulaceae family, not to the metabolism of “crassulacean acid,” which is a chemical entity that does not exist.

Phases of the CAM

Depicts an explanation of the CAM phases according to C.B. Osmond. The main phases of the CAM cycle are divided into two groups:

• The cycle phases I and III, corresponding to the major processes that determine the dark period and light period.
• The cycle Phases II and IV are transitional phases.
• CAM phase expression is influenced by environmental factors such as water availability, irradiance, and sun exposure.

Environmental factors that influence CAM phase expression include:-

The manifestation of CAM phases may also have a relationship with the performance of weak and strong CAM, as well as with the performance of CAM phases. 

• Some plants exhibit a day/night gas exchange similar to C3 photosynthesis, with CO2 uptake occurring exclusively during the day and no CO2 uptake occurring at night. 
• This is accompanied by a nocturnal accumulation of organic acid due to the internal recycling of respiratory CO2. 
• It is referred to as “cam-cycling” or “weak CAM” when this happens. 
• Strong CAM, on the other hand, is characterised by the expression of all four phases of CAM as well as the progressive suppression of the daytime phases of net CO2 uptake (II and IV) as stress levels rise.
• Phase I net  CO2  uptake is gradually suppressed as a result of severe water stress over time, while nocturnal acid accumulation may persist as a result of the recirculation of respiratory  CO2 in the environment. 
• In extreme cases, there is no net CO2  exchange at all during the day and night, and nocturnal organic acid accumulation is solely due to  CO2  recycling during the day and night. This is referred to as “CAM-idling.” 
• As a result, succulent CAM plants can withstand prolonged periods of drought without accumulating new carbon, but they benefit from losing only a small amount of water through cuticular transpiration. 
• They will survive as long as they do not lose more than 50% of the water reserves stored in their water-storing tissues. 
• Additionally, the expression of CAM phases with variation in daytime  CO2  uptake (phases II and IV) and nocturnal CO2  recycling from 0 to 100 percent of malic acid accumulated (phase I) allows for versatile stress responses even in obligate constitutive CAM plants that do not have the option of C3–CAM switchout.

Photosynthesis 

Crassulacean Acid Metabolism (Crassulacean Acid Metabolism)

A photosynthesis adaptation to periodic water supply, known as crassulacean acid metabolism (CAM), occurs in plants from arid regions (such as cacti) or tropical epiphytes (such as ferns and fern-like plants) (e.g., orchids and bromeliads). At night, when the air temperature is lower and water loss is reduced by an order of magnitude, CAM plants close their stomata and take up CO2 from the air during the day. CAM is found in between 5 percent and 10 percent of plants and is always associated with succulence, at least at the cellular level, according to the literature.

However, while the biochemistry of CAM plants is similar to that of C4 plants, the two carboxylations are now separated in time rather than in space, as opposed to the former. In the evenings, malic acid is synthesised from carbohydrates by the enzyme PEP carboxylase, and it is stored in the vacuole. At night, malate is decarboxylated in the cytosol by PEP carboxykinase or NAD(P)-malic enzymes, CO2 is re-fixed by the Calvin-Benson cycle, and carbohydrates are re-formed during the day. This procedure takes place behind closed stomata. In addition, the internal concentration of CO2 can be increased to as high as 10,000 parts per million (ppm), which suppresses photorespiration. CAM plants are characterised by a high degree of metabolic adaptability. The presence of little or no CAM in seedlings and well-watered plants indicates that they are performing C3 photosynthesis, which involves opening their stomata during the day. This allows for increased carbon gain during periods of high water availability or during the establishment of seedlings, for example. Water or salt stress can then cause CAM to be activated, resulting in the activation of gene expression and the synthesis of the component enzymes.

C4 and CAM Photosynthesis in Land Plants, Evolution and Diversification :

Climate-change-adapted (C4) and climate-adaptive (CAM) photosynthesis are complex assemblages of anatomical and biochemical novelties that increase photosynthetic efficiency in a variety of conditions, including heat, salinity, and CO2-poor aquatic environments. Despite their complexity, each of C4 and CAM has evolved numerous times in land plants, each independently of the other. These origins were made possible by the presence of enablers in some plant lineages as well as the presence of evolutionary stable intermediates in the evolutionary process. Because of Miocene aridification and biome opening, both the C4 and CAM lineages have undergone significant expansion and diversification in the years following their initial origins. This diversification is accompanied by a diversity of new ecological strategies resulting from the integration of different photosynthetic types within organisms during the process of evolution.

PHOTOSYNTHESIS AND PARTITIONING | C3 Plants: 

Limitations and Prospects for Improvement in C3 Plants

1.) C3-photosynthesis appears to have evolved much earlier than the CAM or C4 pathways and is found in even the most primitive lower groups.

2.) The majority of the 300,000 plants known on the planet are C3 plants, with CAM and C4 species accounting for about 10% and 1%, respectively. Furthermore, because most crops (particularly cereals, legumes, and oilseed crops) are of the C3 variety, C3 plants have piqued the interest of several scientists.

3.) C3 plant performance and productivity are limited by at least three major factors: high photorespiration (an unavoidable result of rubisco oxygenase activity), a high water requirement, and a preference for temperate regions.

4.) Attempts were made to evolve varieties with low photorespiration or high photosynthetic rates, but single characteristics were unable to improve plant productivity. Another strategy was to introduce a set of C4 traits into C3 plants, but hybridization of C3 and C4 Atriplex species produced only C3-type plants.

5.) Mutants lacking one or more photorespiration enzymes were unable to grow at atmospheric CO2 levels, indicating that photorespiration (as well as rubisco oxygenase activity) was an adaptation to the current CO2/O2 levels. Elevated CO2, on the other hand, is a well-known factor that could improve the photosynthetic performance and productivity of C3 plants.

Throughout the night

CAM plants have their stomata open at night, allowing CO2 to enter and be fixed as organic acids via a PEP reaction similar to the C4 pathway. Because the Calvin cycle cannot function without ATP and NADPH, products of light-dependent reactions that do not occur at night, the resulting organic acids are stored in vacuoles for later use.

Throughout the day

During the day, stomata close to conserve water, and CO2-storing organic acids are released from mesophyll cell vacuoles. An enzyme in the stroma of chloroplasts releases CO2, which enters the Calvin cycle and allows photosynthesis to occur.

Benefits

The ability to close most leaf stomata during the day is the most important benefit of CAM to the plant. Plants that use CAM are most common in arid environments where water is scarce. The ability to close stomata during the hottest and driest part of the day reduces water loss through evapotranspiration, allowing such plants to grow in environments that would otherwise be far too dry. Plants that only use C3 carbon fixation, for example, lose 97 percent of the water they take up through their roots to transpiration – a significant cost avoided by plants that can use CAM.

Aquatic CAM:

CAM photosynthesis is also found in aquatic species from at least four genera, including Isoetes, Crassula, Littorella, Sagittaria, and possibly Vallisneria, and is found in a wide range of species, including Isoetes howellii and Crassula Aquatica.

These plants exhibit the same patterns of nocturnal acid accumulation and daytime deacidification as terrestrial CAM species. However, CAM in aquatic plants is caused by a limited supply of CO2, not a lack of available water. CO2 is limited in water due to its slow diffusion, which is 10000 times slower than in air. The problem is especially acute at acid pH, where CO2 is the only inorganic carbon species present and no bicarbonate or carbonate supply is available. Because of the lack of competition from other photosynthetic organisms, aquatic CAM plants capture carbon at night, when it is abundant.  As a result of less photosynthetically generated oxygen, photorespiration is reduced.

CONCLUSION :

From the following article, we can conclude that A carbon fixation system known as crassulacean acid metabolism (CAM photosynthesis) arose in some plants as an adaptation to arid circumstances. It permits a plant to photosynthesize throughout the day but only exchange gases at night. Crassulacean acid metabolism is also known as CAM photosynthesis.