Plant Physiology and Development

J. Cushman , in Encyclopedia of Applied Plant Sciences (Second Edition), 2017

CAM Phases

CAM phases sensu C.B. Osmond are explained in Figure 10. The main phases are I and III corresponding to the major processes determining the dark period and light period, respectively, of the CAM cycle (Figure 1). Phases II and IV are transition phases. Expression of CAM phases is influenced by environmental factors, such as availability of water, irradiance, and sun exposure (Figure 10). The manifestation of CAM phases may also relate to the performance of weak and strong CAM, respectively. Some plants show a C3 photosynthesis-like day/night gas exchange with exclusively daytime CO2 uptake and no nighttime CO2 uptake, which is nevertheless accompanied by nocturnal accumulation of organic acid due to internal recycling of respiratory CO2. This is referred to as weak CAM or 'CAM cycling.' Conversely, strong CAM is the expression of all four phases of CAM and the gradual suppression of the daytime phases of net CO2 uptake (II and IV) as stress increases. Eventually, i.e., under severe water stress, phase I net CO2 uptake is also gradually suppressed, while nocturnal acid accumulation may remain prominent due to recirculation of respiratory CO2. In extreme cases, there is no net CO2 exchange at all during day and night, and nocturnal organic acid accumulation is exclusively due to CO2 recycling. This is called 'CAM idling.' Thus, succulent CAM plants can overcome extended dry periods without gaining new carbon, but they benefit from losing only a little water via cuticular transpiration. They survive unless they lose more than 50% of the water reserves from their water-storing tissues. Thus, the expression of CAM phases with variation of daytime CO2 uptake (phases II and IV) and nocturnal CO2 recycling from 0% to 100% of malic acid accumulated (phase I) allows versatile stress responses even in obligate constitutive CAM plants without the option of C3–CAM switches.

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Autotrophs

R.F. Sage , in Encyclopedia of Ecology, 2008

CAM Photosynthesis

CAM photosynthesis is a CO2-concentrating mechanism that uses a C4 cycle of PEP carboxylation followed by C4 acid decarboxylation to concentrate CO2 around Rubisco ( Figure 4 ). In CAM plants, stomata open at night when conditions are relatively cool and humid. PEP carboxylase is active at night, fixing inorganic carbon into C4 acids that are stored in large vacuoles. During the day, stomata close to save water, and the C4 acids are decarboxylated, releasing the CO2 that is then refixed by Rubisco in the normal mode of C3 photosynthesis. Both CAM plants and C4 plants use a very similar biochemistry to effect CO2 concentration; however, PEP carboxylation and Rubisco carboxylation are spatially separated in C4 plants, but temporally separated in CAM plants.

Photosynthetic productivity in CAM plants is restricted by the limited capacity of the vacuole for storing organic acids at night. To overcome this limitation, CAM plants form large photosynthetic cells with massive vacuoles to enhance carbon storage. These large cells cause the leaves to have a succulent morphology. Even with this modification, the peak biomass productivity of CAM plants is generally well below that of productive C4 and C3 vegetation. Many CAM plants are not exclusively restricted to CAM metabolism, however, as they can switch into a normal C3 mode when water is readily available. In such species, the CAM mode of photosynthesis is activated during times of severe drought or salinity stress as a survival, rather than a high-productivity mechanism.

The ability to concentrate CO2 at night and close stomata during the day greatly enhances WUE and thus allows for primary productivity in extremely harsh situations. CAM plants exhibit the highest WUE of all plants ( Table 2 ). High WUE enables CAM plants to grow in environments that would otherwise be too dry to support higher plants. CAM plants dominate many of the low-latitude deserts of the world, allowing for species-rich ecosystems in settings that would otherwise have a simple ecosystem of ephemerals plants, mosses, and lichens.

Table 2. Ranges of NPP reported for selected ecosystems of the world

Biome Primary productivity (kg   DM   m−2  yr−1)
C4-dominated systems a
Tropical wetlands 5–14
Sugarcane plantation 6–11
Tropical grassland (high rainfall) 1–4
Tropical grassland (low rainfall) 0.2–1
C3-dominated systems b
Rice plantations 2–5
Wheat fields 1–3
Tropical rainforest 1–3.5
Deciduous temperate forest 0.4–2.5
Evergreen forest 1–2.5
Boreal forests 0.2–1.5
Dry scrub 0.3–1.5
Arctic tundra 0.01–0.4
Deserts 0–0.3
Algae-dominated systems b
Reefs and tidal zones 0.5–4
Coastal zones 0.2–0.6
Upwelling zones 0.4–1
Open ocean 0.0002–0.4
a
C4 NPP values from Long SP, Jones MB, and Roberts MJ (eds.) (1992) Primary Productivity of Grass Ecosystems of the Tropics and Subtropics. London: Chapman and Hall.
b
C3 and algal productivity values from Larcher W (2003) Physiological Plant Ecology, 4th edn. Berlin: Springer.

Most CAM species actually occur in tropical rainforests, growing as epiphytes on the branches of C3 trees. Here, CAM photosynthesis provides primary productivity in microenvironments where the lack of soil allows for substantial aridity between rain events. Many of the orchids and bromeliads of the tropical forest are CAM epiphytes, and they allow for complex aboveground habitat in what could otherwise be a barren branch. In the temperate zone where CAM epiphytes are absent due to cold, much simpler lichens and mosses fill the niche of the CAM epiphyte.

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Role of Trace Elements in Alleviating Environmental Stress

Ghader Habibi , in Emerging Technologies and Management of Crop Stress Tolerance, Volume 1, 2014

14.4.1.2 Sodium and induction of crassulacean acid metabolism

Crassulacean acid metabolism (CAM) is a metabolic strategy to maintain photosynthesis under stress conditions. High salinity (400  μM NaCl) induces a shift from C3 photosynthesis to CAM in Mesembryanthemum crystallinum plants (Niewiadomska et al., 2011). In previous work (Habibi and Hajiboland, 2012), we showed that a strong diurnal rhythm in the activity of some antioxidative enzymes has been observed in C3/CAM intermediate plants. In plants that use C4 or CAM photosynthetic pathways, Na is an essential element (Ohnishi et al., 1990). It was reported that the C4/CAM plants utilize phosphoenolpyruvate (PEP) to fix CO2 for photosynthesis, and Na is needed for the regeneration of PEP from pyruvate.

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Carbon fixation

Tracy Lawson , ... Tanja A. Hofmann , in Photosynthesis in Action, 2022

4.6 Crassulacean acid metabolism

Crassulacean acid metabolism (CAM) plants also exhibit a CO 2 concentrating mechanism, separating initial fixation of atmospheric CO2 from rubisco and the Calvin cycle by time rather than space, with stomatal opening and CO2 fixation taking place at night (Fig. 7). CAM is most common in dry climates and is particularly prevalent in cacti as well as commercially significant crops such as pineapple (Ananas comosus (L.) Merr.) (Davis et al., 2019), representing ~   6% of plant species (Winter and Smith, 1996).

Fig. 7

Fig. 7. Crassulacean acid metabolism (CAM), illustrating the temporal separation of initial uptake of CO2 from the atmosphere from rubisco activity. CO2 is taken up at night-time and stored in the vacuole as malic acid, which is decarboxylated in the light behind closed stomata and CO2 is released for fixation by rubisco in the chloroplast. The closed stomata through the day help CAM plants to maintain water.

 Redrawn from Taiz, L., Zeiger, E., 2002. Plant Physiology, third ed. Sinauer Associates, Sunderland, MA, USA, with permission.

CAM photosynthesis can be divided into four classical phases, phases I–IV, that are interdependent (Osmond, 1978). Phase I consists of initial CO2 fixation in the cytosol into oxaloacetate (Edwards and Ogburn, 2012) at night time using PEP and PEPc (as in C4 metabolism) with open stomata (Osmond, 1978). Nocturnal stomatal opening in CAM plants greatly reduces water loss by up to 80% as evaporative demands are significantly less at night (DePaoli et al., 2014). Oxaloacetate is then converted to malate by NAD(P)-malate dehydrogenase and stored in the vacuole as malic acid until daylight (Borland et al., 2014). Phase II consists of a short transition period where stomata can remain open for a short time (up to about 2   h (Osmond, 1978) depending on the species and the environmental conditions) and atmospheric CO2 diffusing into the leaf through the stomata can be fixed through typical C3 photosynthesis. In phase III, in the light stomata close so preventing water loss and it is under these conditions that the stored malate in the vacuole is decarboxylated and CO2 released behind closed stomata (Borland et al., 2014). This release of CO2 in a 'sealed' compartment results in an increased CO2 concentration, which represents the CAM CO2 concentrating mechanism for fixation by rubisco as part of the Calvin cycle, reducing photorespiration. Finally, in phase IV, late in the light period stomata can re-open once again allowing direct fixation of CO2 by rubisco (Osmond, 1978; Luttge, 2004). Many genes associated with CAM demonstrate a circadian element, such as NAD-ME and PEPC (Wai et al., 2017), indicating circadian regulation may play a vital role in CAM photosynthesis.

Stomatal kinetics are particularly important in CAM photosynthesis, with timings of opening and closure allowing for plants to separate carbon assimilation and initial CO2 fixation whilst also improving water use efficiency. Stomatal conductance is usually highest during the nocturnal phase I, where the uptake of CO2 is essential for the later CO2 concentrating mechanism around rubisco, and lowest during phase III, the light period (Males and Griffiths, 2017), reducing water loss. The timing of stomatal closure in CAM plants has been linked to genes associated with CO2 and ABA-signalling, whilst the specific stomatal blue light response, which is important for early morning and rapid stomatal opening (Matthews et al., 2020) is absent in CAM (Abraham et al., 2016), suggesting that environmental cues aside from light are central to stomatal regulation in CAM.

CAM plants can exhibit varying degrees of CAM photosynthesis. In most CAM plants, expression of the CAM pathway is obligate or constitutive, meaning the CAM photosynthetic pathway is always expressed in mature photosynthetic tissue (Winter, 2019). However, plants in which CAM photosynthesis can be switched on and off have also been described, termed facultative CAM. Facultative CAM, in which expression of CAM photosynthesis is inducible, is thought to be present in over 5% of vascular plant species, and most commonly occurs in plants also expressing C3 photosynthesis (Winter, 2019). Facultative CAM was first described in the C3 plant Mesembryanthemum crystallinum under high salinity, in which water deficit stress induced conversion from C3 to CAM photosynthesis whilst removal of this stress resulted in reversion to C3 photosynthesis (Winter and von Willert, 1972). Since then, facultative CAM has been identified in at least 54 species, mainly from the order Caryophyllales, but is suspected to be present in over 1000 species (Winter, 2019). Nearly all cases of facultative CAM have been noted to occur in response to stress, where a reversible increase in nocturnal CO2 uptake and acidification occurs (Winter, 2019). The transition from C3 to CAM photosynthesis in these facultative CAM plants has been associated with specific changes in gene expression and transcriptional networks. A global gene expression study in Sedum album noted a 73-fold higher expression in the core CAM genes PEPc and PPCK (phosphoenolpyruvate carboxylase kinase) in droughted plants compared to well-watered C3 plants, whilst the networks regulating C3 and CAM co-expression had little overlap (Wai et al., 2019). This demonstrates that there is a large degree of reprogramming associated with the C3-CAM transition.

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Carbon isotope discrimination studies in plants for abiotic stress

Sanket J. More , ... Saravanan Raju , in Climate Change and Crop Stress, 2022

17.4.3 CAM plants

CAM plants use Crassulacean acid metabolism photosynthetic pathway and are very well known to be highly water use efficient plants pertaining to nocturnal and diurnal separation of chemical reactions (O'Leary, 1988). In CAM plants, stomatal conductance is regulated nocturnally to enhance WUE as temperature and water loss are negligible. Hence, CAM plants are prevalent to arid climate. CAM plants uptake CO2 with nocturnal stomatal conductance followed by fixation of carbon into malic acid by PEP carboxylase and malate dehydrogenase. Substantial amount of malic acid is accumulated overnight. Stomata are closed next morning and malic acid is decarboxylated to release CO2. Gradually, released CO2 is fixed by RuBisCO. Following afternoon, many CAM plants open stomata to assign direct C3 photosynthetic pathway (O'Leary, 1988). Nocturnal carbon accumulation in CAM plants brings the delta value near about −11‰, while delta value during the diurnal carbon accumulation is approximately −28‰, the same as C3 plants. Moreover, CAM plants that fix CO2 nocturnally would be expected to show δ13C values more similar to C4 plants because of the involvement of PEP carboxylase, whereas diurnal CO2 fixation would show δ13C values more similar to C3 plants because of involvement of RuBisCO (Nalborczyk, LaCrois, & Hill, 1975; O'Leary, 1981; Cernusak et al., 2013). CAM plants were different than C3 plants owing to their delta values, which were observed to be between −10‰ and −20‰ and then C4 plants on the physiological basis and also on the basis of discrepancy in malic acid quantity accumulation during daytime (O'Leary, 1988). CAM pathway is distinct from C4 pathway in the way that carbon is stored in the form of malic acid overnight in the vacuoles and eventually is transported following morning for carbon fixation. Thus CAM plants temporally concentrate CO2 to improve RuBisCO efficiency, whereas C4 plants spatially concentrate CO2 in bundle-sheath cells. In addition, isotopic fractionation in CAM plants takes place as per the C4 pathway during nighttime, while as per the C3 pathway during daytime. Δ13C in CAM plants enables to track the temporal separation of CO2 fixation as biomass accumulation as a result of nocturnal CO2 fixation is relative to the carbon accumulated during daytime. Δ13C in CAM plants is the function of the balance between diurnal and nocturnal CO2 fixation. Isotopic signatures of CAM plants reveal the proportion of the two CO2 fixation pathways and variation in proportions with changes in environmental conditions (O'Leary, 1988; Osmond, Bender, & Burris, 1976).

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C4/CAM Facultative Photosynthesis as a Means to Improve Plant Sustainable Productivity Under Abiotic-Stressed Conditions

Renata Callegari Ferrari , Luciano Freschi , in Plant Signaling Molecules, 2019

Abstract

Crassulacean acid metabolism (CAM) and the C 4 pathway are photosynthetic adaptations that significantly improve plant water use efficiency. Both involve a prefixation of CO2 into acids prior to the carbon fixation via Rubisco and require refined regulatory mechanisms to synchronize reactions and minimize energy waste. C4 and CAM have been regarded as essentially incompatible, although the existence of C4–CAM facultative species challenges this assumption. Considering the potential emergence of drier areas in many parts of the globe due to climate change, understanding the compatibility between C4 and CAM holds enormous promise for future engineering of both photosynthetic adaptations in crop species. In this chapter, the major biochemical, anatomical, and molecular features of C4 and CAM are compared, and the stress signaling cascades controlling these syndromes are highlighted. The challenges and opportunities in combining the best of both photosynthetic adaptations into crops via genetic engineering approaches are also briefly discussed.

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Photosynthesis

Elhadi M. Yahia , ... Mónica Queijeiro Bolaños , in Postharvest Physiology and Biochemistry of Fruits and Vegetables, 2019

3.7.2 Crassulacean Acid Metabolism

CAM plants have evolved to avoid high evaporative water loss through the stomata, opening them in the night; therefore, CO2 binding is nocturnal, and the resultant organic acids (malate) are accumulated in the photosynthetic cells throughout the night, to be decarboxylated in the next morning. CAM plants need succulent tissues in order to store water; therefore, they have large cells with particularly large vacuoles, where malate can also be stored at night. During the day, the stored malate at night leaves the vacuole to be decarboxylated and CO2 is released and efficiently incorporated into the Calvin–Benson cycle, by the regular C3 path (Fig. 3.9).

Figure 3.9. Temporal separation of carbon fixation in a single photosynthetic cell of a CAM plant.

The temporal separation of CO2 fixation on a day/night basis, in addition to the higher energy cost of the C4 pattern, causes CAM plants to have lower growth rates than C3 plants. Even so, this photosynthetic adaptation allows plants to survive under such harsh and limiting conditions of aridity.

Since CAM metabolism does not require functional and anatomic specialization, it is possible to find facultative CAM plants that shift from C3 to CAM metabolism when water availability decreases. Most agave species or various species from Euphorbiaceae, Portulacaceae, Asteraceae, Vitaceae, Clusiaceae, Apocynaceae, and several other plant families are CAM facultative; whereas most Crassulaceae, Cactaceae, Orchidaceae, and Bromeliaceae species are obligate CAM (Table 3.1). There are also some extreme expressions of CAM metabolism in very dry deserts, where plants are able to maintain their stomata closed for several months, eliminating any possible water loss by evaporation but also disrupting carbon dioxide entry; therefore, they keep recycling CO2 produced by cellular respiration, until water is available again. This is known as the CAM idling mode.

Table 3.1. Some Important Plant Species and Their Photosynthetic Type

Photosynthetic Type Common Name Scientific Name Importance
C3 Apple Malus pumila Horticulture
C3 Wheat Triticum aestivum Agriculture
C3 Pinyon pine Pinus cembroides Forestry, food supply
C3 Tomato Solanum lycopersicum Horticulture
C3 Mistletoe Phoradendron sp. Parasitic plant
C3: Almost 95% of the Total Flora in the World
C4 Corn Zea mays Agriculture
C4 Sugar cane Saccharum spontaneum Agriculture
C4 Rice Oryza sativa Agriculture
C4 Buffel grass Cenchrus ciliaris Animal food, range management
C4 Invasive pink grass Rhynchelitrum repens Ornamental, invasive grass
C4: Most of the Tropical, Invasive Grasses (Poaceae Family)
CAM Pineapple Ananas comosus Horticulture
CAM Tequila agave Agave tequilana Alcoholic beverage
CAM Prickly pear Opuntia ficus indica Horticulture, invasive plant
CAM Vanilla Vanilla planifolia Condiments
CAM: All Succulent Plants, Mainly in Arid Environments

Moreover, it has been demonstrated that cacti, which are obligate CAM plants, grow with a C3 metabolism at their seedling stage, and at a certain moment they shift to a definitive and complete CAM path in a very juvenile stage. Such ability of CAM metabolism to change can be explained by the four phases of PEP-C and Rubisco activity (Fig. 3.10) in all CAM plants. Phase I is characterized by nocturnal PEP-C carbon fixation, whereas phase III is when Rubisco is active in the Calvin–Benson cycle under sunlight. At the very early hours of the morning (phase II) or in the afternoon (phase IV), both PEP-C and Rubisco are active, since stomata can be opened because temperatures are lower and water evaporation diminishes; therefore, CO2 is eventually fixed by the two enzymatic modes. Thus, the plasticity of CAM metabolism depends on the extent to which phases II and IV are extended or contracted, in response to stomata opening.

Figure 3.10. Phases of CAM carbon fixation pathway.

It is also important to note that a type of CAM metabolism occurs in some submerged aquatic plants, where the primary barrier to CO2 leakage is the extremely high diffusional resistance of water. CAM aquatic plants live in shallow temporary pools with extreme diel fluctuations in carbon availability, showing elevated nighttime CO2 levels. PEP-C can catalyze carbon fixation independently to light or oxygen, and even under very low CO2 concentrations in the tissues (contrary to Rubisco, that needs photosynthetic ATP to be activated and certain proportions of CO2 to O2); and besides, PEP-C is also able to recognize as a substrate HCO3, which is the transformation of atmospheric CO2 when it is hydrated.

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The Ecological Water-Use Strategies of Succulent Plants

R. Matthew Ogburn , Erika J. Edwards , in Advances in Botanical Research, 2010

2 Gas exchange: Is succulence a requirement for CAM?

CAM is a photosynthetic mode common in drought-avoiding succulent plants, as well as in some aquatic plants that experience severe diurnal reductions in CO2 availability (Keeley and Rundel, 2003). CAM is essentially an elaboration of the standard C3 photosynthetic pathway. In CAM photosynthesis, stomatal behaviour reverses; atmospheric CO2 is fixed at night and stomata are closed during the day. The initial CO2 fixation is performed by the enzyme phosphoenolpyruvate (PEP) carboxylase. CO2 is fixed into 4-carbon compounds, primarily malate, and stored in vacuoles as an acid. During the day, the 4-carbon acids are shuttled to the chloroplast where they are decarboxylated. The released CO2 is refixed by Rubisco and incorporated into the Calvin cycle, which then proceeds as in the C3 cycle. CAM significantly increases water-use efficiency (WUE; the ratio of carbon fixed to water lost through transpiration) relative to the C3 and C4 photosynthetic modes because nocturnal evaporative demand is lower while the CO2 gradient remains largely unchanged.

CAM is a very flexible photosynthetic system in terms of both degree of expression and variations on the photosynthetic pathway. While some taxa, such as the core cacti, are considered obligate CAM plants, many taxa exhibit facultative CAM in response to drought, switching back to C3 when the stressor is removed (Kluge and Ting, 1978). In other species, CAM is irreversibly induced by drought or salt stress, or simply as a function of ontogeny. Furthermore, a number of "CAM-like" behaviours have been identified. In CAM-cycling, stomatal opening is diurnal as in C3, but PEP carboxylase scavenges respiratory CO2 during the night, helping to maintain a positive carbon budget. In CAM-idling, often induced by extreme drought, stomata remain completely closed, and respiratory CO2 is recycled via the CAM pathway, allowing the plant to maintain basic metabolic function (and possibly mitigate damage to the photosynthetic apparatus) while minimising water loss.

There is no question that CAM and pronounced succulence are highly correlated (Sayed, 2001). CAM requires a large vacuole and thus some degree of succulence, at least at the cellular level (Kluge and Ting, 1978). The question remains, however, whether the correlation of CAM with morphological succulence is because some structural component of one facilitates subsequent evolution of the other, or whether they co-occur simply because of similar selection pressures (i.e. they both evolve independently in response to water-limitation).

Nelson et al. (2005) examined a number of leaf anatomical traits, including cell size, mesophyll thickness, area of inter-cellular air space (IAS), and the ratio of mesophyll surface to IAS (L mes/IAS), in a range of C3, C4, and CAM taxa. They found that cell size and mesophyll thickness were higher in CAM taxa as a group, while IAS and L mes/IAS were lower, confirming that CAM species as a whole tend to have thicker leaves with larger, more tightly packed cells. Within CAM taxa, however, there was no clear relationship between cell size and IAS or L mes/IAS; both IAS and L mes/IAS were generally low while cell sizes varied extensively, including the range of cell sizes in C3 and C4 taxa. These results indicate that tightness of cell packing is constrained in CAM taxa, while cell size is not. Tight cell packing reduces internal conductance (g i) to CO2, affecting rates of both CO2 influx and efflux. The authors argue that the reduction of CO2 efflux, especially during diurnal decarboxylation of malate and re-fixation of CO2 into the C3 cycle, provides a large benefit in carbon gain for CAM species. While low g i also impacts nocturnal carbon fixation, they argue that its impact is less because this step is generally limited by PEP carboxylase activity, not atmospheric CO2. A subsequent study in a group of taxa representing a range of CAM expression supported the importance of these leaf anatomical traits, confirming a negative correlation of both IAS and L mes/IAS with the proportion of nocturnal CO2 fixation (Nelson and Sage, 2008).

A number of studies have also shown a positive relationship between CAM photosynthesis and plant water uptake from the soil. Nocturnally accumulated malate functions as a solute in vacuoles of the chlorenchyma, reducing osmotic potential and providing a stronger driving gradient for soil water uptake. This effect has been demonstrated in Kalanchoe daigremontana (Smith and Lüttge, 1985), Stoeberia beetzii (von Willert et al., 1992), Clusia minor (Herrera et al., 2008), and Senecio medley-woodii (Ruess and Eller, 1985), although malate fluctuations are relatively unimportant in driving soil water uptake in Agave deserti (Smith et al., 1987; Tissue et al., 1991). As malate is consumed during the day and osmotic potential becomes higher again, the water so gained becomes thermodynamically more available to the tissues (Lüttge, 2004).

While it is commonly stated that CAM is adaptive in increasing the WUE of succulent plants in water-limited environments, few examples exist comparing the water relations of CAM and C3 succulents under drought conditions. Eller and Ferrari (1997) compared the daily course of CO2 exchange and WUE for two leaf succulents with similar growth form: Cotyledon orbiculata, which uses the CAM pathway, and Othonna opima, which uses C3. The response of these taxa was measured during a bergwind period, a sustained hot, dry wind (temperatures   >   40   °C) that occurs in the Namib Desert. They emphasised the finding that WUE values were nearly equivalent in these two taxa.

a CAM and productivity

CAM is usually considered to be a way of flexibly dealing with stress rather than as a way of maximising growth, based on many assumptions about the trade-offs between stress tolerance and growth or competitive ability (Lüttge, 2004). CAM and succulence have both been invoked as imposing inherent limitations on growth rates in taxa with these traits, either due to limitation by vacuole space for malate storage (Winter and Smith, 1996), because of a less favourable stoichiometry of ATP use per carbon gain in CAM (Lüttge, 2004), because of unfavourable ratios of photosynthetic assimilatory tissues to non-productive achlorophyllous water storage tissues in many succulent plants (von Willert et al., 1992) or because of limitation on carbon dioxide diffusion rates across low-density stomata and within assimilatory tissues (Borland et al., 2009). Nobel et al. (1992b) have demonstrated, however, that low growth rates and productivities are not intrinsic to CAM or succulence, but are more likely a function of the stressful environments in which they grow. Grown under optimal light and water conditions, productivities for agaves and opuntias can exceed those of most plants recorded (Borland et al., 2009; Nobel et al., 1992b). These high growth rates are attributed to a reduction of photorespiration in the CAM pathway via the high internal CO2 concentrations that occur during daytime decarboxylation when stomata are closed, as well as to the high investment in aboveground biomass in these plants.

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Rising Atmospheric Carbon Dioxide and Plant Responses: Current and Future Consequences

Amit Kumar Mishra , ... Madhoolika Agrawal , in Climate Change and Agricultural Ecosystems, 2019

11.3.3.3 Responses of CAM Plants

CAM is an adaptation observed in some vascular plants, such as prickly pear (Opuntia stricta), agave (Agave salmania), and pineapple (Ananas comosus). Fixation of CO2 and CO2 metabolism are progressively divided in CAM plants. Fixation of CO2 occurs at night, the early morning, and/or the late afternoon catalyzed by the cytosolic enzyme phosphoenol-pyruvate carboxylase (PEPC) to produce malate or aspartate, which is finally stored in the vacuoles (Seneweera and Norton, 2011). Decarboxylation takes place during the daytime and results in the release of CO2 from the malic or aspartic acid, which is finally converted into carbohydrates (Winter and Smith, 1996). Usually, CAM plants possess three to five-fold greater transpiration efficiencies than C3 or C4 plants (Nobel, 1996) and frequently these species exist in environments where water shortages prevail (Drennan and Nobel, 2000). Drennan and Nobel (2000) noticed an increase in both day and nighttime CO2 uptake in 10 species with an average biomass rise of about 35% when CO2 was doubled. They suggested that CO2 fixation by RuBisCO is enhanced in the late afternoon along with nocturnal CO2 fixation, however carboxylation activities of both RuBisCO and PEPC are reduced in response to EC. Under EC, nocturnal malate levels increase with increments in carbohydrate contents (Drennan and Nobel, 2000). Reductions in RuBisCO content in CAM species under EC are compensated by the upregulation of enzyme activities in order to maintain photosynthesis. With diminutive evidence of photosynthetic acclimation against EC, some CAM plants display greater CO2 assimilation (source capacity), higher transport of sucrose in the phloem, and sturdy sink strength (Drennan and Nobel, 2000; Osmond et al., 2008). Due to these adaptations, a better understanding of the mechanisms directing C gain in CAM plants may pave the way for new insights into physiological mechanisms that could help in the genetic manipulation of C3 species for C rich environments.

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Chloroplast evolution and genome manipulation

Arun K. Shanker , ... M. Maheswari , in Climate Change and Crop Stress, 2022

14.3 Crassulacean acid metabolism pathway

Crassulacean acid metabolism (CAM) is one of the mechanisms for concentrating CO 2 at the site of RuBisCo. This pathway is helpful for improving water use efficiency. CAM mechanism is mainly found in desert environment and different angiosperm families. Popular CAM plants are cacti, euphorbias, pineapple, vanilla, and agave. The C4 acids in CAM plants are formed in both temporally and spatially separated mesophyll and bundle sheath cell. PEP carboxylase in the cytosol captures CO2 at night, and the malate that generates from the oxaloacetate product is stored in the vacuole. During the day, the stored malate is transported to the chloroplast and decarboxylated by the NADP-malic enzyme, the released CO2 is fixed by the Calvin cycle, and the decarboxylated triose phosphate product is converted to starch using NADPH.

Cacti, for example, achieve high water use efficiency by opening their stomata during cool desert nights and closing them during hot and dry days. Closing the stomata during the day reduces water loss, but CO2 is taken up at night because H2O and CO2 share the same diffusion pathway. CO2 is incorporated when phosphoenolpyruvate is carboxylated to oxaloacetate, which is then reduced to malate. Malate is accumulated and stored in the huge vacuoles that are a common, but not required, anatomic feature of CAM plant leaf cells. A nocturnal acidification of the leaf is defined as the formation of large levels of malic acid, corresponding to the quantity of CO2 assimilated at night (Bonner & Bonner, 1948). The stomata close as the day progresses, reducing water loss and CO2 absorption. As the stocks of vacuolar malic acid are depleted, the leaf cells deacidify (Drincovich, Casati, & Andreo, 2001). The action of the NADP-malic enzyme on malate usually results in decarboxylation. The internally released CO2 cannot escape the leaf because the stomata are closed, therefore it is fixed and converted to carbohydrate through the Calvin cycle.

The increased internal CO2 concentration efficiently suppresses ribulose bisphosphate photorespiratory oxygenation and encourages carboxylation. The C3 acid produced by decarboxylation is assumed to be transformed first to triose phosphate, then to starch or sucrose, replenishing the original carbon acceptor's source. C4 plants carboxylase is active during the day, whereas CAM plants' carboxylase is active at night. Malate inhibits PEP carboxylase in both C4 and CAM plants, while glucose-6-phosphate activates it. Phosphorylation of a single serine residue in the CAM enzyme reduces malate inhibition and increases glucose-6-phosphate action, making the enzyme more catalytically active (Chollet, Vidal, & O'Leary, 1996). A PEP carboxylase-kinase catalyzes the phosphorylation. The efflux of Ca2+ from the vacuole to the cytosol, and the subsequent activation of a Ca2+/calmodulin protein kinase, stimulates the production of this kinase. (Bakrim, Brulfert, Vidal, & Chollet, 2001) (Fig. 14.5).

Figure 14.5. CAM pathway in plants. CAM, Crassulacean acid metabolism.

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