This allows increased carbon gain during periods of water availability or during seedling establishment. Seedlings and well-watered plants may show little or no CAM and perform C3 photosynthesis, opening their stomata during the day. CAM plants show a high degree of metabolic flexibility. The internal concentration of CO 2 is raised as high as 10,000 ppm, which also suppresses photorespiration. This process occurs behind closed stomata. During the day, malate is decarboxylated, via PEP carboxykinase or NAD(P)-malic enzymes in the cytosol, and CO 2 is re-fixed via the Calvin- Benson cycle and carbohydrates are reformed. Malic acid is synthesized from carbohydrates, via PEP carboxylase, at night, and is stored in the vacuole. CAM occurs in between 5% and 10% of plants and is always associated with succulence, at least at a cellular level.Īlthough the biochemistry of CAM plants is similar to that of C4 plants, the two carboxylations are now separated in time rather than in space. CAM plants close their stomata during the day and take up CO 2 at night, when the air temperature is lower and water loss can be lowered by an order of magnitude. Blankenship, in Encyclopedia of Biological Chemistry (Third Edition), 2021 Crassulacean Acid MetabolismĬrassulacean acid metabolism (CAM) is a photosynthetic adaptation to periodic water supply, occurring in plants in arid regions (e.g., cacti) or in tropical epiphytes (e.g., orchids and bromeliads). Thus, the expression of CAM phases with variation of daytime CO 2 uptake (phases II and IV) and nocturnal CO 2 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 C 3–CAM switches. They survive unless they lose more than 50% of the water reserves from their water-storing tissues. 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. In extreme cases, there is no net CO 2 exchange at all during day and night, and nocturnal organic acid accumulation is exclusively due to CO 2 recycling. Eventually, i.e., under severe water stress, phase I net CO 2 uptake is also gradually suppressed, while nocturnal acid accumulation may remain prominent due to recirculation of respiratory CO 2. Conversely, strong CAM is the expression of all four phases of CAM and the gradual suppression of the daytime phases of net CO 2 uptake (II and IV) as stress increases. This is referred to as weak CAM or “CAM-cycling”. Some plants show a C 3-photosynthesis-like day/night gas exchange with exclusively daytime CO 2 uptake and no night-time CO 2 uptake, which is nevertheless accompanied by nocturnal accumulation of organic acid due to internal recycling of respiratory CO 2. The manifestation of CAM phases may also relate to the performance of weak and strong CAM, respectively. Expression of CAM phases is influenced by environmental factors, such as availability of water, irradiance, and sun exposure ( Figure 8). 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). Lüttge, in Encyclopedia of Applied Plant Sciences, 2003 CAM PhasesĬAM phases sensu C.B.
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