CAM photosynthesis and daily carbon management

Gas exchange and water loss

A plant needs carbon dioxide to produce organic compounds. CO₂ reaches the interior of green tissues through stomata, regulated openings in the epidermis. Water vapour escapes along the same route. Carbon supply and water loss are therefore linked. Closing stomata reduces transpiration but also restricts the use of atmospheric CO₂. [Winter and Smith 2022]

Shifting most gas exchange to a cooler time of day can reduce the water cost of carbon uptake. Evaporative conditions, including air temperature and humidity, matter more than darkness itself. A hot, dry night does not offer the same water-saving opportunities as a cooler, more humid night. CAM therefore needs to be considered alongside the plant's microclimate. [Winter and Smith 2022]

CAM stands for crassulacean acid metabolism. The name is historical and does not restrict the phenomenon to that family. It also occurs in many cacti and other distantly related plant groups. A fleshy organ alone is insufficient to establish the pattern of its gas exchange. [Winter and Smith 2022]

Night-time carbon fixation and vacuolar acidification

In typical CAM, stomata open at night and carbon from CO₂ is incorporated into organic compounds. Phosphoenolpyruvate carboxylase, abbreviated PEPC, plays an important role. The resulting oxaloacetate is converted to malate. Vacuoles accumulate malic acid, and the tissue becomes more acidic during the night. [Winter and Smith 2022]

This is not storage of gas bubbles. Carbon is bound chemically. The skeletons needed to fix it arise from the metabolism of stored carbohydrates. The vacuole also serves as a compartment in which acid can be stored without equally acidifying the entire cytoplasm. The night-time stage therefore requires cooperation between enzymes, membrane transport and resources produced earlier by the plant. [Winter and Smith 2022]

Daytime use of stored carbon

During the day, malic acid is consumed and the released CO₂ becomes available for assimilation reactions in chloroplasts. Rubisco incorporates carbon into the Calvin cycle. These transformations are linked to the supply of energy and reducing power produced by light-dependent reactions. When CO₂ comes from the reserve accumulated at night, stomata can remain closed for much of the day. [Winter and Smith 2022]

Night-time CO₂ uptake therefore does not mean that light has become unnecessary or that all photosynthesis occurs in darkness. CAM separates important stages of carbon management in time. Keeping a cactus in a dark place limits available energy and the production of compounds needed for subsequent cycles. Water reserves may delay visible effects of deteriorating conditions, but do not replace appropriate lighting. [Winter and Smith 2022]

Four phases of the model cycle

The division into night and day is a useful starting point, but actual gas exchange can be more complex. The classical model distinguishes four phases. Their duration and intensity are not fixed, and they need not all be equally distinct in every plant and under all conditions. [Winter and Smith 2022]

This model shows why stating that stomata open only at night may be too categorical. The plant responds to water availability, temperature, light and its developmental stage. The diagram shows the organization of the process, not an unchanging timetable. [Winter and Smith 2022]

Effects of temperature on gas exchange

Nobel's experiment on Opuntia ficus-indica compared CO₂ uptake and night-time acid accumulation at different growth temperatures. The optimum temperature for night-time CO₂ uptake changed with the conditions in which the plants had previously been kept. The response was therefore not determined by one unchanging temperature. [Nobel and Hartsock 1984]

In the system studied, the greatest whole-night CO₂ uptake and acid accumulation occurred at growth temperatures close to 25°C by day and 15°C at night. Hydration and the amount of light also affected the result. These data apply to a particular species and experimental conditions. They do not establish a universal greenhouse setting for all cacti. [Nobel and Hartsock 1984]

The practical conclusion goes beyond this temperature pair. Plant appearance and activity should be related to the entire daily cycle, including the night. Strong daytime lighting may not compensate for unfavourable temperatures during the rest of the cycle. [Nobel and Hartsock 1984]

Severe drought and restricted carbon supply

Prolonged water shortage can cause stomata to close even at night. In the state called CAM-idling, the plant restricts external gas exchange and may refix some CO₂ from respiration. This reduces losses but does not provide the same supply of new carbon as active exchange with the atmosphere. [Brulfert et al. 1987]

Brulfert and co-authors studied this state in Opuntia ficus-indica, using labelled carbon to trace metabolic transformations. After transition to complete drought, tissues retained the ability to metabolize CO₂ behind closed stomata. At the same time, the malate pool and the amplitude of its daily fluctuations decreased. Survival under these conditions did not mean maintenance of the full activity of a well-watered plant. [Brulfert et al. 1987]

This distinction matters in cultivation. A cactus may remain alive for a long time with very limited growth. The ability to survive drought alone does not justify keeping it short of water throughout a period when it could produce healthy growth. [Brulfert et al. 1987]

Methods for identifying CAM metabolism

The daily CO₂ balance can be studied by measuring gas exchange, and acid accumulation by comparing tissue acidity in the evening and morning. These methods describe different parts of the process. Measuring an enzyme, detecting malate or observing a fleshy stem does not replace information on when and from where the plant actually obtains carbon. [Winter and Smith 2022]

Winter and Smith emphasize night-time tissue acidification as a feature distinguishing CAM from the ordinary presence of organic acids in plant metabolism. In practice, measurements must therefore be taken at the appropriate time of day. A single sample taken without information on time and growth conditions may not explain the phenomenon studied. [Winter and Smith 2022]

Implications for watering and growing position

Watering supplies roots with water, while night-time stomatal opening concerns shoot gas exchange. These are linked processes but not identical activities. The occurrence of CAM does not imply a single obligatory watering time. Root condition, substrate temperature, air access and the duration of moisture all matter. [Winter and Smith 2022] [1-WWW]

During active growth, water reserves should be replenished in a way that matches the species' needs and growing conditions. Requirements differ in cool conditions or during a clear reduction in activity. Assessment requires observing new growth and the drying rate of the entire root ball. A dry pot surface should not be equated with a dry interior. [Winter and Smith 2022] [1-WWW]

Good records of light, night temperatures, watering and the appearance of new growth reveal relationships that are invisible during a single inspection. Knowledge of CAM helps interpret these observations. It does not replace habitat information or species identification. [Winter and Smith 2022] [1-WWW]

CAM during winter and after freezing

Studies of the annual cycle of Iowa material identified as Opuntia humifusa showed strong seasonality in gas exchange. Night-time CO₂ uptake dominated from April to September. Acidity changes were small in winter, and the carbon balance could be negative. Measurements were taken on collected stems under conditions reproducing habitat temperature and light. A night-time increase in acidity alone does not demonstrate CO₂ uptake from the air, because it also includes refixation of respiratory carbon. [Koch and Kennedy 1980]

Nobel studied Pelecyphora vivipara material (formerly Coryphantha vivipara var. deserti) from Nevada. In the single plant shown in figure 1, a night at −10°C reduced the maximum rate of night-time CO₂ uptake the following night by about 70%. About 90% of previous activity returned on the second night, and the full level on the fourth. [Nobel 1981, Nobel, pp. 195–196, Fig. 1; POWO, 11 October 2026][2-WWW]

Trials of survival and recovery of gas exchange included five plants for a nine-hour exposure at each temperature level. After −12°C, five of five survived, after −15°C three of five, and after −20°C none. Survival was defined as recovery of positive night-time CO₂ uptake within two months. Survivors of −15°C showed no positive balance during the first ten nights. The same plants underwent successive trials after recovering activity. Exposure conditions and damage indicators are given in the account of frost-hardiness physiology. [Nobel 1981, pp. 195–196, Fig. 2]

CAM and dehydration tolerance in Blossfeldia

In Blossfeldia liliputana, carbon isotope composition indicates a contribution from CAM, while a drying experiment documents survival after extensive mass loss. Stomatal density averaged about 0.6 mm⁻², with stomata concentrated in areole depressions. The account of dehydration physiology distinguishes isotope results, mass changes and observations of rehydration. [Barthlott and Porembski 1996]

Leaves and stems active at different times

In Pereskiopsis porteri, Quiabentia verticillata (Q. chacoensis in the earlier experiment) and Austrocylindropuntia subulata, drought shifted leaf CO₂ uptake towards the night. At the same time, total daily assimilation decreased. A separate account compares day and night balances from the experimental table and explains the role of internal carbon recycling. [Nobel and Hartsock 1987] [Martin and Wallace 2000]

Spines alter the light dose reaching the stem

An experiment with Cylindropuntia bigelovii (Opuntia bigelovii in the publication) showed a shift in the relationship between night-time acidity increase and daily light dose after spine removal. The description of methods and results distinguishes radiation incident on the outer boundary of the spine covering from light available to the photosynthetic surface. [Nobel 1983]

Light and drought in Mammillaria heyderi subsp. gaumeri

In Yucatán populations of Mammillaria heyderi subsp. gaumeri (M. gaumeri in the study), about 97% of the net CO₂ balance during the rainy season occurred at night. The daily balance was 17.6 mmol CO₂/m² in dry forest and 18.8 mmol CO₂/m² in dune scrub. After thirty days of drought, it fell to 1.35 mmol CO₂/m², and new areoles stopped forming. Gas-exchange measurements included four adult plants per site. Low carbon uptake continued while the production of new areoles was inhibited. [Cervera et al. 2007, Cervera et al., pp. 623–626, Fig. 4–5; POWO, 11 October 2026][2-WWW]

CAM and dragon fruit stem age

Climbing dragon fruit illustrates variation in daily gas exchange within a plant. Graham and Nobel observed younger, well-watered Selenicereus undatus segments with substantial daytime CO₂ uptake. Other segments took it up mainly at night. Thickness and transpiration measurements distinguished these rhythms. In Wang’s study, CAM indicators persisted during sixteen weeks of drought, while stomatal opening and metabolic activity decreased. The results concern different segments and procedures and indicate plasticity in the intensity of gas exchange. [Graham and Nobel 2005, Graham and Nobel 2005 p. 15–19, Wang et al. 2019 p. 2–8][Wang et al. 2019, Graham and Nobel 2005 p. 15–19, Wang et al. 2019 p. 2–8]

Evolution and plasticity of CAM among succulents

CAM evolved independently in different plant lineages. It combines stomatal regulation, daily enzyme rhythms and transport of organic acids across the vacuolar membrane. The emergence of this system required coordination of components already present in plant metabolism. Evolution altered, among other features, the timing and intensity of their activity. A comparable daily pattern can therefore occur in distantly related species. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]

Heyduk’s 2022 review considered the emergence of terrestrial CAM lineages in relation to declining atmospheric CO₂ and environmental change. Lower CO₂ availability, high temperature and stomatal closure during drought can increase the cost of photorespiration. CAM supplies CO₂ within photosynthetic tissue and changes the carbon and water balance. Reconstruction of its origins nevertheless depends on analyses of individual lineages rather than a single date applying to the entire group. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]

Constitutive CAM denotes predominant use of nocturnal carbon fixation under typical conditions. Facultative CAM can be upregulated by particular environmental cues, such as water deficit. Some plants also combine substantial daytime CO₂ uptake with weaker CAM. These terms refer to the measured contribution of the processes. Within a plant, organ age and previous growth conditions also matter. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]

Drought responses differ among the species studied. The review contrasts increased nocturnal CO₂ fixation in seedlings of some Clusia and Kalanchoe with reduced fixation in Yucca aloifolia. The different responses may relate to evolutionary lineage and developmental stage. More severe drought can restrict external carbon uptake even in plants with constitutive CAM. An account of an experiment therefore needs to retain the species and conditions. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]

Succulent anatomy favours malate storage in large vacuoles and retention of water reserves. At the same time, densely packed large cells influence CO₂ diffusion within the tissue. Storage benefits therefore interact with constraints on gas transport. Fleshiness and CAM remain associated traits whose strength should be described separately. A leaf section and a daily gas-exchange curve answer different questions. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]

Assessment of future growth conditions considers CO₂, temperature, light supply and water availability together. Higher CO₂ can increase growth in selected CAM plants, but warmer nights also alter respiration and organic-acid metabolism. Research is concentrated in relatively few lineages, including Agavoideae, Cactaceae and Crassulaceae. Species-specific measurements that account for interactions between these factors are needed before the results can support cultivation predictions. [Heyduk 2022, Heyduk 2022 p. 19–21, 24–26]