Two routes to carbon
In C₃ photosynthesis, atmospheric CO₂ is taken up primarily during the light part of the day and enters the Calvin cycle. In CAM metabolism, some carbon is fixed at night into organic acids, stored in vacuoles, then released during the day for photosynthesis. Separating these stages in time limits gas exchange during periods of greater atmospheric water demand. In leafy opuntioids, the contributions of the two pathways can change with conditions. [Nobel and Hartsock 1987] [Martin and Wallace 2000]
Researchers distinguish two measurements. Gas exchange determines CO₂ influx from the atmosphere and its release, while changes in tissue acidity show nocturnal acid accumulation. Acid can also form from CO₂ released by respiration. This internal cycle, termed CAM-cycling when atmospheric uptake occurs by day, limits loss of respiratory carbon. Measuring gases and acids together allows the contribution of each source to be established. [Martin and Wallace 2000]
An experiment with increasing drought
Nobel and Hartsock compared six leafy cacti, including Pereskiopsis porteri, Quiabentia verticillata (in the paper, Q. chacoensis) and Austrocylindropuntia subulata. Plants were maintained at 25°C by day and 15°C at night, with 12 hours of light and 12 hours of darkness. Before drought, they were watered regularly with diluted nutrient solution. The start of the drought period was defined physiologically, when substrate water potential fell below stem water potential. [Nobel and Hartsock 1987]
Leaves and stems were measured separately, usually on two plants with similar gas exchange patterns. Comparison after 0, 7 and 14 days revealed a shift in carbon uptake towards the night. The shift was especially strong in P. porteri leaves, where the nocturnal share increased from around 6% to 92% after one week and 97% after two weeks. [Nobel and Hartsock 1987]
The leaf's full-day balance
A high proportion of nocturnal uptake must be read together with the absolute amount of carbon acquired. Table values are integrals over the twelve-hour day or night phase, expressed in mmol CO₂ per square metre of leaf area. In all three species, substantially less total uptake remained after fourteen days than at the start. [Nobel and Hartsock 1987]
| Species | Drought [days] | Day [mmol m⁻²] | Night [mmol m⁻²] |
|---|---|---|---|
| Pereskiopsis porteri | 0 | 108 | 7 |
| Pereskiopsis porteri | 7 | 6 | 72 |
| Pereskiopsis porteri | 14 | 1 | 35 |
| Quiabentia verticillata (as Q. chacoensis) | 0 | 129 | 9 |
| Quiabentia verticillata | 7 | 24 | 16 |
| Quiabentia verticillata | 14 | 0 | 2 |
| Austrocylindropuntia subulata | 0 | 111 | 2 |
| Austrocylindropuntia subulata | 7 | 6 | 24 |
| Austrocylindropuntia subulata | 14 | 0 | 15 |
In P. porteri the full-day leaf balance fell from 115 to 78 and then 36 mmol CO₂ m⁻². These are sums of the day and night values in the source table. In Q. verticillata 2 mmol m⁻² remained after two weeks. The metabolic shift towards the night thus accompanied reduced activity of the entire organ, especially pronounced in the latter species. [Nobel and Hartsock 1987]
The stem may follow a different rhythm
In well-hydrated A. subulata, leaves took up almost all CO₂ during the day, while the green stem had a positive nocturnal balance. As drought progressed, nocturnal stem uptake weakened. In P. porteri and Q. verticillata, daytime stem exchange was initially positive too, then shifted towards the night under water deficit. The assimilating organ is therefore an important part of describing a species' physiology. [Nobel and Hartsock 1987]
Leaf anatomy of Austrocylindropuntia subulata
Ozerova and Timonin studied the structure of the persistent, cylindrical leaf of A. subulata using light and scanning microscopy. A palisade zone lies immediately beneath the epidermis, with compact non-palisade parenchyma deeper inside. Both zones contain chloroplasts and store water. In the material studied, the leaf interior was chlorenchyma with scattered mucilage cells, rather than a separate central store of chlorophyll-free tissue. [Ozerova and Timonin 2020b]
The leaf appears almost circular in cross-section, but the vascular bundle arrangement retains asymmetry. Its structure represents a compromise between a substantial volume of living, hydrated cells and the gas diffusion path. The authors discuss this anatomy in the context of C₃ and CAM. Anatomical sections themselves describe structure, while gas exchange experiments document metabolic dynamics. [Nobel and Hartsock 1987] [Martin and Wallace 2000] [Ozerova and Timonin 2020b]
Leafy lineages on the evolutionary tree
Analysis of 103 plastid genes in 107 opuntioid taxa indicated independent establishment of long-lived, photosynthetic leaves in different lineages. Present-day Pereskiopsis, Quiabentia and Austrocylindropuntia are therefore considered distinct evolutionary solutions. The physiological flexibility of a leaf and a species' position in the relationship tree describe two complementary levels of organisation. [Majure et al. 2023]
Prepared from scientific publications. Accepted names were checked in POWO on 9 October 2026.
Drought‐induced shifts in daily CO2 uptake patterns for leafy cacti
Nobel P. S., Hartsock T. L., Drought‐induced shifts in daily CO2 uptake patterns for leafy cacti, Physiologia Plantarum, 1987, 70(2):114-118, doi: 10.1111/j.1399-3054.1987.tb06119.x
The experiment compares daily CO₂ exchange in the leaves and stems of six leafy cacti during increasing drought. Three opuntioids showed a shift from predominantly daytime leaf assimilation to predominantly nocturnal assimilation, while total CO₂ uptake declined. The table includes separate day and night balances after 0, 7 and 14 days of drought.
Photosynthetic pathway variation in leafy members of two subfamilies of the Cactaceae
Martin C. E., Wallace R. S., Photosynthetic pathway variation in leafy members of two subfamilies of the Cactaceae, International Journal of Plant Sciences, 2000, 161(4):639-650, doi: 10.1086/314285
The study combines measurements of daily gas exchange with measurements of leaf and stem acidity in eighteen leafy cacti. It distinguishes uptake of atmospheric CO₂ from nocturnal recycling of respiratory carbon. It shows varying contributions of C₃ and CAM photosynthesis in individual organs and species.
Structure of the chlorenchyma in the persistent terete leaves of Austrocylindropuntia subulata (Muehl.) Backeb. (Cactaceae): anatomy beyond functional limitation
Ozerova L. V., Timonin A. C., Structure of the chlorenchyma in the persistent terete leaves of Austrocylindropuntia subulata (Muehl.) Backeb. (Cactaceae): anatomy beyond functional limitation, Wulfenia, 2020, 27:221-232
A microscopic analysis of the persistent, cylindrical leaves of Austrocylindropuntia subulata. It describes outer palisade parenchyma and inner non-palisade parenchyma, mucilage idioblasts and the arrangement of vascular bundles. The discussion concerns the combination of assimilation and water storage in the same tissues.
Phylogenomics of One of the World’s Most Intriguing Groups of CAM Plants, the Opuntioids (Opuntioideae: Cactaceae): Adaptation to Tropical Dry Forests Helped Drive Prominent Morphological Features in the Clade
Majure L. C., Achá S., Baker M. A., Puente-Martínez R., Köhler M., Fehlberg S., Phylogenomics of One of the World’s Most Intriguing Groups of CAM Plants, the Opuntioids (Opuntioideae: Cactaceae): Adaptation to Tropical Dry Forests Helped Drive Prominent Morphological Features in the Clade, Diversity, 2023, 15(4):570, doi: 10.3390/d15040570
An analysis of 103 plastid genes in 107 Opuntioideae taxa. The paper reconstructs geographical origins, habitat shifts and the evolution of large leaves and flattened stems. It indicates the independent establishment of long-lived photosynthetic leaves in different opuntioid lineages.
