Two tissues within one stem
The fleshy stem of Selenicereus undatus contains an outer chlorenchyma, a chloroplast-rich parenchyma, and a deeper water-storage parenchyma. Both tissues hold water, but their volumes change to different degrees during drought. Connecting their properties helps explain how the stem maintains photosynthetic activity as its substrate dries. [Nobel 2006, p. 469–470]
In 2006, Nobel studied rooted stem segments without developing branches. This restricted the influence of water transfer between plant segments. He compared cell dimensions, water potential, osmotic pressure and nocturnal CO₂ uptake. The experiment involved greenhouse plants and progressive drying after irrigation ceased. [Nobel 2006, p. 470]
Water reserves sustain the chlorenchyma
After six weeks without irrigation, the stems had lost about one-third of their water. From cell dimensions and tissue proportions, the author estimated that approximately 95% of the lost water came from the water-storage parenchyma. This value describes a tissue’s contribution to water loss. The distribution of initial reserves and direct flow between tissues describe separate quantities. [Nobel 2006, p. 469, 471–473]
Over the same period, cell length measured perpendicular to the stem surface decreased by 44% in the water-storage parenchyma and by 6% in the chlorenchyma. Because other cell dimensions changed little, length was used to estimate volume changes. More compliant walls allowed the water-storage cells to shrink more substantially. The chlorenchyma retained a much greater proportion of its volume. [Nobel 2006, p. 471–473]
Water potential and wall compliance
Water moves along a water-potential gradient. Dissolved substances and pressure within a cell contribute to that potential. Turgor is the pressure exerted by the cell contents against the wall. In two tissues with different wall compliance, the same progressive water deficit can produce different changes in volume and pressure. [Nobel 2006, p. 469, 472–473]
Nobel tested the response of thin sections to perfusion with distilled water. Water-storage parenchyma cells increased in length more strongly than chlorenchyma cells. This supported an explanation based on differences in wall compliance. Water-potential and osmotic-pressure estimates also indicated concentration of solutes as cell volume decreased. [Nobel 2006, p. 470–473]
CAM under adequate watering and drought
In CAM metabolism, carbon dioxide taken up at night is initially fixed and stored as organic acids. During the day, CO₂ released from these acids supplies carbon for further photosynthesis. Wang and colleagues examined rhythms in stomata, acids, sugars and enzymes in Selenicereus undatus. They observed CAM metabolism in both irrigated and drought-treated plants. [Wang et al. 2019, p. 1–8]
Wang’s experiment lasted sixteen weeks, with greenhouse temperatures ranging from 22 to 29°C. During drought, fewer stomata opened and their apertures were narrower. Daily fluctuations in acid concentrations and the abundance of photosynthetic products declined. The response also involved chloroplasts and enzyme activity. The authors linked reduced assimilation to both stomatal factors and changes within the tissue. [Wang et al. 2019, p. 2–8]
Gas-exchange rhythms depend on the stem studied
In 2005, Graham and Nobel measured stem thickness, transpiration and CO₂ uptake simultaneously. In well-watered plants, they observed stems with gas exchange concentrated at night as well as younger segments with substantial daytime CO₂ uptake. These patterns were accompanied by different daily thickness rhythms. Segment age and measurement conditions therefore help interpret the contribution of individual CAM phases. [Graham and Nobel 2005, p. 13–19]
These observations can be compared with Wang’s results while retaining the scope of each method. Graham and Nobel measured gas exchange directly in individual chamber-grown stem segments. Wang described stomatal rhythms and metabolic indicators during a prolonged greenhouse experiment. Together, the studies describe a CAM plant whose gas exchange varies with conditions, stem age and drought severity. [Graham and Nobel 2005, Graham and Nobel 2005 p. 14–19, Wang et al. 2019 p. 2–8][Wang et al. 2019, Graham and Nobel 2005 p. 14–19, Wang et al. 2019 p. 2–8]
Roots, signals and renewed water uptake
After severing a stem segment or rapidly drying roots, Graham and Nobel observed changes in transpiration. Application of abscisic acid, a hormone associated with water-deficit responses, restricted gas exchange. The experiment supported hormonal involvement in stomatal regulation. The identity and route of the natural root signal remained part of the interpretation because hormone movement from root to stem was not measured directly. [Graham and Nobel 2005, p. 14, 16–19]
Rewatering changed hydration faster than the preceding drying process. In Nobel’s 2006 experiment, after thirty-five days of drought, stem and cell dimensions returned to values recorded under wet conditions within seven days. This result concerns the plants and substrate used in that procedure. In cultivation, the relevant relationship is between the retained root system and the ability to replenish stem reserves. [Nobel 2006, p. 471–473]
Gas exchange in a hot growing environment
Ben-Asher and colleagues measured stem CO₂ exchange in Selenicereus undatus and S. megalanthus in Beer Sheva from May to August 2004. Plants grew on trellises beneath netting providing 70% shade. Measurements covered the nocturnal CAM phase and early-morning CO₂ uptake. Stem temperature was measured directly. The estimated upper limits of gas exchange in both species peaked near 27°C, with uptake declining as temperature rose further. [Ben-Asher et al. 2006, p. 181–184, ryciny 1–3]
After irrigation was withheld, daily net CO₂ uptake in Selenicereus undatus fell by 53% after four days and by 89% after eight days. Mean stem temperatures during this period were 38°C by day and 29°C at night. These values describe a particular eight-day series in a hot, dry environment. Comparison with a longer greenhouse drought shows the importance of water-loss rate and heat exposure. The stem stores water, while its current carbon-assimilation capacity responds to conditions well before the whole plant visibly loses viability. [Ben-Asher et al. 2006, Ben-Asher i wsp. p. 182–184, rycina 2; Nobel 2006 p. 469–473][Nobel 2006, Ben-Asher i wsp. p. 182–184, rycina 2; Nobel 2006 p. 469–473]
Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus
Nobel P. S., Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus, Annals of Botany, 2006, 97(3):469-474, doi: 10.1093/aob/mcj054
Differences in wall compliance and hydration changes in water-storage parenchyma and chlorenchyma during six weeks of drought.
Daily changes in stem thickness and related gas exchange patterns for the hemiepiphytic cactus Hylocereus undatus
Graham E. A., Nobel P. S., Daily changes in stem thickness and related gas exchange patterns for the hemiepiphytic cactus Hylocereus undatus, International Journal of Plant Sciences, 2005, 166(1):13-20, doi: 10.1086/425669
Daily changes in stem thickness compared with transpiration, CO₂ uptake and abscisic-acid experiments.
The highly drought-tolerant pitaya (Hylocereus undatus) is a non-facultative CAM plant under both well-watered and drought conditions
Wang L., Zhang X., Ma Y., Qing Y., Wang H., Huang X., The highly drought-tolerant pitaya (Hylocereus undatus) is a non-facultative CAM plant under both well-watered and drought conditions, Journal of Horticultural Science and Biotechnology, 2019, 94(5):643-652, doi: 10.1080/14620316.2019.1595747
Stomatal, metabolite and enzyme rhythms in irrigated and drought-treated dragon fruit.
Net CO2 uptake rates for Hylocereus undatus and Selenicereus megalanthus under field conditions: Drought influence and a novel method for analyzing temperature dependence
Ben-Asher J., Nobel P. S., Yossov E., Mizrahi Y., Net CO2 uptake rates for Hylocereus undatus and Selenicereus megalanthus under field conditions: Drought influence and a novel method for analyzing temperature dependence, Photosynthetica, 2006, 44(2):181-186, doi: 10.1007/s11099-006-0004-y
Stem CO₂-exchange measurements and drought and temperature responses of two pitahayas under growing conditions.
