Storage in living cells
Water in a fleshy organ is associated with living cells. Large vacuoles occupy much of their volume, and thin primary walls permit reversible changes in shape. Depending on the plant, such cells occur in a leaf, stem or underground organ. The value of a reserve depends on its availability to other tissues as the surroundings dry. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2291, 2295–2297]
In some succulents, parenchyma cells combine water storage with photosynthesis. In others, green photosynthetic parenchyma, termed chlorenchyma, is differentiated from water-storage parenchyma, or hydrenchyma, which is less photosynthetically active or lacks chloroplasts. In an aloe leaf, hydrenchyma forms an extensive internal zone. This division of functions allows water transfer from storage tissue to photosynthetic tissue to be examined. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2291, 2295–2297]
From reserves to flow
Flow direction depends on a difference in water potential, which describes the energetic state of water. Dissolved substances and pressure inside the cell both affect this potential. Water moves towards a lower potential when a transport pathway is available. A water-rich tissue can therefore release water to an adjacent tissue even while both regions remain fleshy. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2291, 2295–2297]
During early drought, hydrenchyma can lose a larger fraction of its water than chlorenchyma. Differences in wall elasticity and the handling of osmotically active solutes favour this response. Stored water then supports the hydration of photosynthetic tissue. Hydraulic capacitance expresses the amount of water released for a change in water potential. It is a functional measure of buffering capacity rather than a measure of leaf thickness alone. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2291, 2295–2297]
Water in the daily rhythm of aloe
The daily carbon cycle can change the distribution of water between tissues. In Aloe vera, nocturnal accumulation of organic acids in the chlorenchyma lowers its osmotic potential. Cabrita interprets this as driving water from the hydrenchyma and reducing turgor in its cells. During the day, consumption of these acids changes the gradient, allowing water to return to the storage tissue. Root uptake also contributes to replenishing the reserves. [Cabrita 2024, Cabrita 2024 p. 2–3, 7–8]
The 2024 study combined gas-exchange measurements with a non-invasive probe clamped onto a leaf patch. The probe signal in aloe followed a daily pattern opposite to that of the studied C₃ banana, Musa acuminata. The author related this pattern to turgor in the extensive hydrenchyma. It was an indirect measurement interpreted through a model of leaf mechanics, rather than direct imaging of the path of individual water molecules. [Cabrita 2024, Cabrita 2024 p. 3–4, 7–8, 10–12]
Measurement scope and volume changes
Some controlled experiments used 25°C during the day and 18°C at night, with sixteen hours of light and eight hours of darkness. Other series were conducted in a greenhouse. Abrupt switching of the lights shortened the transitional gas-exchange phases. The result concerns the Aloe vera leaf, in which hydrenchyma occupies a large proportion of the tissue. Applying the method to a species with a different tissue organisation requires separate validation. [Cabrita 2024, Cabrita 2024 p. 3, 7–8, 12–13]
Release of stored water is accompanied by changes in organ volume. Their extent depends on tissue proportions, wall elasticity and the duration of dehydration. Physiological mobilisation of water is one possible cause of surface wrinkling. Assessment of an individual plant also takes account of its watering history, root condition and surrounding environment. Organ shape alone describes only part of its water balance. [Fradera-Soler et al. 2022, Fradera-Soler i wsp. 2022 p. 2291, 2295–2297]
Prepared from the full texts of the supplied publications. The original diagram illustrates functional relationships and is not drawn to scale.
Elastic and collapsible: current understanding of cell walls in succulent plants
Fradera-Soler M., Grace O. M., Jørgensen B., Mravec J., Elastic and collapsible: current understanding of cell walls in succulent plants, Journal of Experimental Botany, 2022, 73(8):2290-2307, doi: 10.1093/jxb/erac054
Review of succulence, wall biomechanics and cell folding during dehydration. It distinguishes observations from hypothetical regulation of wall remodelling.
Water exchange between the Chlorenchyma and the Hydrenchyma and its physiological role in leaves with Crassulacean acid metabolism
Cabrita P. J. V., Water exchange between the Chlorenchyma and the Hydrenchyma and its physiological role in leaves with Crassulacean acid metabolism, Physiologia Plantarum, 2024, 176(2):e14221, doi: 10.1111/ppl.14221
Gas exchange and indirect continuous assessment of water relations in the Aloe vera leaf. Interpretation of the probe signal depends on the proportion of leaf hydrenchyma.