The root separates two water reserves
Drying soil changes the direction of the forces acting on water in the plant. When its potential becomes more negative than the water potential in the root, the soil can draw water from living tissues. The root system therefore lies between the cactus's well-supplied stem and the dry substrate, and its permeability influences both water uptake and loss. Some changes during drought limit outward flow into the surroundings. After wetting, the same system regains its ability to supply the stem. [North and Nobel 1992, pp. 9–10, 18]
Gretchen North and Park Nobel studied this transition in Ferocactus acanthodes and Opuntia ficus-indica. The ferocacti came from Agave Hill near Palm Desert in California. In the adopted POWO classification, F. cylindraceus is a synonym of F. acanthodes subsp. acanthodes. The material studied concerns this part of the species' range. The comparison included roots of different ages, allowing the response of the youngest portions to be separated from that of older tissues. [North and Nobel 1992][2-WWW]
Two flow pathways
Water first moves from the root surface towards its centre. This is the radial pathway, passing through the tissues surrounding the stele. It then flows along xylem vessels to other parts of the plant. This pathway is axial. Total root hydraulic conductivity depends on both portions. Open vessels can coexist with low permeability in the outer layers. Conversely, easy water penetration into the root remains ineffective when vessel flow is restricted. [North and Nobel 1992, pp. 10–11, 18]
Plants were maintained in a greenhouse for one year and then subjected to drought for 30 days. Water potential in the root zone fell from about −0,5 to −3,5 MPa. The ages of the portions studied at the start of the experiment were estimated as one, three and twelve months, accounting for colour, size, branching order and the plants' growth history. Measurement series during drying included five plants and those after rewetting three. The differences described therefore concern root age, rather than three age groups of whole cacti. [North and Nobel 1992, pp. 10–11, 14]
A young root in a soil sheath
A persistent soil sheath formed around one-month-old roots after about two days of drying. Root hairs and mucilage bound substrate particles into a layer up to 1,5 mm thick. The cortex itself, the tissue between the root surface and its stele, remained continuous. By the end of drought, its cells were less rounded, indicating partial dehydration. Hydraulic conductivity temporarily declined, but after 30 days remained close to its initial value. [North and Nobel 1992, pp. 13–14]
The importance of the sheath was also tested by removing it. Under aqueous conditions, this hardly changed root conductivity. However, when roots still attached to the plant were exposed to air for 24 hours, five roots without a sheath had an average of 41% of the conductivity of five sheathed roots. The sheath therefore mitigated the effects of drying. The authors related its action to protection of young tissues and altered conditions of water exchange at the root–substrate boundary. [North and Nobel 1992, pp. 14, 16–17]
The rupturing cortex of older roots
Three-month-old roots did not retain a similar, firmly attached sheath. After seven and thirty days of drought, ruptured cells and the resulting spaces, called lacunae, were visible in their cortex. The gas-filled areas interrupted part of the radial flow pathway. After 30 days, total root conductivity was 61% lower in the ferocactus and 79% lower in the opuntia. These results show a clear hydraulic cost of changes in tissues surrounding the xylem. [North and Nobel 1992, pp. 13–15]
Lacuna formation was accompanied by greater exposure of deeper tissues to air. Roots of both species already contained periderm, a secondary protective tissue with cells containing suberin in their walls. Suberin limits water penetration. The number of lignified layers increased during drought, although the extent of detectable suberisation in three-month-old roots remained similar. Anatomy thus revealed several overlapping changes, rather than a single new barrier appearing only in dry substrate. [North and Nobel 1992, pp. 14, 18]
An old root changes its properties without major restructuring
In twelve-month-old roots, microscopy showed an almost unchanged structure throughout the month of drought. The periderm had an average of 20 layers in the ferocactus and 18 in the opuntia. Despite this stability, hydraulic conductivity fell by 57 and 63%, respectively. The number of retained layers alone therefore did not determine their current permeability. The authors proposed that dehydration of existing suberised cell walls played an important role. [North and Nobel 1992, pp. 13–14, 18]
This mechanism was also proposed to explain the limited recovery of function after wetting. Dried periderm might retain lower permeability even when water again surrounded the root. This was an interpretation based on the comparison of anatomy and flow measurements. The experiment did not separately measure the permeability of isolated periderm walls in these cacti. A change in the function of older roots was demonstrated directly, while dehydration of a particular layer was identified as its probable basis. [North and Nobel 1992, p. 18]
Gas inside vessels
Drought also affected the axial flow pathway. Embolism developed in vessels, an interruption of the continuous water column by gas. The researchers compared the conductance of excised portions before and after a bubble-removal treatment. After pressure of 0,20 MPa was applied for 20 minutes, flow increased. Dye introduced into the xylem also revealed a larger number of conducting vessels. Both measurements indicated that some blockages were reversible. [North and Nobel 1992, pp. 11, 15–17]
Embolism occurred in all three age groups. It was particularly severe in older roots, but the extent of restriction of total flow also depended on the radial pathway. In the most affected opuntia roots, removing blockages increased axial conductance about 36-fold. Total conductivity increased by only about 17%. Other barriers continued to restrict water entry into the xylem. Opening one portion did not automatically restore the function of the entire root. [North and Nobel 1992, pp. 15, 18]
Water returns and recovery rates differ
After thirty days of drought, the substrate was rewetted. Within a week, the youngest roots recovered almost their initial conductivity. Three-month-old roots reached an average of about 53% of the maximum value, and twelve-month-old roots about 60%. Much of the improvement in older roots occurred on the first day. However, cortical lacunae remained visible. A rapid resumption of water uptake could therefore coexist with persistent effects of earlier dehydration. [North and Nobel 1992, p. 14, Fig. 4]
Axial conductance returned within seven days to the level observed before drought. For the youngest roots, this meant almost complete openness compared with the measurement after gas removal. Older roots already had some non-conducting vessels under initial conditions. Distinguishing the two reference points makes the result understandable. Restoring the pre-drought state and achieving the greatest possible flow through all vessels describe different levels of function. [North and Nobel 1992, pp. 15–16, Fig. 5]
The root and its contact with the substrate
Tissue conductivity was determined in excised portions immersed in solution, with flow driven by a pressure difference. Dry soil adds its own resistance, and a contracting root can lose direct contact with its particles. North and Nobel considered the formation of an air gap as an additional barrier at the root–substrate boundary. After rain, uptake effectiveness is jointly determined by tissue condition, rewetting of the surroundings and restoration of this contact. [North and Nobel 1992, pp. 10–11, 17–18]
In the described experiment, flow recovery occurred in surviving roots. New root formation from the stem is a further developmental process and requires its own documentation. Observations of new roots and measurements of the function of older portions concern two stages in restoring water uptake. North and Nobel's results primarily explain the second. They show how pathways that rapidly recover function coexist with barriers left by drought inside an apparently unchanged root. [North and Nobel 1992, pp. 10, 14–18]
Full text of an experiment on Ferocactus acanthodes from Agave Hill in California and Opuntia ficus-indica. Age refers to root portions at the start of thirty days of drought. Observations of flow in existing roots have been separated from the process of producing new roots from the stem.
Drought-induced changes in hydraulic conductivity and structure in roots of Ferocactus acanthodes and Opuntia ficus-indica
North G. B., Nobel P. S., Drought-induced changes in hydraulic conductivity and structure in roots of Ferocactus acanthodes and Opuntia ficus-indica, New Phytologist, 1992, 120(1):9-19, doi: 10.1111/j.1469-8137.1992.tb01053.x
The experiment compares root hydraulic conductivity and anatomy in Ferocactus acanthodes from California and Opuntia ficus-indica before a thirty-day drought and after rewetting. The sections studied differ in age. Flow measurements and microscopic images describe the significance of rhizosheaths, cortical lacunae, periderm and vessel embolism. The rate and extent of functional recovery in surviving roots were also determined.
Plants of the World Online — Royal Botanic Gardens, Kew
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