The contractile root of Ariocarpus fissuratus. Stem movement and the role of stones

Whole-plant movement can be distinguished from stem wrinkling

In Ariocarpus fissuratus, much of the plant axis remains underground, and the upper stem surface lies close to the ground. Garrett, Huynh and North studied cultivated plants 3–4 cm in diameter. They sought to establish whether lowering of the stem resulted from root contraction and what effects it had on temperature and water loss. To do this, they combined observations of plant position with root preparations and experiments in different substrate types. [Garrett et al. 2010, pp. 1951–1953]

Thin wires placed just above the root–stem boundary allowed changes in stem-base height to be measured. Separate ink marks on the stem recorded shortening of this organ. The decrease in distance between marks accounted for less than 10% of the total lowering. Most movement was therefore associated with root shortening. Visible stem wrinkling during water shortage contributed much less. [Garrett et al. 2010, pp. 1952, 1954–1955, Fig. 1]

In autumn and winter, observations continued for 90 days, with five or six plants per watering treatment. In summer, they continued for 50 days, with 11–12 plants. Movement occurred in both seasons and under different substrate moisture levels, with greater lowering in the autumn–winter series. Extrapolating the short series, the authors reported a rate corresponding to 6–30 mm per year. This is an annual scale calculated from those observations. [Garrett et al. 2010, pp. 1952, 1954–1955, Fig. 1]

The root expands radially and shortens along its axis

The older root portion beside the stem base had transverse wrinkles on its surface. Sections showed bands of xylem separated by parenchyma rays. Ray cells in contracted segments were distinctly wider in the radial direction than in younger segments. Expansion of these tissues increased the root circumference and accompanied axial compression of the connected tracheid system. [Garrett et al. 2010, pp. 1953–1956, Fig. 2]

The tracheids had broad, lignified annular thickenings, with deformable wall portions between them. In older segments, the distance between rings was smaller, indicating approximately 17% shortening of the portions between thickenings. The thickenings protected tracheid lumina from complete collapse, while the wall portions between them could fold. The authors described the whole as a compressible three-dimensional system in which expansion of parenchyma rays coexists with shortening of the xylem. [Garrett et al. 2010, pp. 1954–1956, Fig. 2]

During heat, stones at the surface made the difference

In an experiment on a building rooftop in Los Angeles, plants grew in large containers. Stems maintained at the sand surface were compared with those at the surface of a substrate covered with granite fragments. Each treatment contained five plants, with sensors placed inside their stems. During an eight-day heat episode, the highest measured stem temperature was 60,0°C in sand and 56,5°C among stones. [Garrett et al. 2010, pp. 1953–1954, 1956–1957, Fig. 4–5]

After transfer to milder greenhouse conditions, plants were observed for a further two months. All five from sand died, and all five from the stony treatment survived. The stones covered the substrate surface around the plant, limiting its heating. The result links stem position with a specific thermal environment. Air temperature alone did not represent tissue temperature. [Garrett et al. 2010, pp. 1954, 1957–1958, Fig. 4–6]

The benefit of a low stem depended on surface type

In an earlier treatment with sand lacking stones, plants at the ground surface heated more than stems projecting approximately 30 mm above it. In a later comparison on a stony surface, daily maximum temperatures were lower in plants lying near the ground than in those projecting 30–35 mm above the stones. Heat input from the immediate substrate explains the difference between these arrangements. Root contraction is therefore part of the interaction between growth form and microhabitat. [Garrett et al. 2010, pp. 1953–1954, 1956–1958, Fig. 3 and 6]

Cell-damage tests were conducted separately by staining tissues after controlled heating or cooling. Tolerance depended on the plants' previous temperature acclimatisation. This result helps link a plant's thermal history with its response to a short stress episode. Temperatures measured during the rooftop experiment and damage temperatures of tissue sections describe different exposure conditions. [Garrett et al. 2010, pp. 1953–1954, 1958]

Water loss showed a different pattern from temperature

In a separate twenty-four-hour experiment, six plants each were placed at the sand surface, at the stone surface or above the stones. Water loss did not differ significantly between these treatments. Plants were in a moderately dry substrate, and the authors associated the small nocturnal water loss with limited stomatal opening. In this arrangement, the benefit of stem position was best documented for protection against overheating. [Garrett et al. 2010, pp. 1953–1954, 1957–1958]