Pfeffer's Studies of the Root Growth Pressures Exerted by Plants

Abstract

T HE pressure exerted 3 and work performed by roots during growth are important considerations in evaluating the effect of soil physical properties on plant growth. Data on experiments to measure such pressures appear in a paper by W. Pfeffer, "Druck und Arbeitsleistung durch Wachsende Pflanzen" published in Abhandlungen der Koniglich Sachsischen Gesellschaft der Wissenschaften, 33:235-474, 1893. Since this paper is absent from American soils literature the following review is presented to add to the data and to stimulate further interest in such measurements.The basic principle of Pfeffer's measurements of root pressure was to encase a portion of the root of a seedling plant in a plaster of Paris block in order to provide a base against which the growing root could exert pressure to move a second block that was cast around the exposed tip or side of the root. Movement of the second block would compress a calibrated spring providing a measure of root elongation or expansion and the pressure exerted by the root. Figure 1 shows the arrangement of Pfeffer's apparatus. Measur·ement of Axial Root Growth PressureTo determine the axial pressures of growing roots with this arrangement, (figure 1 (a)), a pot P filled with moist excelsior M, was fastened to a stand S. The lower portion of the root was embedded in a plaster of Paris block A which was anchored securely by the plaster of Paris collar, which extended into the pot through the bottom hole, as shown by the white oval area in the pot. The root tip R extended through block A into a shallow hole •in a second plaster of Paris block B. Block B was firmly secured to a glass disk D which was in turn connected with the stand S through compression spring F. The adjustment of screws E permitted the adjustment of the initial tension in the spring F. Two indicator needles I were mounted on the spring to permit a direct measurement of the compressed spring with the aid of a cathetometer.To prepare a plant for axial pressure measurement, a seedling was placed in the pot filled with excelsior with the root extended 15 to 30 mm. through the bottom hole. The pot was then inverted and a paper cylinder with a radius of about 1 em. placed around the root. The height of the cylinder was cut so that approximately 5 to 8 mm. of the root still extended above the cylinder. A plaster of Paris mixture was poured unto the cylinder, flowing down into the pot through the bottom hole forming a collar within the pot to bind block A to the pot. Then a glass plate, with a hole comparable to the size of the root, was covered with wax paper and pressed over the root tip, permitting the tip to extend upward through the glass and paper. When block A was hard, the glass and paper were removed and replaced by a very thin piece of paper similarly provided with a hole. The plaster of Paris block B was poured on top of this paper, thus separating blocks A and B by a very small gap G, corresponding to the thickness of the thin paper. Results of measurement of axial root growth pressures secured by this me...

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…Although such processes have been studied to some extent, testing hypothesis 1 will require measuring root pressures for relevant species in natural settings in comparison to the strength of rocky material. Of course, laboratory experiments on root strength are poorly suited to real-world bedrock settings in terms of both quantifying stresses over daily or annual timescales and replicating the fracture mechanics that result in actual root-fissure configurations (Gill and Bolt, 1955;Eavis et al, 1969;Misra et al, 1986;McCully, 1995;Gregory, 2006). Thus new techniques are needed to measure external root pressures in situ.…”

“…Specifically, root-related macropores can contain live roots, dead roots, or dead and live roots together (Ghestem et al, 2011). Ecohydrological separation -defined as trees using water of a character different from the gravity-drained water found in soils, in saprolite or in groundwater and streams -has been hypothesized to be common based on a recent meta-analysis of the isotope ecology literature (Evaristo et al, 2015) and global remote-sensing data based on the deuterium composition of atmospheric vapor (Good et al, 2015). These and related studies (e.g., Brooks et al, 2010) suggest that trees rely on water present in the unsaturated zone and this water may have an isotopic composition distinct from the gravitydrained water that transits the hillslope to become groundwater recharge and streamflow.…”

“…The evidence for ecohydrological separation (McDonnell, 2014;Evaristo et al, 2015;Good et al, 2015) suggests that plants sometimes use water from unknown depths and that the water potentials are different from what might be considered the "crop plant" wilting point, e.g., < −1.5 MPa (Kramer and Boyer, 1995). Furthermore, in some cases, Evaristo et al showed that gravity-drained and transpired waters were not isotopically distinct.…”

“…One of the outstanding research questions concerning small catchments is how to predict the relationship of solute chemistry and discharge as a function of variations in precipitation (Godsey et al, 2009). In catchments, many of the nutrients and other solutes added to rain water as it transits through hillslopes to the bounding streams are added from weathering reactions in the soil.…”

“…For example, many numerical models are available to simulate chemical weathering and erosion (Lichtner, 1988;Lebedeva et al, 2007;Minasny et al, 2008;Maher et al, 2009) but most only model trees indirectly by incorporating the assumption that trees can reduce the water flow through the soil through evapotranspiration. Where the impact of trees or biota has been incorporated into models of weathering or landscape development, the models typically focus on one aspect of trees' impact (Gabet and Mudd, 2010;Roering et al, 2010;Corenblit et al, 2011;Reinhardt et al, 2011;Godderis and Brantley, 2014). Many of our hypotheses target these holes in our understanding.…”