Field plants strategically regulate water uptake from different soil depths by spatiotemporally adjusting their radial root hydraulic conductivity

Abstract

Plants modify their root hydraulics to maintain water status and strategically use soil water, but how they achieve this in the field conditions remains elusive. We developed a method to measure and calculate daily root water uptake, root water potential, and radial root water permeability at different depths in a wheat (Triticum aestivum L.) field and a permanent grassland dominated by ryegrass (Lolium perenne L.). During the drying processes, both plant systems reduced the radial water permeability of their shallow roots to limit topsoil water uptake, while increasing the radial water permeability of their roots in the subsoil to enhance water extraction. Conversely, after the topsoil was rewetted, both plant systems increased the radial water permeability of their shallow roots to enhance water extraction, while reducing the radial water permeability of their roots in the subsoil to limit water uptake. Root water uptake in the subsoil was more influenced by the topsoil water than by the subsoil water. The topsoil water serves both as a resource and a signal, coordinating optimal water uptake from different soil depths. These findings have important implications for understanding how plants cope with periodic water stress in the field and for screening drought-tolerant crop varieties.

Keywords: in situ measurements; permanent grass; radial root water permeability; root water potential; root water uptake; winter wheat.

Fig. 1

Schematic showing hourly changes in soil water content over a single day and its preceding and following nights.

Fig. 2

Temporal changes in total root water uptake in the two plant systems (a). Contribution of roots in different soil layers to the total root water uptake in the wheat (Triticum aestivum) field (b) and the grassland (c). The red arrows indicate the two rainfall events.

Fig. 3

Changes in root water uptakes (normalised) with soil water content for different soil layers in the two plant systems: the grassland (a–c) and the wheat (Triticum aestivum) field (d–f). The colour gradients in the symbols represent dates from 1 April (dark blue) to 30 June (bright yellow). The large open triangles indicate the points just before the 5 June rainfall, and the open squares indicate the points just after the 5 June rainfall.

Fig. 4

In both plant systems, root water uptakes (normalised) in the subsoil layers are correlated with the topsoil water content (15‐cm soil layer) more significantly than with the subsoil water content. The plant systems were the grassland (a, b), and the wheat (Triticum aestivum) field (c, d). The colour gradients in the symbols represent dates from 1 April (dark blue) to 30 June (bright yellow).

Fig. 5

Changes in root water uptake rate (volume of water taken daily by roots in a unit volume of soil) with soil matric potential in different soil layers in the two plant systems (a–c for the grass system, and d–f for the wheat system). The colour gradients in the symbols represent dates from 15 April (dark blue) to 25 June (bright yellow). The solid lines are the best‐fitting curves. The blue lines represent the data before the 5 June rainfall, and the orange lines represent the data after the 5 June rainfall.

Fig. 6

Changes in root water potential and radial root water permeability at the depth of 15 cm calculated using the relationships between root water uptake rate and soil matric potential for the two plant systems (Fig. 5(a) for the grassland and Fig. 5(d) for the wheat field). (a) Root water potential: the solid lines and dashed lines represent results calculated using data before and after the 5 June rainfall, respectively. (b, c) Radial root water permeability at the depth of 15 cm: the solid green line and the dashed green line represent the results calculated using the data before the 5 June rainfall when roots did not grow (α = 0) or grew at a rate of α = 0.002 d⁻¹, respectively; the solid blue line and dashed blue line represent the results calculated using data after the 5 June rainfall when the roots did not grow (α = 0) or grew at a rate of α = 0.002 d⁻¹, respectively; ‘a’ marks the onset of the experiment, ‘b’ marks the point just before the 5 June rainfall, ‘b'‘ marks the point just after the 5 June rainfall, and ‘c’ marks the end of the experiment.

Fig. 7

Changes in radial root water permeability with soil matric potential at the depths of 30 and 45 cm before and after the 5 June rainfall in the two plant systems ((a) and (b) for the grass system; (c) and (d) for the wheat system). The solid lines are the results calculated when roots did not grow (α = 0), and the associated dashed lines are the results calculated when roots grew at a rate of α = 0.002 d⁻¹, from 1 April to 30 June.