The water relations of the root–soil interface

Abstract

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non‐destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.

Effects of potassium deficiency on cell water relations and elongation of tap and lateral roots of maritime pine seedlings

“…However the measurements of Steudle (1993) have indicated that in most cases cell membrane reflection coefficients are near unity. We feel that the generally low value of W, compared with the nutrient solution, might result from the presence of solutes in the apoplasm (cell-wall solutes), perhaps as solutes accumulated at the root surface during mineral uptake, or as solutes leak from the cell membranes (Cosgrove & Cleland, 1983;Stirzaker & Passioura, 1996).…”

Effect of drought and heat stresses on plant growth and yield: a review

“…This effect can be further increased due to a decrease in soil-root hydraulic conductance related to root shrinkage, which severely hampers water flow to roots (Carminati et al, 2009 and Willatt, 1983). Recent observations point to a rhizospheric effect onto the water relations of this soil-root interface, involving mucilages, root exudates and possibly solute accumulation (Carminati and Vetterlein, 2013;McCully et al, 2009;Read et al, 2003;Stirzaker and Passioura 1996), which would modulate soil-root contact and water uptake with variations in dry or moist soil (White and Kirkegaard, 2010).…”

“…This effect can be further increased due to a decrease in soil-root hydraulic conductance related to root shrinkage, which severely hampers water flow to roots (Carminati et al, 2009 and Willatt, 1983). Recent observations point to a rhizospheric effect onto the water relations of this soil-root interface, involving mucilages, root exudates and possibly solute accumulation (Carminati and Vetterlein, 2013;McCully et al, 2009;Read et al, 2003;Stirzaker and Passioura 1996), which would modulate soil-root contact and water uptake with variations in dry or moist soil (White and Kirkegaard, 2010).At longer time scales, not only plasticity in water relations but also in root growth will occur during water deficit, with a decrease in root length (reduced growth, increased mortality) in drier parts and an increase in wetter parts (Huang and Eissenstat, 2000;Sekhon et al, 2010). If an increase in root growth can be observed at the onset of water stress, the continuing drought will reduce the overall root growth, resulting from uncoupling between carbon production in leaves and use in root sinks (root apex) (Muller et al, 2011).…”

Diagnosing plant water status as a tool for quantifying water stress on a regional basis in Mediterranean drylands

“…The significance of both Ψ pd and Ψ min as indicators of plant water status has been questioned. As an example, Ψ pd has been reported not to coincide with soil water potential [3,8,43] during dry periods due to an air gap between roots and soil [53]. In other cases (e.g.…”