3.2.4  Observations of water uptake by roots

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(a)  Movement through bulk soil

In field soil, root length density in the topsoil is usually so high (Table 3.1, Figure 3.3) that the local rate of uptake of water is never likely to be limited by soil properties. However, in subsoil, roots become sparse and water flow through soil might limit uptake rates. Even when low root length densities are taken into account, water uptake is often much slower than simple theory would predict.

One possible reason for the discrepancy between theory and observation in water uptake by deep roots is that these roots do not ramify randomly through the soil. Subsoils are sometimes dense and difficult to penetrate so roots grow predominantly in pre-existing fissures or in continuous large pores, biopores, made by previous roots or soil fauna (Figure 3.5). A second reason is extrapolation from laboratory measurements in repacked soil of D to undisturbed soil in the field. The structure of undisturbed soil might inhibit water flow to roots. For example, soil aggregates formed naturally often result in particles of clay, usually in the form of small plates, becoming oriented parallel to the surface. Such orientation would increase greatly the path length for water flow but there are no reliable measurements of D on undisturbed subsoil to confirm this. As a consequence, water in subsoils that might be physically ‘available’ is not necessarily extractable by plant roots.

(b)  Resistance at root surfaces

A substantial resistance to water uptake exists at the interface between soil and root, known as interfacial resistance. The interfacial boundary is only a few hundred micrometres thick and rich in organic substances secreted by the root to form a rhizosphere. Soil particles compressed by the advancing root are also embedded in this zone.

Two properties of an interfacial zone could influence water flow into roots. First, exclusion of ions at root membranes might result in a large build up of these ions outside the membranes. High osmotic pressures outside roots would impede the uptake of water (Section 17.2). This has been confirmed in a study of water uptake by lupin and radish plants (Aylmore and Hamza 1990). Exposure of roots to soil solutions containing 0–100 mM NaCl for only eight hours was used to analyse the impact of osmotic effects of interfacial ion build up in the absence of toxic effects. By increasing NaCl concentrations from 0 to 100 mM, ion concentrations at the root surface rose and water extraction from around the root declined (Figure 3.9). Hydraulic resistances for whole plants about doubled when they were grown in saline soils, presumably due to this interfacial, salt-induced resistance. Gradients are likely to become especially large as soil becomes drier because the diffusion coefficient for solutes falls by a few orders of magnitude as soil dries, so that any excluded solutes will diffuse away from the root surface only very slowly.

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Figure 3.9 (a) Volumetric water content (cm3 cm-3) in soil up to 12 mm from the surface of radish roots and (b) Na+ concentrations at these root surfaces, monitored over an eight-hour period. Radish plants were grown for 18 d in non-saline soil at which time up to 100 mM NaCl was added to the soil and transpiration was elevated by fans. CAT scans were used to measure water distributions near roots and Na+-sensitive (Na+-LIX) microelectrodes to measure Na+ concentrations at root surfaces. Estimates of water and ion levels were made in the top 3 cm of the soil profile. High salt treatment depressed water uptake, leaving root surfaces wetter than those in non-saline soil. Steady build up of salts around roots placed them under ‘osmotic drought’ (Based on Aylmorc and Hamza 1990)

The second possible impediment to water flow from soil to root is that physical gaps might form at the soil–root interface, either through roots growing into pores that are much wider than the root axis or because of roots shrinking within a pore into which they once fitted snugly. What would induce a root to shrink? A fall in water potential of roots could cause shrinkage as thin-walled cortical cells begin to collapse during water deficits. Observations made in rhizotrons (glass-walled tunnels used for observing the behaviour of roots in the field) clearly show a diurnal shrinkage in cotton roots of up to 40% where the roots are growing in large pores but we still do not know whether roots growing in intimate contact with soil particles are similarly prone to shrink. A few observations made using neutron autoradiography have shown no shrinkage in roots growing in the field (Taylor and Willatt 1983).

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