3.6.2  Extracting water and nutrients via the rhizosphere

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Ions taken up into the main axis of a root, a lateral branch, or a fine unicellular root hair must cross a sleeve of rhizosphere up to a few millimetres thick. Even in solution cultures, where turbulence is maximised by air bubbles, unstirred layers of solution exist within 10–200 µm of root surfaces. Young, active roots in soil must draw large quantities of water and nutrients through this complex space in order to generate xylem sap. Balancing solute and water flow into roots is therefore critical, with small variations in flux having a marked effect on solute concentrations around roots. Some outcomes of such an imbalance in uptake rates are described below.


Figure 3.21 Depletion of soil solution phosphate around maize roots over a three-day period in a sandy soil. Note that within the zone explored by root hairs (about 0.8 mm froin the root axis) all phosphate is depleted (Based on Hendriks et al. 1981)

Ions have characteristic rates of release from bulk soil into the transpiration stream, depending on adsorption isotherms and solubilities. Moreover, rates of influx of individual ions into roots vary widely. Hence concentrations of ions in the rhizosphere also vary widely. Ions which are readily released from soil into the transpiration stream, swept to the root surface but taken up more slowly than water, tend to accumulate at the interfacial zone. Figure 3.9 describes this phenomenon when roots were exposed to NaCl, showing that water deficits can result. Roots supplied with essential nutrients rather than NaCl also exhibit solute build up during rapid transpiration. Most striking are plants in calcareous soils where deposits of calcium in the rhizosphere form an encrusted sleeve around roots.

By contrast, plants not experiencing extreme transpiration rates and growing in soils of low to moderate fertility have sustained uptake of ions such as orthophosphate and K+, leading to lower concentrations around roots. This is especially so when low ion diffusivities prevent replenishment of these ions from the bulk soil (Figure 3.21). Local fluctuations in solute concentrations are significant for roots because ion uptake systems respond directly to local concentrations (Chapter 4). Adequate nutrient concentrations in bulk soil might therefore mask local deficiencies at the root surface.