3.6.6  Testing root function

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Roots display marked structural complexity along their axes (dividing, expanding and differentiating tissues), paralleled by gradients in functional capacity. Evidence for longitudinal gradients in ion uptake capacity comes from experiments where separate root zones are supplied individually with nutrient solutions and local ion uptake monitored (e.g. Harrison-Murray and Clarkson 1973). Ions such as K+ and orthophosphate are sometimes taken up more rapidly in the terminal few centimetres of root axes, supplementing phloem in satisfying the large nutrient demand by young, differentiating root apices. Demand for ions by shoots can also be substantial, leading to high rates of ion uptake in mature root tissues and increasing allocation of ions from these tissues to the translocation stream (Figure 3.27). Among the major nutrient ions, uptake of Ca2+ is most consistently localised in young root axes, furnishing these cells with Ca2+ which cannot be delivered in the phloem.



Figure 3.27 Phosphate uptake and translocation from different positions along roots of young barley plants. Roots were supplied with 32P and radioactivity was monitored in root zones to assess the proportion of phosphate which remained in a zone (dashed lines) versus the proportion translocated axially (solid lines) to other plant parts (principally shoots). A peak in untranslocated HP in the zone of root hair formation rellects the efficacy of hairs in phosphate uptake. Mature root zones were effective at translocating 32P, probably reflecting smaller demand for mineral nutrients in mature cells and maturity of the long-distance transport system in basal root zones (Based on Wiebe and Kramer 1954)



Figure 3.28 Local rates of water uptake in 3.5 mm segments of barley (Hordeum vulgare) and marrow (Curcurbita pepo) roots. Water uptake was measured using micropotometers applied to the roots of plants in normal transpiring conditions. The extent of endodermal development was also assessed for these roots and appears as horizontal lines on the figure. Water uptake is fastest in apical, non-endodermal zones under these transpirational conditions (Graham et al. 1974; reproduced with permission of Academic Press)

Water uptake, when measured locally, shows similar gradients (Figure 3.28; Sanderson 1983) although mature root axes with lateral roots contribute significantly to water inflow. Significant osmotically driven water flow (‘root pressure’) occurs in young root tissues during rapid ion influx.

One approach to understanding the significance of these zones for both water and nutrient transport is to place whole roots or segments of root into a root pressure probe (Steudle 1994). Using this method, build up of hydrostatic pressure in xylem vessels (‘root pressure’) caused by energy-dependent solute loading can be measured at the cut end of a root. Also, water potential gradients can be applied across roots, either osmotically or by hydrostatic pressure, to induce water flow and give estimates of radial hydraulic conductivity (Figure 3.29).


Figure 3.29 Root pressure probe for measuring water and solute relations of roots. The living root system is enclosed in a chamber which can be pressurised. After excising the shoot, a water-filled capillary is sealed to the cut stump with silicone; xylem sap can therefore flow directly into this capillary. The capillary connects to a pipette (top) and an oil-filled chamber (right) in which pressure can be monitored by following the oil-water meniscus. Roots in this apparatus remain alive and physiological for long periods, allowing water flow through the root system to be measured in a number of ways. (1) When the pipette is open to the atmosphere, roots under pressure exude sap providing a pressure-flow relationship from which root hydraulic conductivity (Lp) is derived. Unpressurised roots exude at a rate determined by solute uptake (giving ‘root pressure’). (2) Suction can be applied to the cut stump to mimic ‘transpirational pull’, so providing another measure of Lp. (3) The cut stump can even be pressurised, forcing water out through root tissues and generating another pressure—flow relationship. This direction of flow is rare but not unknown in nature. If pressure is applied fairly briefly, osmotic pressures in xylem sap are constant and Lp in (1) to (3) agrees closely. Finally, transient flows can be induced with the aid of the probe to estimate pressure relaxations. In this case, the pipette has to be removed and the system closed. The apparatus also gives information on solutes, in particular reflection coefficients (Equation 4.4) in whole root systems for a range of solutes (Drawing courtesy E. Steudle)

Resistance to radial transport of ions and water are differently distributed. Root pressure probe experiments demonstrate that minor damage to the endodermis, such as pin-pricks, does not substantially alter water flow but does lower root pressure to less than half its original value within two minutes. Therefore, the barrier to ion uptake appears to be principally at the endodermis whereas that to water inflow is probably more generally distributed over the root membranes. Longer-term observations show that root pressures begin to recover 0.5–1 hour after damage, demonstrating repair of endodermal cells. Xylem vessel walls contribute less than one-third of the radial resistance to water flow, reflecting their high degree of leakiness (Section 5.1).

Distinct differences in radial conductivity at various stages of root development can be shown with a root pressure probe. About 20 mm from a root apex, where an endodermis is present but too immature to impose a tight barrier to transport, water and solutes flow to the stele along a low resistance, largely apoplasmic pathway (Frensch et al. 1996). Longitudinal transport meets considerable resistance in these root zones because late metaxylem vessels are immature (Section 3.6.7). Similarly, in mature root axes where secondary laterals emerge, apoplasmic transport sometimes increases, probably because of emerging lateral roots rupturing the endodermal cell layer. Apoplasmic flow past emerging lateral roots of monocotyledons growing in the field is restricted by a lignified adhesive layer between the new epidermis and the cortex of the parent axis. This sealing phenomenon around lateral roots might be widespread but has not been extensively investigated. Overall permeability of mature roots is, however, very small because of increasing suberisation and secondary thickening of endodermal and exodermal cell layers. So, in root pressure probe experiments, the area of ‘apoplasmic bypass’ did not exceed 0.05% of the total cross-sectional area of the endodermis. This might still be significant because the low hydraulic resistance of an apoplasmic pathway (orders of magnitude less than a symplasmic route) coupled with the lack of ion selectivity when membranes are bypassed mean that apoplasmic influx might be important for roots exposed to toxic solutes.

Most knowledge on transport processes in roots comes from herbaceous species which lack the secondary thickening and cell senescence characteristic of mature roots in soil. Solute and water transport in oak and spruce roots show that root pressure contributes little to water flow, reflecting low overall demand for nutrients by mature plants. Transpirational water flow is dominant. When water transport was induced hydraulically, large radial resistances were measured, suggesting that impermeable endodermal and exodermal cell layers and secondary root thickening make roots very impermeable to water.

Sites of water uptake can be visualised by bathing roots in a solution containing membrane-impermeable dyes. Dye accumulates where water enters membranes (Section 5.2), for example at the endodermis and exodermis of roots. These layers are therefore thought to be major points of entry for water into the symplasm. Whether most water enters the symplasm at these cell layers in intact plants in soil is less clear because we know that dyes also accumulate around root hairs, which might therefore be the main site of water uptake into the root symplasm. The exodermis is only a barrier to water transport in mature roots where deposition of hydrophobic substances in the Casparian strips has been extensive. Young root apices do not accumulate dye, suggesting that they are not major sites for water uptake. Mature (non-cytoplasmic) late metaxylem vessels found in older root zones are necessary for full transport capacity to be engaged.