3.6.3  Pathways and fluxes

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Nutrient concentrations are optimised for plant growth in hydroponic solution cultures, favourably modified in fertilised agricultural soils where phosphorus and nitrogen status and pH might be adjusted, and totally unmodified in natural ecosystems. Most essential nutrients for plant growth are present in soil solution at concentrations well below those found in plant tissues (e.g. phosphorus, potassium and nitrogen) while other ions can be in excess around roots (e.g. boron, aluminium and sodium). Plants nevertheless colonise all these environments and produce biomass at impressive rates by controlling water and ion influx. The sensitivity with which roots recognise ions in soil solution is critical to plant survival. For example, exclusion of undesirable ionic species by root membranes will leave ions relatively harmlessly in the rhizosphere whereas passage of these ions to shoots will have more dire outcomes such as leaf abscission and necrosis. Equally, membranes allow root cells to absorb essential ions selectively, even when they are chemically similar to deleterious ions (Section 17.2).

Water and ions move through root tissues along either a symplasmic pathway (intracellular), an apoplasmic pathway (extracellular) or a transcellular path-way involving passage through the tonoplast membranes of vacuoles (Figures 3.22a and b). While each route is explicitly defined, it is, in practice, technically difficult to determine flow rates along each pathway.

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Figure 3.22 (a) Transverse section of a mature maize root. Note an exodermal layer underlying the epidermis, and distinctive enclodermis bounding the stele. Pink Toluidine Blue staining characterises suberised cells of these major barriers to radial flow. Late metaxylem vessels with large diameters are the dominant conduit for axial transport of sap. Smaller xylem elements, xylem parenchyma and phloem bundles lie within tissues adjacent to late metaxylem vessels (× 130). (b) Sketch of a transverse root section showing three proposed pathways of ion and water flow across roots: (1) apoplasmic, (2) symplasmic and (3) transcellular (transvacuolar). Note that ions are predominantly forced to enter the symplasm at the endodermis and no further discrimination in pathway is evident ((a) Photograph courtesy A.W.R. Robards)

By definition, non-apoplasmic flow requires transport across membranes but the intracellular distance traversed and number of membranes crossed when ions travel through cells is variable. Water and ions can move through a series of plasmodesmal connections, thereby remaining in the cytoplasm until reaching the stele. Conductivity in this case is largely regulated by plasmodesmal resistance. Alternatively, water and some ions enter vacuoles and are therefore subject to transport properties of the tonoplast (‘transcellular flow’). Ultimately, most water and ions enter the apoplasm when released into mature xylem vessels, either from xylem parenchyma cells or after rupture of immature xylem elements.

Alternatively, flow across the cortex might be largely apoplasmic as water and ions are drawn through intercellular spaces and cell walls up to the endodermis, where they generally enter the symplasm. Concentrations of ions in the rhizosphere, transpiration rates, ionic species and membrane transport properties all have an effect on the proportion of flow through each pathway. Cells deep within the cortex might have a lower capacity for active uptake of ions into the symplasm than outer cell layers but can none the less absorb K+ when concentrations are high (Clarkson 1996). Entry of anions to deep layers of the cortex is likely to be restricted by charge repulsion from dissociated, negative carboxyl groups in cell walls (Donnan Free Space). In general, cations also pass through cell walls more slowly than through solutions, particularly if many of the carboxyl groups in cell walls are not occupied by Ca2+ ions. None the less, apoplasmic flow of water through roots can sustain large ion fluxes during periods of high transpiration.

Estimates of net flux of water and ions do not reveal the absolute rates of influx and efflux: there is evidence for leakage of many ions (e.g. nitrate and orthophosphate) out of root cells and water can also cross membranes bidirectionally when water potential gradients favour water loss from roots in very dry soil. The case for efflux of orthophosphate, nitrate and sulphate has been made particularly convincingly (see Case study 4.1; Marschner 1995) with evidence that minimum ion concentrations extracted by roots are largely determined by efflux rates. Downregulating efflux of an ion allows roots to extract that ion to a lower concentration. Electrochemical gradients are not the only factors in ion efflux: ion-specific channels and carrier proteins in mem-branes can confer genetic control on efflux rates. Outwardly directed K+ channels and Na+ efflux pumps are two membrane transport proteins likely to play an important role in efflux.

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