3.6.7  Axial versus radial flow

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Analysis of barriers to water and nutrients flowing across roots gradually builds a picture of the radial resistances to transport. Mechanisms within roots that optimise delivery of water and ions to shoots are incompletely understood and will probably be shown to entail subtle controls on internal resistances. Some points in the pathway are certainly more critical than others and should be targeted as sites of control for resistance. Loss of cortical cells, for example, has a small effect on ion uptake, showing that the cortex is not a key site of control of radial resistance to ion flow.

In young roots, radial and axial resistances to water and ion uptake are substantial; with only protoxylem and early metaxylem vessels conducting sap towards shoots, axial resistance can limit flow. Axial resistance decreases up to three orders of magnitude away from root apices as wide late metaxylem vessels mature by death of their cell contents. As barriers to radial flow develop in these mature axes (endodermis, exodermis and secondary thickening), radial transport of water and ions assumes greater importance (Steudle 1994). Flow through symplasmic pathways still occurs but low permeabilities often restrict water and ion delivery to xylem vessels. Xylem sap is therefore a composite of the activities of all these tissue types; utilisation of water and ions gathered by this vast structural complex is discussed in Chapter 4.