The late metaxylem carries the bulk of the water to the shoot because of its greater diameter - four-fold increase in the radius of a tube lead to a 256-fold increase in the volumetric flow rate (3.1.5). However, it does not mature until well back from the root tip, and so the younger part of the root is not functional in providing water to the rest of the plant.
Root tips take up enough water for their own cell expansion but they cannot pass this onto the rest of the plant as their xylem vessels are still alive and not able to function as a conduit. Only when they die, mature, and lose the integrity of their plasma membranes, and the cell walls in the transverse plane disintegrate, can they function as conduits.
Xylem conduits (vessels and tracheids) are dead at maturity. They do not mature until sometime after they are fully elongated, and so they remain alive long after they leave the growing zone of the root. The last-formed xylem vessels in angiosperms, the late metaxylem, may be found alive for some distance from the root tip. In Arabidopsis they remain alive until the root hair zone, but in most species they can only be seen further than 5 or even 10 cm from the root tip.
Figure 3.15 A longitudinal face 15 cm from the tip of a main root of a 21 day-old soybean. Portions of 4 developing elements of what will become a late metaxylem vessel (LMX) are shown. The face grazes a mature LMX element (upper left) with very thick wall. The base of the root is toward the left. Diameter of immature xylem is about 50 µm. SEM image, M. McCully. (Reproduced from Protoplasma 183: 116-125, 1994)
Figure 3.16 Cytoplasmic strands in differentiating LMX elements 50 mm from the root tip of barley. Scale bar, 10 µm. Hand cut section of fresh material, Nomarski optics, C.X. Huang and R.F.M. van Steveninck. (Reproduced from Physiol Plant 73: 525-533, 1988)
In soybeans, immature vessels were found as far as 150 mm from the root tip (McCully 1994). In barley the late metaxylem vessel (LMX) was still differentiating 100-150 mm from the tip (Huang and van Steveninck, 1988). Light microscopy of hand-cut sections showed the presence of cytoplasmic strands and intact cross walls in LMX up to 100 mm from the tip (Figure 3.16).
Living/immature xylem vessels can be recognised by their high K+ concentrations, 100 mM or more. In contrast, xylem sap that flows through the roots has K+ concentrations of only 5-10 mM, as shown in Table 3.2 in the following section.
When the vessels mature, and their end walls disintegrate, their cellular contents are carried away by the transpiration stream. This leaves hollow tubes that greatly increase the conductance of water flow through the xylem.
Figure 3.17 shows the effect of xylem differentiation and maturity on axial conductance in roots of two crop species, wheat and lupin. Xylem are continually differentiating in lupin within a short region of a young root, which results in dramatic increase in axial conductance. In contrast, once the central metaxylem of wheat has matured there is little change in the root’s axial conductance.
Figure 3.17 Cross sections of wheat (A) and lupin (B) roots stained with berberine-aniline blue and viewed under UV optics. Scale bars, 50 µm. Mature xylem vessels fluoresce due to lignification of their cell walls, which increases with root development (compare upper panels taken 2 cm from the root tip with lower panels taken 18 cm from root tip). (C) shows change in axial conductance as vessels mature in young roots. Images and graph, H. Bramley. (Modified from Plant Physiol 150: 348-364, 2009)