Figure 3.10 Cross section from a seminal wheat root stained with toluidine blue, showing the cortex, endodermis (black arrow), late metaxylem (LMX) central vessel, and peripheral xylem (red arrow). Many root hairs can be seen. Scale bar, 100 µm. (Image courtesy H. Bramley)
In vascular plants, water absorbed by roots is transported up the plant in the mature (dead) tracheary elements (xylem vessels and tracheids) of roots and stems (Figure 3.10).
Plants are capable of rapidly transporting water to heights in excess of 100 m, even from extremely dry soils and highly saline substrates. They can transport water from soils to leaves at velocities of up to 16 m per hour (4 mm per second) if they have wide xylem vessels in the range of 100 µm. With the more common xylem vessel size of 25-75 µm, peak velocities are 1-6 m per hour. What biophysical mechanism allows plants to achieve this? We know that plants do not possess a pump to move water to the canopy under positive pressure. Instead, plants suck!
Plants have evolved a transport system that relies on water sustaining a tensile force while under suction. The xylem sap in transpiring plants is under negative pressure. This elegant, but counter intuitive mechanism, described by the Cohesion-Tension theory, allows plants to move large quantities of water from the soil to the transpiring leaf surface with little input of metabolic energy. The following section describes the experimental history of how the Cohesion Theory came to be accepted.
Figure 3.11 (a) Mercury (black) sucked into tracheids of pine (Pinus radiata) by transpirational pull generated in the shoots. The water—mercury interface is powerful enough to hold this vertical column of mercury in stems. The height to which the dark column of mercury rises is used to calculate suctions created in xylem vessels. Note the generally small heights, reflecting the high specific gravity of mercury. About 2 MPa suction is produced in these xylem vessels. (b) Mercury enters bordered pits but remains connected to the vertical column of mercury in xylem vessels. While mercury can pass through the pit apertures, it cannot pass the finely porous ‘pit membranes’ because it is much more cohesive than water. Seen laterally, the bordered pits appear as discs.
Around 1905, great plans were made to resolve the mystery of the ascent of sap in trees by Professor E.J. Ewart in Melbourne, using eucalypts as a model plant. At that time, Australian mountain ashes (Eucalyptus regnans) vied with American coast redwoods (Sequoia sempervirens) as the tallest trees in the world, being well over 100 m tall. Using special scaffolding, Ewart climbed eucalypt trees, removed lengths of branch and measured the pressures required to push water through these stems. These investigations led him to conclude ‘The ascent of water is, therefore, a vital problem in so far as it depends upon conditions which hitherto can only be maintained in living wood’. If water transport required living cells, it could not be supported by discovery of a pump akin to that in animals. Even roots, which sometimes could pump water by root pressure, lacked the necessary positive pressures to push water so far aloft, especially around midday when water was most needed.
Suction from the shoots was an alternative explanation, but manmade suction pumps cannot do this without inducing formation of air bubbles (embolisms) in the xylem and blocking flow. One clue to the solution came from Dixon and Joly (1894) who claimed that very pure water molecules would be held together by powerful cohesive forces provided the water was especially clean (much cleaner than in manmade pumps).
Ewart did not agree with the unorthodox proposal that the suction of pure water through xylem vessels underpinned transpiration. However, Dixon (1914) ultimately postulated the Cohesion Theory, based on those properties of water which distinguish it as an ideal biological solvent. Cohesion (due to hydrogen bonding between molecules of water), adhesion to walls of the vessels, and surface tension, are central features. In short, in the absence of microscopic gas bubbles water could withstand quite enormous tensions.
Evaporation from wet cell walls of substomatal cavities in leaves creates a large tension (also called negative pressure or suction), which is transmitted via xylem conduits, pulling more sap from roots to leaves. Fine pores in cell walls provide sufficient suction to draw water to the crown of even a lofty tree: a curved interface in a 10 nm pore can store a pressure of -30 MPa. This value can be derived from equation (5) in the previous section: DP = 0.15/r where P is in Pa and r in this case is 5 x 10-9 m.
Through the evaporative power of the atmosphere, a continuous ‘chain’ or ‘catena’ of water, well below atmospheric pressure, could be drawn up to a leaf canopy. The tensions created in this way could even suck water from the surrounding soil. We now recognise that the evaporative energy is supplied as the latent heat of vaporisation ultimately derived from solar radiation. This cohesive property of water gave rise to the ‘Cohesion Theory for the Ascent of Sap’.
Two other properties of water are also essential for long-distance water transport: surface tension, and the adhesion of water to solid surfaces such as the xylem vessels within trees. Dixon claimed that if water could ‘hang together’, the enormous evaporative energy of the air (the same power which dries the washing hanging on a line) could be harnessed to lift sap, which is mainly water, vertically. This would entail no metabolic energy on the part of the plant. This theory of sap flow accorded with earlier experiments by Professor E. Strasburger in 1893 showing that a tall oak tree trunk, severed at the base, could draw poisons and dyes up to the leaves by some wick-like action. If metabolism energised sap flow, poison should have inhibited it. This was well illustrated in later experiments (Figure 3.11a) in which mercury was drawn through fine tracheids of pine stems purely through the suction created by transpirational water loss from the shoot above. The tension required to achieve this is about 2 MPa.
However, the physical properties of plants had to be more complex than those of simple pipes conducting water. As mentioned, manmade pumps failed through embolisms if used to suck water higher than 10 m, whereas hundreds of litres of water reaches the canopies of tall trees daily. Even overlapping sawcuts in tree trunks, which should allow a massive quantity of air to flow into xylem vessels when under suction and cause trees to die from embolisms, did not stop all water flow to leaves. If water was under such suction, how could trees keep air bubbles out of the sap when the trunk was cut? This additional problem was not resolved until the very complex anatomical structures of trunks were much better understood. The highly compartmentalized, extensively redundant structure of the xylem network performs the critical role of isolating gas voids while water transport continues in adjacent conduits. In reality, the complex structure of the xylem network is what makes reliable water transport under tension possible.
Xylem is not composed merely of pipes: it is made up of partially sealed units (technically vessels, tracheids and fibres, called collectively conduits), which most effectively limit the spread of introduced gases and thus, maintain water flow in some conduits despite very severe disruption from embolisms in others.
Figure 3.12 Cross section from barley root grown in soil; coleoptile node axile root, bar 100 µm. C is cortex, arrow points to peripheral xylem and arrowhead points to inner xylem. Extensions from the epidermis (red) are root hairs. Section was stained with rhodamine B and viewed with UV fluorescence optics. Micrograph, M. Watt. (Reproduced from New Phytol 178: 135-146, 2008)
The diameter of xylem vessels can be as small as 10 µm as in Arabidopsis, 60-100 µm in the roots of wheat and rapid growing annuals like maize, to over 100 µm in trees. Remembering that trees can be over 100 m in height, the conductive efficiency of xylem conduits is essentials for plants to move water to the canopy at rates that satisfy the transpirational water loss at the leaf surface. These dimensions are for the vessels with maximum diameter, the late metaxylem in the central part of the stele (e.g. Figure 3.12).
The diameter of xylem vessels in a given species varies greatly with root type (Watt et al. 2008). For example, in wheat and barley, the diameter varies from 10 to 60 µm depending on position within the stele (central or peripheral), and the type of root (seminal or nodal). Figure 3.12 shows a section of a nodal root from barley.
A wider xylem diameter translates to an increase in conductive efficiency that can be appreciated by revisiting equation (6). From the Hagen-Poiseuille Law, which shows that flow increases with the fourth power of the radius, we can see that a four-fold increase in the radius of a tube leads to a 256 fold increase in the volumetric flow rate.
In addition to allowing for high rates of water flow, the xylem must also protect the plant against formation and spread of gas bubbles. For xylem sap to sustain tensions required in tall trees, there must be no gas bubbles in the system. Cohesion breaks down if there is a single ‘nucleation site’ on which bubbles can form and enlarge. On the other hand, sap normally contains dissolved gases which, surprisingly, do not disrupt the system provided there are no nucleation sites available. Even the rigid walls of xylem vessels are compatible with high xylem tensions, attracting water by adhesion, which is essential for transport.
Surface tension acts as an interfacial water–air stopper, preventing air from being sucked into the many millions of tiny pores present in all plant cell walls. For example, water delivered to leaf cells by xylem vessels passes through these tiny menisci, which act effectively as non-return valves, so preventing air from being sucked into the xylem (Section 3.3). Surface tension also explains how water in leaves remains under strain within an essentially porous system through which water flows.
Figure 3.13 (a) Scanning electron micrograph (SEM) shows a transverse section of xylem tissue in Brachychiton australis. Large xylem vessels are surrounded by fibres and parenchyma; scale bar, 500 μm. Xylem vessels are dead at maturity and form long hollow tubes that minimise the resistance to water flow through the plant. Connecting intervessel walls contain bordered pits, cavities in the lignified secondary cell walls that allow for transfer of water between vessels. (b) Longitudinal section showing vessels in the xylem tissue of Fraxinus americana. Vessels are made up of repeated individual units (vessel elements) that are joined end to end by perforation plates; scale bar, 400 μm (c) SEM of the finely sculptured scalariform perforation plates in Betula ermanii xylem. Water passes easily from one xylem vessel to another by this route; scale bar, 20 μm (d) SEM showing surface view of the pit membrane with secondary wall removed by sectioning; scale bar, 2 μm. Tiny pores allow the movement of water between vessels but limit the movement of gas and pathogens. Bordered pits act as the safety valves of the plant hydraulic system. (Images courtesy B. Choat and S. Jansen).
Vascular transport systems have evolved to become amazingly reliable despite the metastable condition of the sap (existing as a liquid below its vapour pressure). From primitive, thickened, hollow cells, increasing specialisation has produced greater elongation and thickening of the tubes (Figure 3.13a,b). Xylem walls contain pits, in which zones of the primary wall known as ‘pit membranes’ allow water to be transmitted between vessels efficiently, while preventing a gas phase spreading through the interconnected system of vessels and blocking transport through embolisation (the blockage of a fluid channel with a bubble of gas) (Figure 3.13d). No living membrane is present in these wall structures. The efficiency with which pit membranes isolate adjacent vessels is shown in Figure 3.11b (in the previous section) where mercury, a highly cohesive liquid, is drawn into specialised bordered pits of pine tracheids without being able to exit into neighbouring tracheids.
Vascular systems have evolved from plant species possessing only fibres and tracheids, for example the more primitive Tasmannia, to more advanced plants possessing vessels which resemble the unicellular tracheids in structure but are much wider and longer and originate from a number of cell initials fused together. Lignin thickening patterns have also evolved. Some thickening designs, such as annular and spiral, allow the tubes to extend longitudinally while supplying growing organs.
When elongation growth has ceased, an organ can be provided with more efficient pipes of larger bore and with stronger thickenings, in reticulate and scalariform patterns (Figure 3.13c). Pit fields which allow water transport across vessel walls can also be simple, unreinforced structures (simple pits) or more elaborate bordered pits in which secondary cell walls mechanically support the pit membrane. All these forms of pits can prevent air in an air-filled conduit from spreading to adjacent conduits which are conducting water under strong suction. Reinforcement of the walls around pits allows pit membranes to be as large as possible and thereby maximise water exchange between vessels.
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)
Xylem sap contains all the inorganic nutrients needed for plant growth, in the proportions in which they are needed. The concentrations of some nutrients are dilute when compared to phloem sap, in particular potassium and nitrogen-containing solutes (Table 3.2). The concentrations also vary at different times of day, being lowest in the middle of the day when transpiration is highest, and quite high at night when stomates are closed so there is very little flow of sap to the shoots. The osmotic pressure of xylem sap therefore ranges from less than 0.05 MPa during the day to about 0.15 MPa (60 mOsmol L–1) during the night. The flux of solutes (the concentration multiplied by the flow rate of the sap) is maintained at a steady rate over the whole 24 hours.
Most solutes in xylem sap are inorganic ions (e.g. nitrate, potassium, magnesium and calcium), but organic solutes are also present (Table 3.2). Organic acids and amino acids in xylem sap can be present in substantial concentrations, and sugars (but not sucrose) reaching 5 mM in some perennial species. Many trees including eucalypts are host to boring insects at particular times of the year, when sugar and nitrogen content of the sap is nutritionally valuable. Even though sugar concentrations in xylem sap are much lower than in phloem sap, the high nitrogen to sugar ratios and low osmotic pressures make it a good substrate for many herbivores. More extreme examples of the carbohydrate content of xylem sap are temperate deciduous trees such as maple, which have traditionally been tapped to yield a sugary solution in the period prior to budburst. This indicates that xylem can be a conduit for carbon remobilisation in addition to its central role as a pathway for water and nutrient transport. Xylem parenchyma cells load ions into xylem vessels in roots (Section 3.6) and also contribute to modification of ion levels along the xylem pathway. In secondary tissues, rapid transfer of solutes into and out of the xylem is partly achieved through close association of living ray cells and xylem vessels.
Other organic molecules act to transport inorganic nutrients to the shoots. Nitrate and ammonium are assimilated into organic forms, such as amino acids, in the roots of many plants. In legumes, nodules deliver an even wider selection of nitrogenous compounds to the xylem, including ureides and amides. These often constitute the dominant form of nitrogen reaching shoots and are therefore, a major component of the sap. Other examples of complexed forms of inorganic nutrients in xylem sap are metal ions such as zinc, copper and iron, which are almost exclusively chelated to organic acids.
Phytohormones such as abscisic acid and cytokinins are present in xylem sap and serve as signals from roots to shoots affecting growth and development, and other physiological responses.
