How is root pressure produced

Futile cycling consumes metabolic energy, but its benefit for the plant seemed to be elusive and so far has remained an open question. Water secretion may be part of the answer. Hypothetical interplay of membrane transporters in the plasma membrane of xylem parenchyma cells for water secretion. Note that this transport is electrically silent. Aquaporins may to some extent short-circuit co-transport-driven water flow if their activity is not down-regulated. Note that all transporters have been demonstrated to co-exist in the plasma membrane of root stelar cells.

For more details, see the text. This figure is available in colour at JXB online. In his quantitative biophysical analysis of coupled ion and water transport by CCC transporters, Zeuthen did not make use of the framework of the thermodynamics of irreversible processes.

However, this formalism can conveniently be applied to arrive at a quantitative expression for the hypothesis on root pressure developed in this communication. For this simplified model, we can write:. It is instructive to have a closer look at this simple equation. Inhibition of the CCC transporter e.

A formal criterion for energetically uphill water transport is given by the following relationship:. This parameter can be expressed in terms of the parameters well known from thermodynamics of irreversible processes by comparison with Equation It has to be kept in mind that the volume excreted into the xylem via CCC co-transporters has a defined salt concentration.

A critical aspect of the putative role of cation—chloride co-transporters in water secretion into the xylem is their dependence on the availability of Cl — as a substrate. In glycophytes growing in natural habitates, tissue chloride concentration is said to range from 7mM to 70mM White and Broadley, , but cytosolic concentration may be lower to evade toxic effects.

This may limit the activity of CCC transporters in glycophytes. It is as yet unknown whether in plant co-transporters Cl — could be replaced by other monovalent anions that do not play a role in animal tissues e. On the other hand, AtCCC-deficient mutants had a clear phenotype even under low salt conditions, and salt stress did not affect the expression in the wild type Colmenero-Flores et al.

Co-transport of water together with one or more substrates appears not to be a unique property of the CCC transporters. Sugar transporters play an important role in plants in the context of assimilate allocation in sinks, such as roots.

Notably they are involved in phloem unloading and in the retrieval of monosaccharides, for example by root cells after sucrose splitting by apoplastic acid invertases. This water, being secreted into the stelar apoplast, could contribute significantly to root pressure, at least in intact plants. Note that the energetically uphill transport of water is not restricted to the transporters discussed so far although they seem to be most effective with respect to the number of water molecules transported per substrate.

Few studies have been undertaken to study this phenomenon quantitatively, but in one case data on an ion channel of plant origin are available. At present, this is purely based on speculation since no experimental evidence is currently available showing that channels differ in that respect.

Indeed, short-term oscillations in root exudation Zholkevich et al. Another putative candidate for ion-coupled water flow across the membrane of xylem parenchyma cells is the NORC, a channel that is outwardly rectifying and poorly selective among cations and anions Wegner and De Boer, Conspicuously, this channel type has also been found in other secretory tissue. Its role in water translocation across the membrane remains to be tested.

This is still hampered by our lack of information on the genetic basis of this ion channel. Both rectifiers operate alternately, co-ordinated by membrane potential oscillations. Transporters of neurotransmitters were also found to be involved in water translocation in brain cells McAulay and Zeuthen, Interestingly, Zholkevich and co-workers have repeatedly demonstrated that neurotransmitters such as acetylcholine, adrenaline, and serotonin stimulate root pressure exudation e.

Zholkevich et al. It is worth testing whether water secretion coupled to the transport of these substances is involved in the generation of root pressure. Possibly, several pathways for co-transport of water and ions solutes may co-exist in the membrane to achieve the required rate of water secretion under various conditions, as previously also described for epithelia Zeuthen, It should be noted that the hypothesis developed here refers to isotonic radial water flow i.

In other cases, however, the osmotic pressure of the xylem sap is higher than that of the ambient medium e. Miller, ; under these conditions, the osmometer model holds water, and aquaporins come into play. The most obvious and serious objection against the hypothesis described in the previous paragraph is the presence of aquaporins in the plasma membrane of xylem parenchyma cells e.

Postaire et al. Aquaporins conduct H 2 O molecules passively along with the driving force. Aquaporin activity contributes strongly to the hydraulic conductance of the membrane and, together with the natural water conductivity of the bilayer, tends to dissipate existing water potential gradients. To bring this aspect into perspective, it has to be remembered here that under daylight conditions—even at a low light intensity—in most cases the hydrostatic pressure gradient across the membrane i.

Under these conditions, water transport will become more efficient at an increased hydraulic conductance due to aquaporin activity, and water secretion into the vessels that is indirectly coupled to the consumption of metabolic energy is likely to be down-regulated. But what about those processes of water secretion into xylem vessels that are the focus of this study?

Is net water flow against a transmembrane gradient in water potential compatible with the existence of aquaporins? In the case of plants, a quantitative treatment of this issue is hampered by our lack of information on the rate of water secretion, for example by CCC transporters, that depends on both their density in the plasma membrane and the ion gradients.

For a model calculation, a value of 3. Moreover, in plant cells, the hydraulic membrane conductance can vary greatly. Depending on species, cell type, time of the day, nutrient availability, etc. At lower water potential gradients, indicated by the shaded area in Fig. As expected, the range of water potential gradients allowing water secretion depends strongly on L P. At maximum aquaporin activity, however, the water potential gradient would be kept close to the equilibrium.

Nevertheless, isotonic water efflux would still be possible if water secretion and resorption of the transported ions were orchestrated in such a way that gradients in water potential across the membrane remain very small because the secreted volume is isoosmotic to the xylem sap and passive backflow of water is diminished by the lack of a driving force!

After all, conclusive data on mammalian epithelia that also have aquaporins co-located with CCC transporters e. Double-logarithmic plot of the water potential gradient against which water secretion could be maintained shaded area as a function of the hydraulic conductance of the plasma membrane range of values following Maurel, A value of 3.

From an energetic point of view, it is favourable to keep futile water cycling across the membrane at a minimum. Hence, it is most likely that water secretion and passive transport via aquaporins are inversely regulated and that aquaporin activity is low when water secretion by a co-transport mechanism is active, for example under conditions of isotonic radial water transport.

Aquaporin activity has been demonstrated to be kept under tight physiological control e. This finding is at variance with water flow being driven by osmotic gradients. Clearly, there are several studies reporting a close correlation between root pressure exudation, expression of aquaporins, and hydraulic conductivity of the root e. Lopez et al. Energetically uphill water transport is most probably not restricted to the root, but can occur everywhere along the vascular system.

Stem pressure analogous to root pressure is a well-known phenomenon, and AtCCC expression in Arabidopsis was not restricted to the root but was also found in the vascular tissues of the shoot Colmenero-Flores et al.

The ability of plants to refill void xylem vessels that underwent cavitation and, as a consequence, became embolized has frequently been documented even for individual xylem vessels, for example by magnetic resonance imaging Holbrook et al. Embolism repair requires vessels to be filled with xylem sap secreted by adjacent cells. It is generally believed that some overpressure has to be built up in a vessel during the refilling process in order to dissolve residual inclusions of gas completely; removal of cavitation nuclei appeared to be a prerequisite for a vessel to regain functionality Holbrook and Zwieniecki, ; Zwieniecki and Holbrook, ; Nardini et al.

Usually refilling occurs overnight when transpiration is low and little or no tension prevails in adjacent, functional vessels. However, even during the day, repair of embolized vessels has been observed when pressures below vacuum were apparently established in those parts of the xylem that were still conductive Nardini et al. The puzzling aspect of this phenomenon even bordering on a miracle Holbrook and Zwieniecki, is how a steep pressure gradient between vessels undergoing repair and others under tension is maintained; water being secreted into the void vessels should immediately be swept into the functional vessels unless those vessels during refilling are hydraulically isolated from their environment.

The problem becomes somewhat less dramatic if one takes into account that tensions in the xylem of transpiring plants have been grossly overestimated and are likely not to increase beyond a few tenths of a megaPascal Zimmermann et al.

Clearly, pits connecting the vessels have to be occluded, and the hydraulic conductivity of the walls separating the vessels has to be extremely low to ensure that water influx into vessels under repair coming from adjacent cells exceeds water loss to adjacent, conductive vessels Zwienicki and Holbrook, In search of a mechanism that could explain the movement of water from cells into void xylem vessels, osmotic forces have most frequently been considered.

Release of sugars or salts into the apoplast by vascular tissue could provide the driving force for water movement into the vessels, and indeed this mechanism was confirmed for refilling of embolized vessels during springtime in deciduous trees. In birch, for example, osmotically active sugars are mobilized by starch breakdown and released into the vessels providing the driving force for passive water movement Westhoff et al.

Moreover, longitudinal gradients are established in the stem that could induce water ascent provided that the xylem is sectioned by solute-reflecting barriers.

However, springtime refilling is a special case, and previous attempts to establish a similar mechanism for embolism repair in transpiring plants have failed Tyree et al. Recently, the involvement of sugars as osmotica in the refilling process was reconsidered Zwieniecki and Holbrook, ; Nardini et al. Indeed, residual water in embolized vessels contained measurable amounts of sugars, in contrast to non-embolized ones; however, under many conditions, the overall osmotic pressure of this vessel content was much too low to provide a sufficient driving force for attracting water from adjacent cells by a passive mechanism Secchi and Zwieniecki, Alternatively, some authors have suggested a refilling mechanism that involves tissue pressure e.

Enns et al. It is suggested here that water may be secreted into gas-filled vessels by solute—water co-transport Fig. In fact, a common mechanism for both phenomena has been suspected before Holbrook and Zwieniecki, ; McCully, ; Enns et al. High-resolution computed tomography imaging has demonstrated that droplets form at certain sites along the vessel walls, preferentially where xylem walls border on xylem parenchyma cells Brodersen et al.

Water secreted into embolized vessels may pressurize and dissolve the gas phase so that those vessels become refilled with fluid again Brodersen and McElrone, Circumstantial evidence points to proton—sucrose antiporters being involved in this process Secchi and Zwieniecki, , but detailed information is still lacking.

Schematic representation of embolism refilling according to the water secretion mechanism introduced in this communication. Water is transported into the central, void vessel. This simple mechanism drives water transport into the xylem and eventually allows generation of an overpressure in the vessel sufficient to dissolve the vapour phase.

Again, the role of aquaporins in this process needs some attention. Evidence for the involvement of aquaporins in the refilling process has been obtained Secchi and Zwieniecki, , but, as previously pointed out by Nardini et al. Xylem parenchyma cells themselves cannot provide the water required for vessel refilling; secreted water has to be retrieved from storage water in fibres, from the phloem, or from below or above the site of refilling.

Aquaporins may be required to ensure water supply from these resources to the site of secretion. There is general agreement that root pressure can contribute significantly to long-distance water transport in plants, varying strongly with species, environmental conditions, and time of the year e.

Fisher et al. However, according to most textbooks, ascent of sap is supposed to be driven predominantly by transpiration, as postulated by the cohesion—tension CT theory Dixon and Joly, , ; Tyree, Water loss to the atmosphere is thought to induce a hydrostatic pressure gradient in the continuum of the network of xylem conduits that extends down to the roots. The main problem of this theory is that pressure has to decrease with tree height by some 0. As a consequence, pressures in the vessels are supposed to drop below vacuum in the canopy of trees exceeding a height of 3—5 m.

This is feasible from a thermodynamic point of view, but water in the fluid phase at below-vacuum negative pressure is jeopardized by spontaneous gas formation, known as cavitation. Since water at a negative pressure is in a metastable state, and the existence of hydrostatic pressure gradients in the vessels of tall trees, as postulated by the CT theory, could not be demonstrated convincingly by experimental methods, the validity of this theory has repeatedly been questioned, and alternative hypotheses have been proposed Canny, ; Laschimke et al.

In constrast to the CT theory, that postulates continuous water columns extending from the top of the canopy to the root tips even in tall trees, Zimmermann et al. The CT mechanism itself is not altogether dismissed since it satisfactorily explains water ascent e. However, in stems of tall trees, the CT mechanism fails to provide a comprehensive explanation of water supply to the canopy Zimmermann et al.

Therefore, Zimmermann et al. Zimmermann et al. An obvious candidate, in the light of the above discussion, would again be co-transport-driven water secretion into vessels against the chemical potential of water. Most probably, putative watergates are closely related to the process of refilling of embolized vessels as discussed in the previous paragraph. From an energetic point of view, it would be most efficient if uphill water secretion was initiated only once cavitation had occurred in a group of vessels in a way that affects water supply to the canopy of a tree.

However, when local cavitation in a few vessels has occurred, a feedforward process is initiated, since the resistance of the stem increases and more tension is built up in those vessels that are still conductive, thus increasing their cavitation probability.

When most vessels are embolized at a particular site, water tends to bypass the cavitated xylem area, moving through adjacent cells. Note that water flow through the distal xylem segment is no longer driven by transpiration, but by water—ion or water—sugar co-transport involved in the secretion process and the metabolic energy required to keep this mechanism going.

A particular role for the phloem in this process is likely Nardini et al. Local water storage sites provided by non-conducting vessels e.

In these non-functional vessels, pressure is at above-atmospheric values. When vessels are substantially embolized at a particular site, water will be retrieved from these stores and be swept upwards. The buffer is refilled by secretion of water originating from vessels under tension; however, this refilling is not necessarily a simultaneous process but may start with some delay e.

Same molecular process as in Fig. Flat water potential gradients prevail up to the ray cell at position 3 i. Although a conclusive network of evidence is not available yet, some information in support of this model can be obtained from the literature. In a very recent study, Melcher and Zwieniecki have shown that water can easily bypass embolized xylem vessels in leaf petioles, obviously flowing through cells.

Moreover, Westhoff et al. This may have been due to a higher susceptibility to air seeding via the pits. Also at the base of many branches, most xylem vessels appeared to be cavitated. Apart from these general features, distribution of xylem water appeared to be highly variable, which is in line with a highly dynamic pattern of segmentation.

Evidence for the involvement of water secretion by xylem parenchyma in long-distance water transport was also obtained for grapevine, using high- resolution computed tomography imaging Brodersen et al.

Clearly, we are just beginning to understand how long-distance water transport in plants is organized. The hypothesis put forward here to explain root pressure with implications for refilling of embolized vessels and long-distance transport in trees needs further rigorous experimental testing.

Proteins of the CCC family are the most likely candidates for mediating water secretion by water—salt co-transport in plants.

More information is urgently required on members of this family in the plant genomes, and their expression patterns in vascular tissue, particularly in roots and in the stem of trees.

Their biophysical properties have to be tested by heterologous expression, for example in oocytes, to establish that these proteins can mediate water transport against the chemical potential gradient for water in plants, much as was done for similar transporters in animals.

Moreover, the ion selectivity and stoichiometry of these transporters need further testing. If these transporters play the role ascribed to them here, knockout or antisense mutants of Arabidopsis and rice that lack the CCC transporter s should lose the ability to generate root pressure and to repair embolized vessels. In parallel, the ability of outward-rectifying ion channels to participate in water secretion should be quantified, for example those of the outward-rectifying channel, SKOR, and of the NORC.

Experiments of this kind will hopefully contribute to overcome the current deadlock in our understanding of long-distance water transport that we are facing despite tremendous experimental effort being invested in this field of research. I would like to thank two anonymous reviewers for their encouragement and constructive criticism that helped to improve the manuscript.

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The main contribution of root pressure is to establish the continued movements of water molecules in the xylem that may be affected by sweating.

When the roots are pressed, the water passes freely through the root tissues, but not the minerals the root is a semi-permeable barrier. As per the natural phenomenon of osmosis, the water molecules naturally flow from the area of low mineral concentration to the area of high mineral concentration, and this flow of water into the root pressurizes it.

This osmosis process occurs very frequently in all other animal and plant cells. For example, in non-timber plants, osmosis allows plant cells to collect water and be sufficiently plump to keep the plant upright. Root pressure can be readily seen when trees are cut down during the spring season. When a tree is cut or sawn, a stump can generally be seen bleeding sap. The bleeding of sap from strains and other wounds in some tree species is a result of root pressure, a phenomenon that takes place only in limited circumstances at certain times of the year.

When various ions from the soil are actively transported into the vascular tissues of the roots, it is known that water also follows and this tends to increase the pressure inside the xylem. This positive pressure is known as Root pressure.

The root pressure has the ability to push water up to small heights in the stem. Most commonly positive pressure is observed as guttation from leaves or bleeding from cut stems.

Root pressure may occur in fine roots where it uses soil water as the source, or in woody roots and stems, using water stored in living cells, fibres, cell walls, and intercellular spaces as the source. We can define root pressure as the positive pressure that develops in the roots of plants and this happens by the active absorption of nutrients from the soil.