Ascending current of water

Last updated: July 07, 2026 Русский Español

1. Why Is Water Rise a Physical Problem?

Imagine a 30‑meter pine tree. In its crown, millions of needles evaporate hundreds of litres of water daily. Yet the tree has no heart—no mechanical pump to push water upward. Nevertheless, water continuously rises from the roots to the very top leaves. How is this possible?

This question has occupied scientists for centuries. Van Helmont in the 17th century, after his famous willow experiment, concluded that water is the sole food for plants (Medvedev, 2012). He was wrong in his conclusions, but he raised the key question: how does water reach the upper parts of the plant, and what drives it?

Today we know that for normal functioning, a plant must maintain a continuous stream of water from roots to leaves. Without this, neither photosynthesis, nor mineral transport, nor leaf cooling would be possible. But let us examine the physical difficulties a plant encounters when raising water to a height.

Why Is Root Pressure Insufficient?

Let us start with an obvious assumption: perhaps water is simply pushed out of the roots under pressure? Indeed, many plants exhibit root pressure—a positive hydrostatic pressure in xylem vessels, arising from active ion uptake into the vessels and subsequent osmotic water entry (Schopfer & Brennicke, 2016).

This phenomenon is easy to observe: if you cut a stem at the base, a fluid—bleeding sap—will exude from the stump. In spring, birch trees may exude up to 5 litres of sap per day. This is a manifestation of root pressure (Medvedev, 2012).

However, let us look at the numbers. Root pressure typically does not exceed 0.1–0.3 MPa, and only in exceptional cases reaches 0.6 MPa (Medvedev, 2012; Schopfer & Brennicke, 2016).

Now imagine we need to raise water to a height of 30 metres. What pressure would be required? Recall a simple physical fact: a 10‑metre column of water creates a pressure of about 0.1 MPa (one atmosphere). Therefore, to raise water 30 metres, at least 0.3 MPa is needed just to overcome gravity.

But that is not all. In real xylem vessels, water does not move unimpeded: it encounters resistance from vessel walls, bordered pits, and perforation plates. This resistance, due to water viscosity, requires additional pressure. For a 100‑metre tree, considering both gravity and hydraulic resistance, a pressure difference of about 2 MPa is required (Taiz et al., 2023).

Thus, even for a 30‑metre tree, a pressure of about 1 MPa is needed, while root pressure supplies only 0.1–0.3 MPa. The conclusion is obvious: root pressure is insufficient to lift water to height, especially in tall trees. Moreover, root pressure disappears during active transpiration or when the soil dries out (Connor et al., 2011). Therefore, another, more powerful mechanism must exist.

How Much Pressure Must Be Generated?

Let us perform simple but telling calculations. They will help us appreciate the scale of the physical problem the plant solves.

First: to raise water to height \( h \), we must overcome the gravitational potential \( \Psi_g = \rho g h \), where \( \rho \) is water density (1000 kg/m³), \( g \) is gravitational acceleration (9.8 m/s²). For 30 m height this gives:

\Psi_g = 1000 \times 9.8 \times 30 = 294\,000 \text{ Pa} \approx 0.3 \text{ MPa}

For 100 m — already 1 MPa.

Second: water in vessels does not move ideally. According to Poiseuille’s law, the flow rate is proportional to the fourth power of the vessel radius and directly proportional to the pressure gradient (Taiz et al., 2023). In real xylem, with its pores and wall irregularities, flow resistance is about twice that of an ideal tube. This means that to drive water at a typical speed of 4 mm/s through a vessel of radius 40 μm, a pressure gradient of about 0.02 MPa per metre is required (Taiz et al., 2023). For a 30‑metre tree, this adds another 0.6 MPa.

Thus, for a 30‑metre tree, the total required pressure is about 0.9–1.0 MPa. For a 100‑metre redwood, about 2 MPa or more (Taiz et al., 2023).

Third: what does 2 MPa mean in physical terms? That is 20 times normal atmospheric pressure! Such pressure cannot be created by any mechanical pump at ground level. But the most astonishing thing is that such pressure in the plant is... negative. Water in xylem vessels is under tension, i.e., under negative hydrostatic pressure.

This sounds paradoxical: water under negative pressure should boil and turn into vapour. However, as we will see later, the unique properties of water and the structure of xylem allow this state to exist.

Where Is the “Engine”?

We now come to the key conclusion: the engine that lifts water in the plant is not in the root, but in the leaves. This discovery was made as early as the beginning of the 20th century and confirmed by numerous experiments.

As shown by studies using thermoelectric methods for measuring sap flow velocity (the Huber method), in the morning the flow accelerates first in the branches, and only later in the trunk. In the evening, when stomata close, the flow slows down first in the branches and only then in the trunk (Schopfer & Brennicke, 2016). This is direct evidence that the “motor” is located in the crown.

But how exactly do leaves generate a force capable of lifting water tens of metres? The answer is given by the cohesion‑tension theory, which will be the central topic of the next part of our lecture.

Why Water Rise Is a Physical Problem: Summary

So, we have established that:

1. The plant has no mechanical pump. Unlike animals, plants have no organ like a heart that creates positive pressure to pump fluid.

2. Root pressure is insufficient. Its magnitude (0.1–0.3 MPa) is too low to lift water more than a few metres, especially considering hydraulic resistance.

3. The required pressures are huge. For a 30‑metre tree, a pressure difference of about 1 MPa is needed; for a 100‑metre tree, about 2 MPa or more. No pump at ground level can generate such pressure.

4. The engine is in the crown. The sap flow velocity changes first in the branches and then in the trunk, indicating the driving force originates in the leaves.

5. Water in the vessels is under tension (negative pressure). This state seems to contradict the laws of physics, but turns out to be possible thanks to the unique properties of water and the specialised structures of xylem.

It is this unusual situation—water under negative pressure—that will be the key theme of the next section. How is water kept in the liquid state when it should boil? How does evaporation from leaves create a pulling force sufficient to lift water 100 metres? And what price does the plant pay for this efficient but vulnerable mechanism?

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2. The Cohesion‑Tension Theory

From Paradox to Solution

In the previous part, we concluded that root pressure cannot explain the rise of water to tens of metres. The engine is in the crown, and water in xylem vessels must be under negative pressure—tension. But how exactly does evaporation from leaves create this pulling force? And why does water not boil under such a vacuum?

The answer is provided by the cohesion‑tension theory, which was proposed independently by Böhm in 1893 and by Dixon and Joly in 1894 (Schopfer & Brennicke, 2016). This theory remains the main explanatory model for water transport in plants today, although some aspects continue to be debated.

The central idea is simple and elegant: the plant does not push water from below—it pulls it from above. Evaporation of water from leaves creates tension, which is transmitted down the continuous water column to the roots. Imagine a rope pulled from the top—the whole rope becomes taut and moves upward.

Three Physical Principles Underpinning the Theory

The cohesion‑tension theory rests on three fundamental properties of water and its interaction with the environment:

1. Cohesion—Attraction Between Water Molecules

Water molecules are held together by hydrogen bonds—weak electrostatic interactions between the oxygen atom of one molecule and the hydrogen atom of another. Each water molecule can form up to four hydrogen bonds with neighbouring molecules (Medvedev, 2012).

These hydrogen bonds give water a high cohesion—the ability of molecules to stick to each other. Thanks to cohesion, water behaves like a single column: when one water molecule evaporates from the leaf, it “pulls” its neighbours, which pull the next ones, and so on down to the roots. Water cohesion is so strong that the water column can withstand tension up to 25–30 MPa under ideal conditions (Taiz et al., 2023; Schopfer & Brennicke, 2016).

It is cohesion that explains why water does not break into separate drops in xylem vessels even under strong tension.

2. Adhesion—Sticking of Water to Vessel Walls

Water not only coheres to itself, but also adheres to the hydrophilic walls of xylem vessels through adhesion. The cell walls of tracheids and vessels contain cellulose and other hydrophilic polymers to which water molecules are attracted via hydrogen bonds (Medvedev, 2012).

This adhesion is critically important for two reasons:

First, adhesion transmits tension from water to the vessel walls. When water in vessels is under tension, it tends to contract, but adhesion keeps it attached to the walls, preventing detachment and bubble formation.
Second, adhesion creates capillary forces. Because water adheres to the walls of narrow capillaries (and xylem vessels are indeed capillaries), a capillary rise occurs. However, as we will see later, capillary forces are insufficient to raise water to the height of tall trees—their contribution is relatively small compared to transpirational tension.

3. Surface Tension—Meniscuses in Cell Walls

The third key principle is the surface tension of water, which generates negative pressure at the sites of evaporation. When water evaporates from the cell walls of the mesophyll, the water–air interface (meniscus) retreats into the narrow pores between cellulose microfibrils (Taiz et al., 2023; Lambers & Oliveira, 2019).

Because water tends to minimise its surface area, the meniscus becomes curved. According to the Laplace–Young equation, such a curved meniscus creates a negative hydrostatic pressure in the liquid phase:

\Psi_p = -\frac{2T}{r}

where \( T \) is the surface tension of water, and \( r \) is the radius of curvature of the meniscus.

The stronger the evaporation, the deeper the meniscus enters the pores, the smaller the radius of curvature, and the more negative the pressure becomes. It is this negative pressure in the cell walls that creates the “pulling force” that is transmitted down the water column to the roots. The pores in cell walls are extremely small (about 5 nm) and can generate tremendous tension—up to 25 MPa or more (Lambers & Oliveira, 2019)!

How It Works: From Leaf to Root

Now let us trace the entire path of water—from the moment of evaporation in the leaf to uptake from the soil. This will help us see the cohesion‑tension theory in action as an integrated system.

Step 1. Evaporation from Mesophyll Cell Walls

Inside the leaf, in the substomatal cavity, the air is nearly saturated with water vapour (relative humidity about 99–100%). This is because mesophyll cells have a huge internal surface area (7 to 80 times the external leaf area!) from which evaporation occurs (Lambers & Oliveira, 2019).

When stomata are open and let in CO₂ for photosynthesis, water vapour diffuses from the substomatal cavity into the drier atmospheric air. The concentration gradient of water vapour is enormous: even at 90% relative humidity, the water potential of the atmosphere is about –14 MPa (Schopfer & Brennicke, 2016).

Evaporation of water from the substomatal cavity causes the menisci to retreat deeper into the cell‑wall pores. This creates increasingly negative pressure (tension) in the liquid water of the cell walls.

Step 2. Transmission of Tension to the Xylem Water Column

Because water is continuous—present both in cell walls and in xylem vessels—the negative pressure from the cell walls is transmitted to the water column in the xylem. The water in vessels is under tension (negative hydrostatic pressure).

This tension is the main driving force of the ascending flow. It creates a hydraulic gradient that “pulls” water from roots to leaves.

Step 3. Rise of Water Through Xylem Vessels

Water moves through xylem vessels down the pressure gradient. It is important to understand that this is not diffusion—molecules do not “make their way” one by one. It is a bulk flow, where a huge number of molecules move together like a single river.

The water column does not move as a rigid body—different parts may move at different speeds, but the direction is uniform: upward. Adhesion of water to vessel walls and cohesion between water molecules preserve the integrity of the column, preventing it from breaking.

Step 4. Water Uptake from the Soil

The tension created in the leaves is transmitted through the root system to the soil. As a result, the water potential of root cells becomes more negative than that of the soil solution. Water enters the root along the water potential gradient (Kuznetsov & Dmitrieva, 2006).

It is important to emphasise: water enters the root passively, according to physical laws, not as a result of active work by root cells (though ions that create the osmotic gradient are actively accumulated). The driving force of the entire process is evaporation in the leaves, which “sucks” water from the soil through the plant.

Why Does Water Not Boil Under Such Tension?

Let us return to the question we raised at the beginning: how can water remain liquid at negative pressures far below the saturation vapour pressure?

Metastable State

Water in xylem vessels is in a metastable state—a state that is thermodynamically unfavourable but can persist for a long time if there is no trigger for a phase transition (Taiz et al., 2023).

Imagine supercooled water that remains liquid below zero degrees as long as there is no crystallisation nucleus. Similarly, water under tension does not turn into vapour if there are no centres of vapour formation (gas bubbles).

Role of Xylem Structure

The plant uses several mechanisms to prevent the formation of vapour‑formation centres:

1. Blockage of pores in vessels. In the walls of xylem vessels there are bordered pits—very narrow channels with a diameter of less than 0.2–0.4 μm (Lambers & Oliveira, 2019). These pores are so small that water cannot boil even under large tension, because a bubble nucleus requires a certain size, and the pores are smaller than the critical size.

2. Filtration of water by the endodermis. In the root, before entering the xylem vessels, water must pass through endodermal cells with Casparian strips—suberinised regions of cell walls that are impermeable to water. This prevents gas bubbles from the soil from entering the vessels (Schopfer & Brennicke, 2016; Taiz et al., 2023).

3. Pit membranes with a torus. In conifers, bordered pits have a central thickening—the torus. When a pressure difference arises between adjacent vessels, the torus is displaced and occludes the pit, preventing gas bubbles from spreading from one vessel to another (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Thanks to these mechanisms, water in xylem vessels can withstand tensions of 2–5 MPa under real conditions, and up to 25–30 MPa in laboratory experiments (Taiz et al., 2023).

Experimental Evidence for the Cohesion‑Tension Theory

The cohesion‑tension theory is not a purely speculative construct. It is supported by numerous direct and indirect experiments.

Classic Experiments of Böhm and Dixon

As early as the late 19th century, Böhm and Dixon performed a simple but convincing experiment with capillaries (Schopfer & Brennicke, 2016). If you connect a water capillary to an evaporating surface (e.g., a porous clay cylinder), you can raise mercury in a manometer to a height significantly exceeding 76 cm—the height that normal atmospheric air can raise mercury (Fig. 13.13 in Schopfer & Brennicke, 2016). This is direct evidence that evaporation creates tension in the water column sufficient to lift liquid to great height.

If gas bubbles appeared in the system, the mercury column instantly fell to the usual barometric level—a vivid demonstration of how cavitation destroys the water column.

Tension Measurements Using a Pressure Chamber

In 1965, Scholander and colleagues developed the pressure chamber method (or “Scholander bomb”), which allows measurement of tension in xylem vessels (Schopfer & Brennicke, 2016; Taiz et al., 2023).

The principle is simple: a cut branch is placed in a sealed chamber so that the cut end protrudes outside. The pressure in the chamber is then increased until water appears on the cut surface. The pressure required equals the tension that was present in the xylem vessels of the living plant.

Using this method, the following data have been obtained:

In moderately moist plants (mesophytes), tension is –0.5…–1.5 MPa.
In drought‑stressed plants, up to –10 MPa (Kuznetsov & Dmitrieva, 2006).
In a 108‑metre redwood (Sequoia sempervirens) at a height of 110 m, the measured tension was –1.84 MPa (Schopfer & Brennicke, 2016).

These direct measurements confirm that a significant negative pressure indeed exists in xylem vessels, and that it increases with height (which is consistent with the need to overcome gravity).

Observations of Trunk Diameter Changes

Another elegant proof is the measurement of diurnal fluctuations in trunk diameter using a dendrometer. As early as 1897, Friedrich noticed that the diameter of the upper part of a tree trunk begins to decrease in the morning (when transpiration starts) slightly earlier than the lower part (Schopfer & Brennicke, 2016).

This is explained by the fact that tension in the vessels arises first in the crown and only gradually spreads downwards. Water under tension “compresses” the vessel walls (because it tends to contract), and the trunk diameter decreases. At night, when transpiration subsides, tension relaxes and the trunk expands again.

MacDougal in 1924 showed that in redwoods, diurnal fluctuations in trunk diameter can be appreciable (Kuznetsov & Dmitrieva, 2006), indicating significant changes in tension over the course of the day.

Acoustic Detection of Cavitation

In 1966, Milburn and Johnson developed an acoustic method for detecting cavitation in plants (Taiz et al., 2023; Hopkins & Hüner, 2009). When a water column breaks in a vessel, microscopic “clicks” occur that can be detected with sensitive microphones.

Counts show that the number of such clicks roughly corresponds to the number of vessels in the studied segment. This confirms that cavitation is a real phenomenon that occurs under high tension. Furthermore, this method has shown that cavitation occurs first in the upper, thinner shoots, and only later in the trunk, which agrees with the cohesion‑tension theory.

Limits of Applicability of the Theory

The cohesion‑tension theory is not universal in the sense that it describes the main, but not the only, mechanism of water ascent. There are two important exceptions:

1. When transpiration is weak or absent (e.g., at night or under high humidity), positive pressure may develop in vessels—root pressure, which we have already discussed. This occurs due to active ion uptake into vessels and subsequent osmotic water uptake (Schopfer & Brennicke, 2016).

2. In spring, in some deciduous trees (e.g., sugar maple), stem pressure is observed that exudes sap when wounded. This pressure is due to starch hydrolysis in wood parenchyma cells and is not related to either root pressure or transpiration (Schopfer & Brennicke, 2016).

However, under conditions of active growth and normal transpiration, the cohesion‑tension theory remains the primary and most supported model of the ascending water flow.

Limits of Possibility: Why Do Trees Not Grow to the Sky?

The cohesion‑tension theory also answers the question of why trees cannot be infinitely tall. Studies by Koch and co‑workers (2004) on the tallest trees in the world—redwoods over 110 m high—showed that at such heights, tension in vessels reaches critical values (Schopfer & Brennicke, 2016).

As height increases:

Leaves experience increasing water deficit.
Transpiration requires greater tension.
Stomata close to avoid cavitation.
Photosynthesis declines.
At tensions around –1.9 MPa at the treetops, signs of water supply disruption appear.

Extrapolation of the data shows that at heights above 125 m, photosynthesis in leaves would become negative (Schopfer & Brennicke, 2016)—that is, the tree would spend more energy on maintaining life than it would gain. This is why redwoods, the tallest trees on Earth, do not exceed 112–115 m: above this limit, the physics of water and plant biology come into conflict.

Summary: The Cohesion‑Tension Theory

We have now examined the central concept of plant water relations—the cohesion‑tension theory. Its main points are:

1. The driving force of the ascending flow is evaporation from leaves. It creates negative pressure (tension) in the xylem water column.

2. Water rises as a single column thanks to cohesion (hydrogen‑bonding between water molecules) and adhesion (sticking to vessel walls).

3. Tension in vessels can reach 2–5 MPa under normal conditions and 10 MPa under drought, sufficient to lift water to heights of up to 120 m.

4. Water does not boil at such low pressures because of the absence of vapour‑formation centres, the narrowness of pores in vessels, and specialised structures (bordered pits with a torus in conifers).

5. The theory has been experimentally confirmed by pressure‑chamber measurements, dendrometry, acoustic detection of cavitation, and direct observations.

6. The theory has its limits. Cases of positive pressure in vessels (root pressure, spring stem pressure) exist, and tree height has a natural limit due to the inability of water to rise above a certain height without cavitation.

In the next part, we will examine the structure of xylem as a pipeline and learn why there is a fundamental trade‑off between conducting efficiency and system resistance to drought.

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3. Xylem as a Pipeline: Efficiency vs. Safety

From Physical Principle to Anatomical Structure

In the previous part, we established that water rises through the plant due to tension created by evaporation from leaves. But for this tension to be effectively transmitted from roots to crown, a specialised conducting system is needed that can withstand huge negative pressures while providing sufficient water flow for transpiration. That system is xylem.

In this part, we will consider the structure of xylem as an engineering construction and analyse the fundamental trade‑off faced by all plants: high throughput versus resistance to failure. This trade‑off determines not only the plant’s internal physiology, but also its ecological strategy, distribution across climatic zones, and ability to withstand droughts.

Main Elements of Xylem: Tracheids and Vessels

The water‑conducting system of xylem consists of two types of dead conducting cells—tracheids and vessel elements (in angiosperms they are assembled into vessels). Both are hollow tubes with lignified walls that can withstand tension and prevent collapse (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Tracheids

Tracheids are elongated, spindle‑shaped cells (usually 1–10 mm long, 10–50 μm in diameter) with tapering ends (Taiz et al., 2023). They occur in all vascular plants, including gymnosperms (conifers) and angiosperms. In conifers, tracheids are the only type of water‑conducting cell.

Water moves through tracheids via numerous pits in their lateral walls. Pits of adjacent tracheids are usually opposite each other, forming pit pairs. Through the pit membrane (a thin primary wall and middle lamella), water passes from one tracheid to another.

A distinctive feature of conifer tracheids is bordered pits with a torus (Taiz et al., 2023; Medvedev, 2012). The pit membrane has a central thickening—the torus, surrounded by a porous and flexible region—the margo. When a pressure difference arises between adjacent tracheids (e.g., one is air‑filled), the torus is displaced and occludes the pit, preventing gas bubbles from spreading into neighbouring tracheids. This makes conifers exceptionally resistant to the spread of embolism (Taiz et al., 2023).

Vessels

Vessels are multicellular conducting elements made up of vessel elements arranged end‑to‑end. During maturation, the end walls of vessel elements break down, forming perforation plates—openings through which water can flow with almost no resistance (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Vessels are characteristic of angiosperms (and some gymnosperms—gnetophytes). They are much larger and longer than tracheids. Vessel diameters can reach 40–60 μm in maple, 100–300 μm in oak, and up to 500 μm in lianas (Taiz et al., 2023; Lambers & Oliveira, 2019). Vessel length in trees varies from a few centimetres to 10 metres or more (in oak) (Taiz et al., 2023).

Along the lateral walls of vessels there are also pits (usually simple or bordered) through which water can flow from one vessel to a neighbouring one, which is important for bypassing embolised sections (Taiz et al., 2023; Medvedev, 2012).

Hydraulic Conductivity: What Does “Ease of Water Passage” Mean?

To understand why xylem structure is so important, we introduce the concept of hydraulic conductivity—the ability of the conducting system to allow water to pass with minimal resistance.

Poiseuille’s Law: Radius Decides Everything

Water flow through a cylindrical tube is described by Poiseuille’s law (Taiz et al., 2023; Schopfer & Brennicke, 2016):

Q = \frac{\pi r^4}{8\eta} \cdot \frac{\Delta P}{\Delta x}

where \( Q \) is the volumetric flow rate (m³/s), \( r \) is the tube radius, \( \eta \) is water viscosity, and \( \Delta P/\Delta x \) is the pressure gradient.

The key takeaway: water flow is proportional to the fourth power of the vessel radius. This means that even a small increase in radius leads to a colossal increase in conductance. For example, if the vessel radius is increased 5‑fold (from 20 to 100 μm), the volumetric flow increases by \( 5^4 = 625 \) times! (Taiz et al., 2023).

This is why angiosperm vessels are far more efficient than conifer tracheids. For comparison: in conifers, maximum flow velocity is about 0.3–0.6 mm/s, while in lianas it can reach 150 m/h (about 42 mm/s) (Schopfer & Brennicke, 2016; Lambers & Oliveira, 2019).

Resistance of Pits and Perforations

However, the actual conductivity of xylem is determined not only by vessel radius, but also by the resistance created by:

Perforation plates—even if simple, they create some flow resistance; in some species they may be scalariform (ladder‑like) or reticulate (net‑like), increasing resistance (Taiz et al., 2023).
Pits in lateral walls—they narrow the path of water when passing between vessels or tracheids. In angiosperms, pit membranes have very small pores (less than 0.01 μm), creating significant resistance but protecting against embolism. In conifers, pits are larger (especially the margo), and resistance is lower, but this comes at the cost of lower protection against embolism (Taiz et al., 2023).

Thus, real xylem conductivity is a compromise between efficiency and safety.

Efficiency vs. Safety: A Fundamental Trade‑off

We now come to the central idea of this section: wide vessels give high conductivity but make the plant more vulnerable to drought and freezing; narrow tracheids give lower conductivity but greater resilience.

Why Are Wide Vessels More Vulnerable?

The reason lies in the probability and rate of gas bubble formation under tension. Wide vessels have:

1. Larger pores in pit membranes (or, in conifers, a larger torus), which facilitates air seeding at high tension (Taiz et al., 2023; Lambers & Oliveira, 2019).

2. A smaller wall surface area per unit volume, which reduces adhesion holding the water column.

3. A higher probability of defects and centres of vapour formation that can trigger cavitation.

Consequently, wide vessels embolise at lower tensions than narrow tracheids. This has been confirmed experimentally: the tension at which 50% loss of conductivity occurs (\( P_{50} \)) correlates with vessel diameter (Lambers & Oliveira, 2019).

Examples from Different Ecological Groups

Comparison of various plant groups vividly illustrates this trade‑off (Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016):

Plant Group	Xylem Characteristics	Conductivity	Embolism Resistance
Conifers (pine, spruce)	Only tracheids (narrow, 10–50 μm)	Low	High
Angiosperms diffuse‑porous (maple, beech)	Medium‑diameter vessels (40–60 μm)	Medium	Medium
Angiosperms ring‑porous (oak, ash)	Large vessels (up to 300 μm)	High	Low
Lianas	Very large vessels (up to 500 μm)	Very high	Very low

Conifers sacrifice transport speed for reliability. Their narrow tracheids with torus‑margo allow them to withstand severe droughts and frosts without embolism. This is why conifers dominate boreal and high‑altitude forests, where extreme temperatures and droughts are common.

Ring‑porous angiosperms (oak, ash) have large vessels formed in spring, providing fast water transport early in the growing season. However, these same vessels are extremely vulnerable to embolism, and such trees often show branch dieback after severe droughts or frosts (Lambers & Oliveira, 2019).

Diffuse‑porous (maple, beech) occupy an intermediate position: they form vessels evenly throughout the season, giving moderate conductivity and moderate resistance (Lambers & Oliveira, 2019).

Lianas are an extreme example: their very wide vessels provide enormous conductivity (up to 150 m/h), but at the cost of extreme vulnerability to embolism, and they grow only in humid tropical forests where droughts are rare (Taiz et al., 2023; Lambers & Oliveira, 2019).

Bordered Pits as “Safety Valves”

In conifers, bordered pits with a torus function as automatic valves. When tension in one tracheid becomes too high and a gas bubble forms, the pressure in that tracheid drops sharply. The resulting pressure difference displaces the elastic torus to the pit aperture, occluding it. This prevents the bubble from spreading into neighbouring, still‑functional tracheids (Taiz et al., 2023; Medvedev, 2012).

Angiosperms lack such a developed valve system. Their pit membranes simply have very small pores (less than 0.01 μm) through which air cannot pass until tension becomes very large (Taiz et al., 2023). However, when that happens, a gas bubble can quickly fill the entire vessel, and such a “failure” affects a large number of vessel elements at once.

Xylem Efficiency and Resource Investment

Xylem structure affects not only transport speed but also the energy efficiency of the plant. Narrow tracheids require less biomass per unit length, but provide less conductivity per unit cross‑sectional area. Wide vessels require more biomass (thicker walls to withstand tension), but provide tremendous conductivity (Taiz et al., 2023; Lambers & Oliveira, 2019).

Leaf Area to Xylem Area Ratio

An important indicator of xylem efficiency is the ratio of the leaf area served by a given xylem segment to the cross‑sectional area of the conducting tissue (\( Af/As \)). In species with high conductivity (e.g., ring‑porous), this ratio is high—a large leaf area is served by a relatively small xylem cross‑section. In conifers, with their low conductivity, this ratio is significantly lower (Lambers & Oliveira, 2019).

In drought‑tolerant species, this ratio is usually lower than in mesophytes, reflecting lower water availability and the need for a more “safe” xylem (Lambers & Oliveira, 2019).

Seasonal and Age‑Related Changes in Xylem

In woody plants, xylem is not static—it changes over the season and with age.

Spring and Summer Vessels

In ring‑porous trees (oak, ash), large vessels (earlywood) are formed in spring, providing high water flow early in the growing season when leaves are just unfolding and transpiration is still low. However, these vessels are vulnerable to embolism and are often blocked by mid‑summer. During summer, small vessels (latewood) are formed, which provide more modest but stable conductivity (Lambers & Oliveira, 2019).

In diffuse‑porous trees (maple, beech), vessels are formed evenly throughout the season, giving more stable, but less peak, conductivity.

Secondary Xylem and Ageing

As a tree ages, the central part of the trunk accumulates heartwood, in which vessels are blocked by tyloses (outgrowths of parenchyma cells) and/or embolised. This “dead” core does not participate in water transport but provides mechanical strength. Water is transported only through the sapwood—the peripheral part of the xylem (Lambers & Oliveira, 2019).

Thus, the tree constantly builds new, younger xylem at the periphery, while old xylem gradually “retires” to serve a supporting function. This is an important long‑term mechanism for maintaining hydraulic integrity.

How Hydraulic Conductivity Affects Photosynthesis and Growth

Finally, it is important to understand that xylem hydraulic conductivity directly affects the rate of photosynthesis and plant growth. The higher the conductivity, the more water can reach the leaves, the higher the transpiration rate, and hence the higher the potential photosynthesis rate (provided other factors are not limiting).

However, as we have seen, high conductivity makes the plant more vulnerable. Therefore, under drought, plants with wide vessels must close their stomata earlier to avoid critical tension and cavitation. This reduces photosynthesis but allows survival.

In drought‑tolerant species (sclerophytes), xylem is generally more “safe”—it has narrow vessels, giving lower transport speed, but allows photosynthesis to be maintained at lower soil water potentials (Lambers & Oliveira, 2019). This is one example of how xylem anatomy determines a species’ ecological strategy.

Summary: Trade‑off as a Basis of Evolutionary Strategy

We have now examined the organisation of xylem as a hydraulic system and analysed the fundamental trade‑off between efficiency and safety. The main conclusions are:

1. Xylem consists of tracheids and/or vessels—dead tubular cells with lignified walls capable of withstanding tension.

2. Conductivity depends strongly on vessel radius (to the fourth power!). Wide vessels give high conductivity but make the plant vulnerable to embolism.

3. Bordered pits with a torus in conifers function as automatic valves that prevent embolism spread.

4. Different ecological groups (conifers, ring‑porous, diffuse‑porous, lianas) exhibit different strategies of balancing conductivity and resistance, corresponding to their ecological niches.

5. Xylem changes over time—spring vessels differ from summer vessels, and old xylem becomes heartwood that does not participate in transport.

6. Hydraulic conductivity directly affects photosynthesis and growth—high conductivity offers more potential but requires greater caution in water use.

In the next part, we will examine what happens when this trade‑off breaks down—the phenomena of cavitation and embolism, which are the “price” for high hydraulic efficiency.

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4. Cavitation and Embolism: The Price of High Tension

Introduction: Paying for Efficiency

In previous parts, we established that the ascending water flow relies on enormous tension created by evaporation in leaves. We also learned that high hydraulic conductivity, provided by wide vessels, comes at the cost of reduced system stability. This vulnerability manifests in the phenomena of cavitation and embolism—the main “failures” of the plant’s water‑conducting system.

Imagine a water pipe under high vacuum. If an air bubble appears, it can expand and completely block the flow. Something similar happens in xylem, but with an important twist: water here is in a metastable state, and the appearance of a gas bubble (cavitation nucleus) can occur spontaneously, without external intervention, if the tension becomes too strong.

Let us examine what cavitation and embolism are, why they occur, what consequences they have for the plant, and how plants try to cope with them.

What Are Cavitation and Embolism? Distinguishing the Concepts

In plant physiology, these two terms are often used interchangeably, but there is an important difference (Taiz et al., 2023; Lambers & Oliveira, 2019):

Cavitation (from Latin cavus — hollow) is the process of gas bubble formation (or void) in liquid water under tension. During cavitation, water abruptly changes from liquid to vapour in a local region inside a vessel or tracheid. This occurs when the tension exceeds the tensile strength of water at that point. Cavitation is the moment of “rupture” of the water column.
Embolism (from Greek embolos — plug, stopper) is the state of a vessel or tracheid when it is filled with gas (air or water vapour) and can no longer conduct water. Embolism is the result of cavitation. After a bubble forms, it can expand, fill the entire vessel lumen, and the vessel becomes “blocked” with gas. It is embolism that causes the loss of hydraulic conductivity.

Thus, cavitation is an event, and embolism is its consequence. In the literature, however, the term “embolism” is often used to denote both the process and the result when the context does not require strict distinction.

Mechanisms of Cavitation

There are two main mechanisms leading to bubble formation in xylem vessels.

1. Air Seeding (Air Entry Through Pores)

This is the main mechanism of cavitation under water stress (Lambers & Oliveira, 2019; Taiz et al., 2023). It involves air from outside the vessel (e.g., from an adjacent embolised vessel or from intercellular spaces) being drawn through pits in the vessel wall under high tension.

How it happens:

The pit membrane (primary wall and middle lamella) has pores of a certain size (from 0.05 to 0.4 μm, depending on the species) (Lambers & Oliveira, 2019).
When the tension in a vessel becomes very large (water potential highly negative), a pressure difference arises between the air in an adjacent embolised vessel (atmospheric pressure) and the water in the functioning vessel (negative pressure).
This pressure difference “sucks” air through the pit membrane pores if the tension exceeds a critical value determined by pore size.
Once inside the vessel, the air bubble expands rapidly (because the pressure in the vessel is negative) and fills the entire lumen.

The critical tension at which air seeding occurs depends on the pore diameter in the pit membrane: the smaller the pores, the higher the tension required for air entry. In conifers, the torus‑margo effectively blocks pores, preventing air seeding and making them more resistant (Taiz et al., 2023).

2. Homogeneous Nucleation (Spontaneous Vaporisation)

The second mechanism is spontaneous formation of water vapour bubbles within the liquid itself at very high tension. Water in vessels is in a metastable state, and at tensions exceeding ~25–30 MPa (under ideal conditions), it can boil at room temperature (Taiz et al., 2023).

However, in real xylem such extreme tension is almost never reached (usually no more than 2–10 MPa). Therefore, homogeneous nucleation is more of a theoretical possibility than an actual mechanism of cavitation in natural conditions. In practice, air seeding plays the dominant role.

3. Freezing‑Induced Cavitation

This mechanism is particularly important for plants in cold climates (Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016).

When water freezes in vessels, dissolved gases (nitrogen, oxygen, carbon dioxide) are expelled from the ice because their solubility in ice is extremely low.
These gases form microbubbles that remain inside the frozen vessel.
Upon thawing, if the water in the vessel is under tension, these microbubbles can expand and cause embolism.
Importantly, freezing cavitation does not require strong tension—it can occur even at relatively high water potentials, as long as some tension arises after thawing.

Wide vessels are more susceptible to freezing cavitation because they contain larger volumes of dissolved gas, which form larger bubbles. Therefore, in cold regions, species with narrow vessels or tracheids predominate (Lambers & Oliveira, 2019).

Factors Affecting Vulnerability to Cavitation

A plant’s vulnerability to cavitation is determined by several factors:

1. Vessel (or Tracheid) Diameter

As already noted, wide vessels are more vulnerable. This is confirmed by numerous studies: the tension at which 50% conductivity is lost (\( P{50} \)) correlates with vessel diameter (Lambers & Oliveira, 2019). In species with very wide vessels (lianas), \( P{50} \) may be only about –1…–2 MPa, while in conifers with narrow tracheids it can be –6…–10 MPa or more.

2. Pit Membrane Structure

In conifers, the presence of the torus and margo allows effective blocking of air passage through pits when the torus is displaced. This gives them high resistance to air seeding (Taiz et al., 2023).
In angiosperms, pit membranes are homogeneous, with very fine pores. However, the size of these pores and the membrane thickness determine the critical tension. Species with thicker pit membranes and finer pores are generally more resistant (Lambers & Oliveira, 2019).

3. Vessel Length

Long vessels are more vulnerable because:

They have a greater chance of containing defects or nucleation centres.
When one vessel becomes embolised, conductivity is lost over a large length, which is harder to compensate through bypass routes.
Long vessels contain more dissolved gas, which can be released upon freezing.

Lianas have very long vessels (up to 10 m), making them extremely vulnerable (Lambers & Oliveira, 2019; Taiz et al., 2023).

4. Plant Physiological State

Drought increases tension, bringing it closer to critical.
Damage to roots or trunk can introduce air into the xylem.
Tissue ageing—old vessels often have thinner walls and larger pores, increasing their vulnerability.

Physiological Consequences of Cavitation and Embolism

When a vessel or tracheid becomes embolised, it ceases to conduct water. This initiates a cascade of negative consequences for the plant.

1. Reduction in Hydraulic Conductivity

The primary and immediate consequence is a decrease in overall hydraulic conductivity of the xylem. The more vessels are embolised, the less water can be supplied to the leaves at a given tension (Lambers & Oliveira, 2019; Taiz et al., 2023).

This loss of conductivity is described by vulnerability curves, which show the percentage loss of conductivity as a function of tension (or water potential). For each species, the \( P{50} \) value can be determined—the tension at which 50% conductivity is lost. In drought‑tolerant species, \( P{50} \) is more negative (Lambers & Oliveira, 2019; Taiz et al., 2023).

2. Leaf Water Deficit

Reduced conductivity leads to a decline in water supply to the leaves while transpiration continues. As a result, leaf water potential becomes more negative, and they may lose turgor. This manifests as visible wilting (Schopfer & Brennicke, 2016; Kuznetsov & Dmitrieva, 2006).

3. Stomatal Closure

When leaf water potential falls below a certain threshold, or when abscisic acid (ABA) is produced in roots in response to drought, stomata close (Connor et al., 2011; Lambers & Oliveira, 2019). This is a protective response aimed at reducing transpiration to prevent further drops in water potential and cavitation.

However, stomatal closure comes at a cost: it limits CO₂ entry, and photosynthesis drops sharply.

4. Reduced Photosynthesis and Growth

Decreased photosynthesis due to stomatal closure, along with direct damage to the photosynthetic apparatus under dehydration, leads to a slowdown or cessation of growth (Lambers & Oliveira, 2019; Connor et al., 2011). Under prolonged drought, the plant may shed some leaves to reduce evaporative surface, but this further reduces productivity.

5. Risk of Dieback and Mortality

If cavitation progresses and affects an increasing number of vessels, the plant may fail to supply water even with closed stomata. As water potential continues to fall, large vascular bundles in the trunk may become embolised, leading to dieback of entire branches or even plant death (Lambers & Oliveira, 2019; Taiz et al., 2023). Particularly dangerous is when embolism spreads to the root system, disrupting the entire hydraulic connection.

How Plants Avoid Cavitation: Defence Strategies

Plants have evolved several strategies to minimise the risk of cavitation or limit its consequences.

1. Structural Adaptations

Narrow vessels or tracheids—the main strategy of xerophytes and plants in cold regions. The cost is reduced conductivity (Lambers & Oliveira, 2019).
Thick pit membranes with fine pores—increase the critical tension for air seeding.
Torus‑margo in conifers—provides additional protection (Taiz et al., 2023).
Short vessels—limit the spread of embolism because each vessel is isolated, and bypass routes through neighbouring vessels allow conductivity to be maintained (Lambers & Oliveira, 2019).

2. Physiological Mechanisms

Early stomatal closure—a preventive measure: stomata close before leaf water potential becomes critical, preventing excessive tension (characteristic of isohydric species) (Lambers & Oliveira, 2019; Connor et al., 2011).
Osmotic adjustment—accumulation of osmotically active substances in cells allows turgor and water potential to be maintained at lower water contents (Connor et al., 2011; Lambers & Oliveira, 2019).
Reduction of leaf area—through leaf shedding or reduced leaf size (Lambers & Oliveira, 2019).

3. Behavioural (Motor) Responses

Some plants (e.g., mimosa) change leaf orientation to reduce solar radiation absorption and transpiration.
Leaf rolling in grasses reduces evaporative surface and increases water vapour concentration around stomata (Lambers & Oliveira, 2019).

Measuring Embolism and Vulnerability Curves

To quantitatively assess plant vulnerability to cavitation, vulnerability curves are constructed. They are obtained by measuring hydraulic conductivity of xylem samples at different levels of tension. The most common methods include:

Centrifuge method—a sample is placed in a centrifuge and subjected to known tension, then conductivity is measured (Lambers & Oliveira, 2019; Taiz et al., 2023).
Air‑injection method—a pressure gradient is applied to simulate tension.
Acoustic method—clicks corresponding to cavitation of individual vessels are recorded (Taiz et al., 2023).

Vulnerability curves show at which tension a certain percentage of conductivity is lost. The key parameter is \( P{50} \) (the tension at which 50% conductivity is lost). Across species, \( P{50} \) varies from –0.5 to –14 MPa or more (Lambers & Oliveira, 2019). Species with more negative \( P_{50} \) are more drought‑resistant.

Linking Cavitation to Ecology and Species Distribution

Vulnerability to cavitation is closely linked to a species’ ecological niche. Generalising data from many species, we can identify patterns (Lambers & Oliveira, 2019; Taiz et al., 2023):

Species from humid tropical forests (lianas, many palms) have wide vessels, high conductivity, and low cavitation resistance (\( P_{50} \) ≈ –1…–2 MPa). They cannot tolerate drought.
Temperate‑zone species (oak, maple) occupy an intermediate position (\( P_{50} \) ≈ –2…–4 MPa).
Species from arid regions (pines, junipers, many sclerophytes) have narrow vessels or tracheids, high resistance (\( P_{50} \) down to –10…–14 MPa), and low conductivity.

A global analysis (Choat et al., 2012) showed that \( P{50} \) correlates with mean annual precipitation in a species’ habitat: the drier the climate, the more negative the \( P{50} \) (Lambers & Oliveira, 2019). This confirms that cavitation resistance is a key adaptive trait determining species distribution.

Interestingly, in many species the hydraulic safety margin (the difference between the minimum water potential reached in nature and \( P_{50} \)) is quite small—only 1–2 MPa (Lambers & Oliveira, 2019). This means that plants often function at the limit of their capabilities, and even a small additional drought can lead to massive cavitation and mortality.

Special Case: Freezing‑Induced Cavitation and Its Ecological Consequences

In cold regions, freezing‑induced cavitation can be even more dangerous than drought (Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016). Conifers, with their narrow tracheids and torus‑margo, are better protected from this type of embolism than broad‑vessel angiosperms. This is why conifers dominate in the taiga and high mountains.

In angiosperms, especially ring‑porous species, the large spring vessels often become embolised during freezing and thawing in winter. Such trees must form new xylem in spring, which requires additional resource expenditure.

Some species, such as birch and maple, can recover conductivity of embolised vessels through root pressure in spring, which “pushes” gas bubbles back into solution. However, this mechanism works only in short trees and under moist soil conditions (Schopfer & Brennicke, 2016).

Summary: Cavitation and Embolism as the “Price” of Efficiency

We have examined the phenomena of cavitation and embolism—dangerous disruptions of the water‑conducting system that occur under high tension in the xylem. The main conclusions are:

1. Cavitation is the process of gas bubble formation in water under tension; embolism is the result of cavitation, when a vessel or tracheid becomes gas‑filled and loses conductivity.

2. The main mechanism of cavitation during drought is air seeding: air entry through pit membrane pores at high tension.

3. Freezing and thawing also cause embolism, especially in wide‑vessel species.

4. Vulnerability is determined by vessel diameter (wider are more vulnerable), pit membrane structure, and vessel length.

5. Physiological consequences are cascading: loss of conductivity → water deficit → stomatal closure → reduced photosynthesis → growth slowdown → risk of mortality.

6. Plants defend themselves through structural adaptations (narrow vessels, torus‑margo), physiological mechanisms (early stomatal closure, osmotic adjustment), and behavioural responses (leaf movement).

7. Vulnerability curves and the \( P{50} \) parameter allow quantitative assessment of species resistance; \( P{50} \) correlates with climatic conditions of the habitat.

8. Freezing‑induced cavitation is particularly dangerous for angiosperms, determining the dominance of conifers in cold regions.

In the next, concluding part, we will discuss whether a plant can “repair” damaged xylem, and what recovery mechanisms exist in nature.

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5. Can a Plant “Repair” Xylem?

In previous sections, we established that cavitation and embolism are inevitable companions of high tension in xylem. Wide vessels give high conductivity but pay for it with vulnerability. Drought, frost, and even normal daytime transpiration can cause blockage of some conducting elements.

A natural question arises: can a plant restore conductivity of embolised vessels, or is it doomed to lose them forever?

The answer is more complex than it might seem. For a long time, it was thought that embolised vessels in woody plants are “dead weight,” and the only way to restore conductivity is to form new xylem. However, in recent decades, evidence has accumulated that some plants are capable of local recovery of embolised vessels, and in herbaceous plants and some trees, root pressure plays an important role.

In this section, we will consider three main mechanisms by which a plant can “repair” its water‑conducting system: root pressure, local recovery involving living parenchyma cells, and formation of new xylem. We will also discuss how effective and universal these mechanisms are, and why the main “repair” mechanism in woody plants is still constant renewal.

1. Root Pressure—Pushing Bubbles Out

We already mentioned this mechanism in the first part of the lecture. Root pressure is a positive hydrostatic pressure generated in xylem vessels through active ion uptake and subsequent osmotic water uptake. It can reach 0.1–0.3 MPa, and occasionally 0.6 MPa (Medvedev, 2012; Schopfer & Brennicke, 2016).

How Does Root Pressure Help Restore Xylem?

When root pressure exceeds the tension in the vessels (e.g., at night, in moist soil, and with low transpiration), it creates positive pressure in the water column.
This positive pressure acts on gas bubbles trapped in embolised vessels and forces them to compress and dissolve in water (according to Henry’s law, gas solubility increases with pressure).
Thus, the bubble gradually disappears, and the vessel refills with water.

Where and When Does This Mechanism Work?

Root pressure is most effective in short herbaceous plants and in some trees during certain seasons:

Herbaceous plants (grasses, many dicots) often exhibit strong root pressure, especially at night or under high humidity. In maize, for example, root pressure can restore embolised vessels after nighttime rest when transpiration is minimal (Schopfer & Brennicke, 2016).
In trees, root pressure is particularly pronounced in spring, before leaf emergence, when transpiration has not yet started and the soil is moist. Birch sap, maple sap—these are all results of spring root pressure (Schopfer & Brennicke, 2016; Medvedev, 2012). It helps refill vessels that were embolised over winter before the growing season begins.

Limitations of Root Pressure

However, this mechanism has serious limitations:

1. Small magnitude of pressure. 0.1–0.3 MPa is too low to “push” bubbles in vessels at great heights (more than 10–30 m). For a 100‑metre tree, root pressure cannot overcome the hydrostatic pressure of the water column (Schopfer & Brennicke, 2016). Therefore, root pressure is effective only in short plants or in the lower part of the trunk.

2. Dependence on soil moisture. Root pressure disappears when the soil dries, because active ion uptake into roots weakens (Connor et al., 2011). Under drought, this mechanism fails.

3. Requires active metabolism. Root pressure depends on active ion transport, which requires energy (ATP). At low temperatures or under oxygen deficiency in the soil, root pressure drops (Kuznetsov & Dmitrieva, 2006).

Thus, root pressure is a useful but not universal recovery mechanism. It is especially important for herbaceous plants and for spring “priming” of the water‑conducting system in trees, but it cannot rescue from embolism during summer drought.

2. Local Recovery Involving Living Parenchyma Cells

This mechanism is much more subtle and long remained a mystery to physiologists. In recent decades, experimental evidence has emerged that living parenchyma cells surrounding xylem vessels can actively participate in the recovery of embolised vessels, even when root pressure is absent (Lambers & Oliveira, 2019; Taiz et al., 2023).

Who Are These “Repairers”?

In the xylem of woody plants, besides dead conducting elements (vessels and tracheids), there are living parenchyma cells—these are wood parenchyma cells arranged in vertical strands (parenchyma strands) or horizontal rays (medullary rays). These cells retain protoplasts, contain vacuoles, mitochondria, and other organelles; they are metabolically active.

In angiosperms, parenchyma cells are adjacent to vessels through pits (usually simple or slightly bordered), allowing exchange of substances between living cells and the vessel lumen (Medvedev, 2012).

How Does Local Recovery Occur?

Several hypotheses have been proposed to explain how living parenchyma cells can “pump out” gas bubbles from embolised vessels. The most substantiated is the osmotic hypothesis (Lambers & Oliveira, 2019; Taiz et al., 2023):

1. In an embolised vessel, water is absent, and gas pressure is near atmospheric (or lower).

2. Parenchyma cells adjacent to this vessel actively pump ions (K⁺, Cl⁻) and/or organic osmolytes (sugars) into their vacuoles, creating a very negative osmotic potential in the cell.

3. Water from neighbouring functioning vessels or from the apoplast enters these parenchyma cells along the osmotic gradient. The cells swell, and their turgor pressure increases.

4. Because the parenchyma cells are connected to the vessel lumen through pits, the increased hydrostatic pressure in them “pushes” water into the vessel. Water fills the vessel, compressing the gas bubble and forcing it to dissolve.

5. Once the vessel is filled with water, it resumes transport.

This process requires metabolic energy (for active ion transport and osmolyte synthesis) and therefore depends on respiration of the parenchyma cells.

What Evidence Exists for This Mechanism?

Staining experiments. Several studies have shown that embolised vessels can recover conductivity within hours or days after the plant is returned to normal conditions. Traces of metabolic activity of parenchyma cells are found in the vessels (Lambers & Oliveira, 2019).
Inhibitor studies. If parenchyma cell respiration is inhibited (e.g., by cyanide) or ion transport blocked, recovery does not occur (Taiz et al., 2023).
Modern imaging techniques (X‑ray tomography, magnetic resonance) have allowed observation of individual vessel recovery in real time (Lambers & Oliveira, 2019).

Limitations of Local Recovery

Despite its elegance, this mechanism also has limitations:

1. High energy cost. Active ion transport and osmolyte synthesis require ATP. Under drought, when root and parenchyma respiration are depressed, recovery may be hindered.

2. Effective only under moderate tension. If the tension in neighbouring functioning vessels is too high, water will be “sucked” out of parenchyma cells rather than the reverse, and recovery becomes impossible.

3. Not present in all species. In some species, especially conifers, parenchyma cells are less active, and local recovery is weaker. In conifers, the main role is played by torus‑margo, which prevents embolism spread rather than its elimination (Lambers & Oliveira, 2019).

4. Slow rate. The process can take from several hours to several days, and during this time transpiration may cause new embolisms.

3. Formation of New Xylem—The Main Long‑Term Mechanism

For woody plants, especially tall ones, local recovery and root pressure are often insufficient. Therefore, the main long‑term “repair” mechanism of xylem is the formation of new, fresh xylem (Taiz et al., 2023; Lambers & Oliveira, 2019).

How Does This Mechanism Work?

In woody plants, a cambium functions—a meristematic tissue located between xylem and phloem. The cambium continuously produces new cells: outward—phloem, inward—xylem (secondary xylem, or wood).

Every year (or season), new layers of xylem—annual rings—are formed. Newly formed vessels and tracheids are initially free of embolism and have high conductivity. Old, embolised xylem layers are gradually pushed towards the centre of the trunk, where they may serve only a mechanical (support) function or be filled with tyloses (outgrowths of parenchyma cells) (Lambers & Oliveira, 2019).

Strategies of New Xylem Formation in Different Groups

In ring‑porous trees (oak, ash), most water is transported through large spring vessels, which often embolise by mid‑summer. Therefore, the tree must annually form new large vessels in spring to supply water for the coming season. This is energy‑intensive, but allows rapid restoration of hydraulic conductivity after winter (Lambers & Oliveira, 2019).
In diffuse‑porous trees (maple, beech), vessels are formed evenly throughout the season, allowing gradual renewal of the conducting system without sharp peaks of expenditure. However, overall conductivity of such trees is lower than in ring‑porous species (Lambers & Oliveira, 2019).
In conifers, tracheids serve for decades, and their renewal occurs gradually over many seasons. Thanks to torus‑margo, tracheids rarely become fully embolised, so conifers can manage without annual complete replacement of the conducting system.

Advantages of This Mechanism

1. Long‑term sustainability. The tree constantly “refreshes the fleet” of water‑conducting elements, replacing old, damaged ones with new ones.

2. Independence from drought. Unlike root pressure and local recovery, formation of new xylem does not require moist soil or low transpiration—it can occur even during dry periods, provided there is sufficient assimilate supply.

3. Combination of functions. Old xylem, even if it does not conduct water, continues to provide mechanical support, which is important for tall trees.

Disadvantages and Limitations

1. Energy cost. Building new xylem layers requires significant expenditure of assimilates (carbohydrates) and minerals. Under stress, the tree may reduce wood increment.

2. Time lag. Newly formed xylem must “mature” (lignify, form pits) before it can conduct water effectively. Early in the season, when leaves have already unfolded but new vessels have not yet formed, the tree may experience water deficit.

3. Gradual reduction of sapwood. With tree age, the central part of the trunk becomes heartwood, which does not conduct water. The proportion of functional sapwood may decrease, and to maintain the same conductivity, the tree must form increasingly wide annual rings.

Comparison of the Three Mechanisms and Their Roles Under Different Conditions

For clarity, we summarise the characteristics of the three mechanisms in a table:

Mechanism	Effectiveness	Energy Cost	Conditions for Operation	Typical Plant Groups
Root pressure	Up to 0.3 MPa, height‑limited	Moderate (active ion transport)	Moist soil, low transpiration, night/spring	Herbaceous plants, some trees (birch, maple)
Local recovery by parenchyma	Up to 2–3 MPa (ideally), local	High (osmolyte synthesis, transport)	Moderate tension, oxygen for respiration	Many angiosperms (especially herbs and shrubs)
Formation of new xylem	Unlimited (new layers)	Very high (cell wall synthesis)	Availability of assimilates, active cambium	All woody plants, especially ring‑porous

It is important to emphasise that these mechanisms are not mutually exclusive; they often work together. For example, in oak, root pressure in spring helps fill new spring vessels with water, and local recovery may “clean up” individual embolised sections. However, under severe drought, the main role falls on the tree’s ability to form new, narrower, and more resistant vessels in subsequent seasons.

Modern Views and Open Questions

Despite significant progress, many aspects of xylem recovery remain controversial. For example:

Can local recovery occur under high tension? Some researchers believe that for a vessel to refill, parenchyma cells must overcome the tension in neighbouring vessels, which is thermodynamically extremely difficult. Others argue that recovery is possible only when tension temporarily relaxes (e.g., at night) (Lambers & Oliveira, 2019).
What is the role of aquaporins? Water channels (aquaporins) in parenchyma cell membranes may facilitate water movement, but their role in recovery is still insufficiently studied (Medvedev, 2012).
Universality of the mechanism. Not all angiosperms show equal local recovery. It is possible that in some species, embolised vessels are indeed “written off” without repair (Lambers & Oliveira, 2019).

Summary: Xylem “Repair” Is a Survival Strategy

In conclusion, a plant has several ways to cope with embolism, but none is universal and absolute:

1. Root pressure—effective in short plants and certain seasons, but limited in magnitude and dependent on soil moisture.

2. Local recovery by parenchyma cells—a subtle, energy‑intensive mechanism that allows “repair” of individual vessels, but works only under favourable conditions (moderate tension, oxygen supply).

3. Formation of new xylem—the main long‑term mechanism in woody plants, but it requires time and resources and does not provide immediate effect.

Thus, the plant is constantly in a state of “hydraulic balancing”: it must ensure sufficient conductivity to support transpiration and photosynthesis, while minimising the risk of embolism. Each species has evolutionarily found its compromise, and understanding these mechanisms is critically important for breeding drought‑tolerant cultivars and for predicting forest responses to climate change.

In the next, concluding part, we will give a general summary of the lecture and build a bridge to the next topic—transpiration and its role in plant life.

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6. Lecture Conclusion

From Paradox to Understanding

We began this lecture with a question that has occupied researchers for centuries: how does water, without a mechanical pump, rise tens of metres to the tops of trees? Through our exposition, we have travelled from recognising the physical problem to understanding the elegant but vulnerable solution that evolution has proposed.

Now that we have covered all the key elements, let us assemble them into a single picture and formulate the final answer to the main question of the lecture.

Brief Summary: The Path of Water from Root to Leaf

The ascending flow of water is a continuous stream that starts in the soil and ends in the atmosphere. It involves several stages, each governed by its own physical laws:

1. Water enters the root from the soil along a water potential gradient. In the root, it moves through the apoplast (cell walls) and symplast (via plasmodesmata), and at the endodermis with Casparian strips it is forced to cross plasma membranes (Schopfer & Brennicke, 2016; Taiz et al., 2023).

2. Water enters xylem vessels due to active ion transport into parenchyma cells of the central cylinder, creating an osmotic gradient and, under low transpiration, may generate root pressure (Medvedev, 2012).

3. The main rise of water through xylem vessels occurs via tension created by evaporation in leaves. Water rises as a single column thanks to cohesion (hydrogen bonds between molecules) and adhesion (sticking to vessel walls) (Taiz et al., 2023; Schopfer & Brennicke, 2016).

4. Water enters the leaves, where it evaporates into substomatal cavities and then through stomata into the atmosphere. This process—transpiration—creates the very tension that “pulls” the entire water column upward (Connor et al., 2011; Lambers & Oliveira, 2019).

Answer to the Main Question: How and Why?

How does water rise tens of metres upward without a mechanical pump?

Answer: water rises by a physical principle, not a biological pump. Evaporation of water from leaves creates a negative hydrostatic pressure (tension) in the capillaries of mesophyll cell walls. This tension is transmitted down the continuous water column to the roots. Water is “pulled” upward because the evaporation of one molecule pulls neighbouring molecules through hydrogen bonds (cohesion), and adhesion to vessel walls helps hold the column and transmit the force.

Thus, the engine of the ascending flow is not in the root but in the leaf, and this engine runs on solar energy, which heats the leaves and creates a water vapour gradient between the leaf and the atmosphere.

Why is this mechanism both efficient and vulnerable?

Efficiency of this mechanism is remarkable:

It is passive—does not require direct metabolic energy to lift water (energy is spent only on building xylem and maintaining membranes, not on the movement itself).
It exploits a natural gradient of water potential between the soil (high, ~0 MPa) and the atmosphere (very low, down to –100 MPa or less). The plant essentially “plugs into” this ready‑made gradient (Schopfer & Brennicke, 2016; Taiz et al., 2023).
It allows water to be lifted to heights of up to 110–120 m (in redwoods), which is unattainable for any mechanical pump (Schopfer & Brennicke, 2016).

Vulnerability of this mechanism is equally fundamental:

Water under tension is in a metastable state—thermodynamically unstable and can suddenly turn into vapour if a vapour‑formation centre (gas bubble) appears. This state is like a “taut string” that can break (Taiz et al., 2023).
Under high tension (drought, strong transpiration), cavitation occurs—formation of gas bubbles in vessels, leading to embolism—blockage of a vessel by gas and loss of conductivity (Lambers & Oliveira, 2019).
Embolism triggers a cascade of negative consequences: reduced hydraulic conductivity → leaf water deficit → stomatal closure → reduced photosynthesis → growth slowdown → risk of desiccation and death (Connor et al., 2011; Taiz et al., 2023).
Particularly vulnerable are wide vessels of angiosperms, which give high conductivity but embolise easily. Narrow tracheids of conifers are safer but give lower conductivity. This is the fundamental trade‑off of “efficiency vs. safety” (Lambers & Oliveira, 2019).

How Does the Plant Cope with Vulnerability?

Evolution has produced several ways to minimise risks:

1. Structural adaptations—narrow vessels, thick pit membranes, torus‑margo in conifers that prevent embolism spread (Taiz et al., 2023; Lambers & Oliveira, 2019).

2. Physiological regulation—early stomatal closure when soil dries, osmotic adjustment of cells, changes in leaf orientation (Connor et al., 2011; Lambers & Oliveira, 2019).

3. Recovery mechanisms—root pressure (especially in herbaceous plants and in spring for trees), local recovery involving living parenchyma cells (energy‑intensive but subtle), and the main long‑term mechanism—continuous formation of new xylem by the cambium (Taiz et al., 2023; Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016).

None of these mechanisms is perfect, and their combination determines how resistant a species is to drought, frost, and other stresses. This is why different ecological groups (xerophytes, mesophytes, hygrophytes) exhibit different water‑use strategies (Medvedev, 2012; Kuznetsov & Dmitrieva, 2006).

Practical Significance for Agronomy and Ecology

Understanding the mechanisms of ascending water flow has enormous practical value:

Breeding drought‑tolerant cultivars requires consideration of xylem hydraulic properties: breeders aim to create varieties with an optimal balance of conductivity and embolism resistance, especially for regions with erratic rainfall (Connor et al., 2011; Lambers & Oliveira, 2019).
Predicting forest responses to climate change relies on knowledge of which species are more vulnerable to drought (through vulnerability curves and the \( P_{50} \) parameter). This allows prediction of which forest ecosystems may suffer first (Lambers & Oliveira, 2019).
Irrigation and water management in agroecosystems must account for the fact that even brief water stress can cause irreversible embolism in some vessels, reducing productivity in subsequent periods (Connor et al., 2011).

Bridge to the Next Lecture

We have established that water rises through the plant due to evaporation from leaves—a process we call transpiration. But what is transpiration from a physiological perspective? Why does a plant that needs water constantly lose it? And why can it not function properly without these losses?

We will answer these questions in the next lecture, which will be devoted to transpiration and plant water balance. We will consider:

Mechanisms and types of transpiration (stomatal and cuticular).
The role of stomata in regulating gas exchange and water status.
Factors affecting transpiration intensity (light, temperature, humidity, wind).
The physiological significance of transpiration (cooling, transport of substances, maintenance of tension).
The concept of water deficit and its consequences for the plant.

Transpiration is not just a “loss” of water; it is a key process linking water relations, photosynthesis, and plant growth into a single system. Understanding this process will complete our picture of plant water relations and prepare the ground for studying ways to manage water exchange in agroecosystems.

Closing Words

In this lecture, we have journeyed from wonder at the ability of plants to lift water tens of metres to an understanding of the physical principles underlying this phenomenon. We have seen that:

Water rise is a physical problem solved by evolution through the use of solar energy and the unique properties of water.
The cohesion‑tension theory provides an elegant explanation but also reveals the fundamental vulnerability of the system.
Xylem as a pipeline exemplifies a classic engineering trade‑off between efficiency and safety.
Cavitation and embolism are the “price” for high conductivity, and the plant must constantly balance on the edge of hydraulic collapse.
“Repair” of xylem is possible, but imperfect, and the main strategy of woody plants is constant renewal of the water‑conducting system.

This system is not just a passive “plumbing” but a dynamic, regulated, and highly adaptive structure that allows plants to dominate the land, despite all the difficulties associated with water deficit.

References

Connor, D.J., Loomis, R.S., Cassman, K.G. (2011). ‘Water relations’, in Crop Ecology. Productivity and Management in Agricultural Systems. Cambridge, UK: Cambridge University Press, pp. 229-261.
Hopkins, W.G., Hüner, N.P..A. (2009). ‘Whole Plant Water Relations ’, in Introduction to Plant Physiology. Hoboken, NJ: John Wiley & Sons, pp. 19-38.
Lambers, H., Oliveira, R.S. (2019). ‘Plant Water Relations’, in Plant Physiological Ecology. Cham: Springer International Publishing, 187-263.
Schopfer, P., Brennicke, A. (2010). ‘Ferntransport von Wasser und anorganischen Ionen’, in Pflanzenphysiologie. Berlin, Heidelberg: Springer Berlin Heidelberg, 311-331.
Taiz, L., Møller, I.Max., Murphy, A., Zeiger, E. (2023). ‘Water Balance of Plants’, in Plant Physiology and Development. New York, NY: Oxford University Press, pp. 169-188.
Кузнецов, В.В. (2006). ‘Водный обмен растений [Water exchange in plants]’, in Физиология растений [Physiology of plants]. Москва: Высшая школа, pp. 143-202.
Медведев, С.С. (2012). ‘Водный режим растений [Water regime of plants]’, in Физиология растений [Physiology of plants]. Санкт-Петербург: БХВ-Петербург, pp. 145-174.