Water as a physiological environment

1.1. The Plant Consists Predominantly of Water

Water is the most abundant molecule in the plant body. Its content varies depending on tissue type, age, and physiological state, but for most actively growing organs the following values are typical (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006):

In non‑woody (herbaceous) plants, water usually accounts for 70–95% of the mass. This means that out of every 100 grams of a living plant, only 5–30 grams are dry matter—organic and mineral compounds that make up the cells. Water fills cell walls, cytoplasm, organelles, and, most importantly, the vacuole, which in a mature plant cell can occupy up to 90% of the volume. If a plant loses water below a certain level (for most crops, about 45–60% of the fully hydrated state), it loses the ability to maintain homeostasis and dies (Kuznetsov & Dmitrieva, 2006; Medvedev, 2012).

1.2. Most Physiological Processes Are Impossible Without Water

Water participates in all key life processes of the plant—directly or indirectly.

1.3. Water Unites All Sections of the Plant Physiology Course

In the plant physiology course there are several major topics: photosynthesis, respiration, mineral nutrition, growth and development, stress tolerance, and transport of substances. Through all these sections, as along a single axis, runs water.

Link to photosynthesis. Water is not only an electron donor but also the medium in which photosynthetic pigments and proteins function. Water deficit causes stomatal closure, reduces CO₂ uptake, and consequently decreases photosynthetic intensity. We will discuss this in detail when studying plant drought tolerance.

Link to respiration. Hypoxia (oxygen deficiency) in roots due to waterlogging leads to respiratory impairment and, consequently, to cessation of water uptake. This is a classic example of “physiological drought”—when water is present but the plant does not take it up. We will consider this phenomenon in the section on anaerobic stress.

Link to mineral nutrition. Mineral ions move through the plant exclusively with the water flow in the xylem. The water potential gradient creates the driving force that “pulls” ions from the root to the leaves. Therefore, water and mineral nutrition are inseparable.

Link to growth. Cell expansion—the main mode of vegetative organ growth—depends entirely on turgor pressure. Without water, a cell cannot increase in volume, and without volume increase, growth does not occur.

Link to stress resistance. Water deficit is the most common stress factor in agriculture. Crop losses from drought exceed those from all other abiotic and biotic stresses combined (Lambers & Oliveira, 2019). Understanding water relations is the basis of agronomic practices aimed at improving drought tolerance.

1.4. Water Relations Are Not an Isolated Topic

Students often ask: “Why do we need water relations as a separate section if we have already studied everything about water?” The answer is: water relations are not just the properties of water, but the set of physiological processes that ensure the uptake, transport, and loss of water by the plant under changing environmental conditions.

Water relations include:

• Uptake—how and why the root absorbs water from the soil
• Transport—how water rises through the stem for tens of metres (a key question of our lecture)
• Loss—how and why water evaporates from the leaf surface
• Regulation—how the plant balances these three processes to avoid dehydration

These processes permeate all of physiology, and today’s lecture is only an introduction where we lay down the fundamental physicochemical foundations. Without them, it is impossible to understand growth, nutrition, or stress tolerance.

Conclusion of Section 1

Water is a central factor in plant life for three reasons:

1. Quantitative. The plant is 70–95% water, making water the main component of its biomass.

2. Functional. Water participates in cell structuring, serves as a reactant and product in biochemical reactions, provides transport, thermoregulation, and mechanical strength (via turgor).

3. Integrative. Water relations link all sections of physiology: photosynthesis, respiration, mineral nutrition, growth, and stress tolerance—through a unified aqueous phase and a common physicochemical mechanism—water potential.

That is why we begin the module on water relations with the fundamental question: “Why is water able to move through a plant without a pump?” We will answer this question step by step, starting with an understanding of the plant as part of a single hydraulic system.

Transition to the next section: Water in a plant is not isolated—it is part of the soil–plant–atmosphere system. It is in this context that we must consider water relations, and we shall now explain why this perspective is fundamentally important for physiology.

2.1. The Plant as an Open System

Unlike an animal organism, which can actively control its internal water balance via excretory systems, the plant is an open thermodynamic system. This means that it constantly exchanges with its surroundings not only energy (heat, light) but also matter—primarily water and gases (Schopfer & Brennicke, 2016).

Water continuously enters the plant from the soil through the roots, moves through the conducting tissues, and is released in huge quantities into the atmosphere as water vapour through the leaves. Water thus flows through the plant—it is not stored (except in some special cases, e.g., succulents) nor reserved for future use. In this sense, the plant is like a tube through which water flows: the inlet is the roots, the outlet is the leaf stomata.

This fundamental idea was formalised in the mid‑20th century as the concept of the Soil‑Plant‑Atmosphere Continuum (SPAC). The idea belongs to the British physiologist J. R. Philip and was developed in the work of P. Scholander and others. Today, SPAC is a widely accepted paradigm in plant water relations (Lambers & Oliveira, 2019; Taiz et al., 2023).

2.2. What Is SPAC?

SPAC (Soil‑Plant‑Atmosphere Continuum) is a model according to which water, soil, plant, and atmosphere represent a single hydraulic system in which water moves down a gradient of water potential from the soil (where the potential is high) through the plant to the atmosphere (where the potential is extremely low) (Hopkins & Hüner, 2009).

In simple terms: water is not “sucked up” by the root; it is “pulled” through the entire plant because of the difference in free energy of water between the soil and the air. The plant is merely a conduit—a kind of “hydraulic cable” connecting two reservoirs: the soil solution and the atmosphere.

Schematically, the continuum can be represented as:

SOIL WATER (source, high Ψ) → ROOT (entry, Ψ drops) → XYLEM (transport, Ψ drops) → LEAF (evaporation, Ψ drops) → ATMOSPHERE (sink, very low Ψ)

2.3. The Three Phases of the Continuum

The soil–plant–atmosphere continuum can be viewed as a sequence of three main phases, in each of which water moves under different physical forces, but always down the water potential gradient (Medvedev, 2012; Lambers & Oliveira, 2019).

2.4. Why Is the SPAC Concept Fundamentally Important?

For an agronomy student (and for anyone studying plant physiology), understanding SPAC provides the key to thinking correctly about water relations. Here is how the perception changes:

2.5. Illustrative Examples

Example 1. Capillary and wick. Imagine a fabric wick dipped at one end into water and hanging down at the other. Water rises up the wick by capillary forces until it reaches the end. If that end is now placed in dry air, water will evaporate, and new water will continuously flow through the wick to compensate for evaporation. The plant is like that wick, only much more complex, with valves (stomata) and active control (Medvedev, 2012).

Example 2. Siphon. If you dip one end of a tube into water and place the other end below the water level, water will flow through the tube even if it rises above the water level in the vessel. This happens because of the difference in hydrostatic pressure. In the plant, the “lower end” is played by the atmosphere with its extremely low water potential—it “pulls” water like a siphon (Lambers & Oliveira, 2019).

Conclusion of Section 2

The plant is not an isolated object but a link in the unified hydraulic system “soil – plant – atmosphere” (SPAC).

This means:

• Water is not actively absorbed by the plant; it moves passively down the water potential gradient.
• The main driving force is transpiration (evaporation in leaves), not root activity.
• A disturbance in any link of the system (soil, root, stem, leaf, atmosphere) disrupts the entire water flow.
• Understanding SPAC is necessary for competent management of water regimes in agronomy: irrigation, root protection, stomatal regulation—all are interventions at different links of the same chain.

Now that we have established where and in what direction water moves, we turn to the main question: why does water move? What physical force causes water molecules to move from the soil to the air? The answer lies in the concept of water potential, to which we shall now turn.

Transition to the next section: We know that water moves down a gradient. But what is this gradient? Why does water “prefer” to move from one place to another? To answer, we must turn to the fundamental concept of free energy and how it manifests in the behaviour of water.

3.1. An Intuitive Question: What Makes Water Flow?

We all know that water flows “downhill”—from a mountain, from a tap, along a sloping surface. But in a plant, water moves “upwards”, against gravity. So there must be something stronger than gravity. What is it?

Consider a few simple examples familiar to everyone.

Example 1. Tea with sugar. If you drop a lump of sugar into a cup of tea and do not stir, after some time the sugar will be evenly distributed throughout the volume. Why? Because sugar molecules tend to occupy all available space—they diffuse from a region of high concentration to one of low concentration. This movement occurs spontaneously, without external influence.

Example 2. Dry sponge. If you put a dry sponge into water, it quickly absorbs water. Why? Water penetrates the pores of the sponge because it is attracted by the surfaces (capillary forces). Again—spontaneous movement.

Example 3. Dehydrated cell. If you place a wilted leaf in water, it regains turgor. Water enters the cells because inside the cells salts and sugars are dissolved, which “attract” water (osmosis). Again spontaneous movement.

What do all these examples have in common? In all cases, there is disequilibrium: a difference in concentration, pressure, or wettability. And water always moves so as to reduce this disequilibrium—toward equilibrium. This is a manifestation of the second law of thermodynamics: any isolated system tends toward a state of maximum entropy (disorder), which means that the free energy of the system decreases (Schopfer & Brennicke, 2016; Taiz et al., 2023).

3.2. Free Energy—a Key Concept

In thermodynamics, there is an important quantity—free energy (more precisely, Gibbs free energy). It is the part of the internal energy of a system that can be converted into work at constant temperature and pressure.

For any substance, including water, free energy is determined by its chemical potential (μ). Chemical potential is the free energy per mole of substance. It indicates how “active” a substance can be in chemical reactions or in movement.

Key law: any substance spontaneously (without energy expenditure) moves from a region where its chemical potential is higher to a region where it is lower. This movement continues until the chemical potential is equalised—then equilibrium is reached (Schopfer & Brennicke, 2016).

For water, this rule reads:

Water always moves from a region of higher chemical potential of water to a region of lower chemical potential of water.

But chemical potential has dimensions of energy per mole (J/mol), which is not very convenient for plant physiology. Therefore, physiologists introduced a special quantity—water potential (Ψ), which is the chemical potential of water divided by the partial molar volume of water (≈ 18·10⁻⁶ m³/mol) (Taiz et al., 2023; Hopkins & Hüner, 2009).

where μ_w is the chemical potential of water in the given state, μ_w° is the chemical potential of pure water under standard conditions (atmospheric pressure, temperature 25 °C), and V_w is the partial molar volume of water.

The dimension of water potential is energy per volume, which is equivalent to pressure (Pascal). This makes water potential physically intuitive: it can be measured in the same units as pressure, and it behaves like pressure in hydraulic systems.

3.3. What Does “Water Potential” Mean?

Water potential is a measure of the free energy of water at a given location relative to pure water under standard conditions. Pure water at atmospheric pressure and 25 °C has a water potential defined as zero (Ψ = 0). This is the reference point.

If any substances (salts, sugars, acids) are dissolved in water, the chemical potential of water decreases because water molecules are partially “bound” by hydration shells around the dissolved particles. Consequently, the water potential of the solution becomes negative (Ψ < 0). The more dissolved particles, the more negative Ψ becomes (Hopkins & Hüner, 2009; Medvedev, 2012).

If positive pressure acts on water (e.g., turgor pressure in a cell), its chemical potential increases, and Ψ becomes positive. If water is under tension (negative pressure, as in the xylem), then Ψ becomes more negative (Taiz et al., 2023).

Thus, water potential combines two main factors:

• Concentration component (osmotic effect)—dissolved solutes lower Ψ.
• Pressure component—positive pressure raises Ψ, negative pressure (tension) lowers Ψ.

3.4. Why Does Water Move in Only One Direction?

From the definition of water potential follows a simple rule:

Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential.

This movement occurs spontaneously, without energy expenditure, because it leads to a decrease in the free energy of the system. It is analogous to a ball rolling down a hill: it moves from a region of higher potential energy to one of lower potential energy.

Important: water never moves spontaneously in the opposite direction. To make water move against a Ψ gradient (from more negative to less negative), energy must be expended. In plants, such processes occur but are always coupled with active transport and ATP expenditure (e.g., phloem loading with sugars creates a local decrease in Ψ, and water enters the phloem, even though overall water moves from xylem to phloem—this is a separate mechanism).

3.5. Water Potential in the Soil–Plant–Atmosphere System

Now we can return to SPAC and understand why water moves in the direction we described.

(Values according to Lambers & Oliveira, 2019; Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006)

Notice that water potential monotonically decreases from soil to atmosphere. This is the very gradient that drives water. The atmosphere has such a low (huge in magnitude negative) Ψ that even the tallest trees (>100 m) can raise water solely because of this gradient.

3.6. Why Is This Not a “Pump” but a Gradient?

Now we can clearly answer the main question of the lecture:

Water moves through the plant without a pump because it moves passively down the water potential gradient, which is created by the difference in free energy between the soil and the atmosphere. The plant merely “plugs into” this pre‑existing gradient, providing a continuous path for water.

There is no “pump”. Transpiration creates a very low (negative) Ψ in the leaves, and water is “pulled” from the soil, where Ψ is higher. Water is not pushed by the root—it is pulled by transpiration. Root pressure (which we will discuss later) is only an auxiliary mechanism that helps restore water columns in the xylem after embolism or during dormancy. The main work is done by the atmosphere, which possesses a colossal “water‑absorbing” potential.

Conclusion of Section 3

• Water moves spontaneously because of the system’s tendency to reduce free energy.
• The measure of free energy of water is water potential (Ψ)—a quantity measured in pressure units and indicating how much the water in a given state differs from pure water under standard conditions.
• Water always moves from higher Ψ to lower Ψ.
• In the soil–plant–atmosphere system, Ψ monotonically decreases: soil (–0.01 MPa) → root (–0.5) → xylem (–1.0…–2.5) → leaf (–1.5…–3.0) → atmosphere (up to –100 MPa).
• This gradient is the driving force of the water flow. The plant does not actively create this gradient (except for root pressure); it merely uses it for passive water transport. The main “engine” is the atmosphere.

Transition to the next section: Now that we know what water potential is and why it determines the direction of water movement, we must analyse what it consists of. What physical forces create the particular Ψ values in different parts of the plant? This will allow us to understand how the plant can regulate its water status by modifying these components.

4.1. From Chemical Potential to Water Potential

In thermodynamics, the state of any substance in a system is described by its chemical potential (μ). The chemical potential is the partial derivative of the Gibbs free energy with respect to the amount of substance (number of moles) at constant temperature, pressure, and amounts of other components. Simply put, it is the free energy per mole of substance under given conditions (Schopfer & Brennicke, 2016; Taiz et al., 2023).

For water, the chemical potential (μw) indicates how “active” the water is in that state. Water molecules always tend to move from a state of higher μw to a state of lower μ_w. This tendency is the driving force for diffusion, osmosis, and all other water transport processes.

However, chemical potential has dimensions of energy per mole (J/mol), which is inconvenient for physiological calculations involving flows of liquid through membranes and pores. Therefore, plant physiologists, following the work of Slatyer and Taylor (Slatyer & Taylor, 1960), introduced the quantity water potential (Ψ), defined as the chemical potential of water divided by the partial molar volume of water (V_w ≈ 18·10⁻⁶ m³/mol) (Taiz et al., 2023; Hopkins & Hüner, 2009).

where μ_w° is the chemical potential of pure water under standard conditions (atmospheric pressure, temperature 25 °C), and V_w is the partial molar volume of water (≈ 18 cm³/mol).

This definition gives dimensions of energy per volume, which is equivalent to pressure (1 J/m³ = 1 Pa). Water potential is measured in pascals (Pa), but in practice megapascals (MPa) or bars are more often used (1 MPa = 10 bar ≈ 9.87 atm).

4.2. Standard State: Pure Water at Atmospheric Pressure

For water potential to have practical meaning, a reference point is needed. That point is the standard state of pure water at atmospheric pressure (0.1 MPa) and temperature 25 °C (or the temperature at which measurements are made). In this state, the chemical potential of water equals the standard (μw = μw°), hence the water potential is zero (Ψ = 0) (Hopkins & Hüner, 2009; Medvedev, 2012).

Important: Ψ = 0 is not “absence of water” but the maximum possible water potential. Any deviation from the pure state (addition of solutes, pressure change, change in height) makes Ψ either negative or positive. However, for water in natural and physiological conditions, Ψ is almost always negative because solutes dominate over positive pressure.

4.3. Why Do Solutes Lower Water Potential?

When we dissolve any substance—salt, sugar, amino acid—in water, water molecules start interacting with ions or polar groups of the dissolved molecules. Around each solute particle, a hydration shell forms—a layer of oriented water molecules bound by hydrogen bonds or electrostatic forces. These water molecules lose some mobility and become “bound” (Hopkins & Hüner, 2009; Medvedev, 2012).

From a thermodynamic viewpoint, this means that entropy (a measure of disorder) decreases because the water molecules become more ordered. The Gibbs free energy increases, but importantly for us, the chemical potential of free (unbound) water decreases—because some water molecules can no longer freely participate in movement or reactions.

Consequently, in a solution the mole fraction of water (X_w) is less than 1, and the chemical potential of water (μ_w) becomes lower than that of pure water (μ_w°). Correspondingly, the water potential becomes negative (Schopfer & Brennicke, 2016).

where Ψ_s is the osmotic component of water potential (discussed in Section 5), R is the universal gas constant (8.31 J·mol⁻¹·K⁻¹), T is absolute temperature (K), c_s is the concentration of solute particles (osmolarity, mol/L). The minus sign indicates that solutes lower Ψ (Hopkins & Hüner, 2009; Taiz et al., 2023).

Physical meaning: the more solute particles in water, the fewer free water molecules are capable of moving. The water potential becomes more negative, and water will tend to move to where it is “freer”—that is, to a region of higher Ψ. This is why water moves from the soil (dilute solution) into root cells (more concentrated solution).

4.4. Why Does Pressure Affect Water Potential?

Water in the plant can be under various hydrostatic pressures. This pressure changes the chemical potential of water because it does work on the system. When pressure increases, water molecules are compressed (though slightly), and their free energy increases because energy is required to keep the water compressed. Consequently, positive hydrostatic pressure raises water potential (makes it less negative or even positive) (Taiz et al., 2023; Lambers & Oliveira, 2019).

Conversely, negative pressure (tension) lowers water potential, making it more negative. This is what happens in xylem vessels, where water is under tension created by transpiration. This tension “pulls” water upward, increasing the Ψ gradient between root and leaf.

In general, the pressure contribution to water potential is denoted Ψ_p and equals the hydrostatic pressure (with sign). In living cells, Ψ_p is turgor pressure, which is positive and raises Ψ. In the xylem, Ψ_p is tension, which is negative and lowers Ψ (Medvedev, 2012).

4.5. Why Is Water Movement Spontaneous and Directed in One Direction?

Now we can formulate the fundamental law of water relations:

Water always moves spontaneously from a region of higher (less negative) water potential to a region of lower (more negative) water potential.

This movement occurs without energy expenditure because it corresponds to a decrease in the free energy of the system. As soon as the difference in water potentials (ΔΨ) becomes zero, the system reaches equilibrium, and net water flow ceases (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Why is movement directed only in one direction? Because a spontaneous process always goes toward decreasing free energy. If water were to move against a Ψ gradient (from more negative to less negative), that would mean an increase in free energy, which is impossible without external work. To force water to move “uphill” along a Ψ gradient (e.g., for phloem loading or ion pump operation), energy must be expended, usually in the form of ATP (Lambers & Oliveira, 2019).

4.6. Relationship of Water Potential to Other Quantities

It is important to understand that water potential is not some abstract quantity but a physically measurable characteristic. It can be determined experimentally in several ways:

• Psychrometrically—by measuring the dew‑point depression or changes in air humidity above a sample.
• Osmometrically—by the osmotic pressure of a solution in equilibrium with the sample.
• By volume change of cells or tissues when immersed in solutions of different concentrations (incubation method).
• Using a Scholander pressure chamber—by measuring the pressure needed to force xylem sap out of a cut shoot (this method measures Ψ in the xylem, which is usually close to leaf Ψ) (Hopkins & Hüner, 2009; Medvedev, 2012).

These methods allow numerical values of Ψ to be obtained in different parts of the plant and to construct a water potential map that clearly shows where water has maximum free energy and where it has minimum.

4.7. Why Is Water Potential Not Simply Concentration?

It is important to note that water potential is not equal to water concentration. The concentration of water in a solution is always lower than in pure water, but water potential takes into account not only concentration but also pressure and other factors (which we will discuss in the next section). This is why in a plant water can move from a region of lower water concentration (but higher Ψ) to a region of higher water concentration (but lower Ψ). A classic example is osmosis: water moves from a dilute solution to a concentrated one, even though the water concentration in the dilute solution is higher. This happens because the water potential in the concentrated solution is lower (more negative) due to the greater osmotic effect (Taiz et al., 2023; Lambers & Oliveira, 2019).

Thus, water potential is an integral indicator that combines all factors affecting the free energy of water: solute concentration, hydrostatic pressure, capillary forces, and gravity. That is why it is so convenient for describing plant water relations.

4.8. The Significance of Water Potential for Plant Physiology

For a physiology student and agronomist, understanding water potential provides the key to managing plant water regimes. Knowing Ψ of soil, root, leaf, and atmosphere allows one to predict whether water will enter the plant, at what speed it will move, and what stresses the plant may experience.

Main practical conclusions:

1. Soil salinisation lowers soil Ψ, hindering water uptake by roots even if the soil is physically wet. This is called physiological drought.

2. Fertilisers increase the osmotic concentration of the soil solution, temporarily lowering soil Ψ. Therefore, applying fertilisers to dry soil can cause root burn—water will start moving out of roots into the soil, not vice versa.

3. Stomatal closure reduces transpiration and, consequently, decreases tension in the xylem, which can lead to a drop in leaf Ψ and cessation of water flow.

4. Irrigation raises soil Ψ, restoring the gradient and stimulating water uptake by roots.

All these processes will be discussed in detail in subsequent lectures, but their understanding is impossible without a clear grasp of what water potential is and how it determines water movement.

Conclusion of Section 4

• Water potential (Ψ) is a quantitative measure of the free energy of water, expressed in pressure units.
• It is defined as the difference between the chemical potential of water in a given state and that of pure water in the standard state, divided by the partial molar volume of water.
• The standard state is pure water at atmospheric pressure and 25 °C (Ψ = 0).
• Solutes lower Ψ (make it negative) by reducing the mole fraction of water and decreasing entropy.
• Positive hydrostatic pressure raises Ψ; negative pressure (tension) lowers Ψ.
• Water always moves spontaneously from higher (less negative) Ψ to lower (more negative) Ψ.
• This movement requires no energy expenditure and proceeds until Ψ equalises (equilibrium).
• Water potential is an integral parameter combining concentration, pressure, capillary, and gravitational effects.

Transition to the next section: Now that we know what water potential is and how it determines the direction of water movement, we must answer the question: “What exactly does Ψ consist of in different parts of the plant?” Water potential is not a monolithic quantity but a sum of several components, each reflecting a particular physical force. Understanding these components will allow us to explain why Ψ is one value in the soil, another in the root, another in the leaf, and yet another in the atmosphere. We now move to the analysis of the components of water potential.

5.1. Osmotic Potential (Ψₛ) — “Solute Force”

What is this force? When any substances (salts, sugars, organic acids, amino acids) are dissolved in water, water molecules interact with ions or polar groups of the solute molecules, forming hydration shells around them. These “bound” water molecules lose some mobility and can no longer freely participate in movement or chemical reactions. As a result, the free energy of water decreases—water becomes less “active” (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

How does it manifest? The more solute particles per unit volume, the more water molecules become “bound”, and the more negative Ψₛ becomes. Osmotic potential is always negative (or zero for pure water). It is never positive (Taiz et al., 2023).

Simple formula (for dilute solutions):

where R is the universal gas constant (8.31 J·mol⁻¹·K⁻¹), T is absolute temperature (K), c_s is the total concentration of all osmotically active particles in the solution (osmolarity, mol/L). The minus sign indicates that solutes lower water potential (Hopkins & Hüner, 2009; Taiz et al., 2023).

Where is this important in the plant?

• In the cell sap of the vacuole—salts, sugars, organic acids are accumulated there, creating a low (negative) Ψₛ. This is the main driving force of osmosis—water tends to enter the cell where Ψₛ is more negative than outside.
• In the soil solution—salt concentration determines the Ψₛ of the soil. Soil salinisation makes Ψₛ very negative, hindering water uptake by roots (physiological drought).
• In the xylem sap—the concentration of salts and organic substances is relatively low (Ψₛ about –0.1…–0.2 MPa), so the main contribution to xylem Ψ comes from pressure (tension), not osmosis.

Physical meaning: Osmotic potential is a measure of how strongly solutes “attract” water. The more salts, the stronger they “pull” water, the more negative Ψₛ becomes, and the more actively water tends to enter that solution (Lambers & Oliveira, 2019).

5.2. Pressure Potential (Ψₚ) — “Pressure Force”

What is this force? Water in the plant can be under hydrostatic pressure. This pressure is created either by the elasticity of cell walls (turgor pressure) or by tension in xylem vessels. Hydrostatic pressure changes the free energy of water because it does work: compression raises the energy of the system, while stretching (tension) lowers it (Taiz et al., 2023; Lambers & Oliveira, 2019).

How does it manifest?

• Positive pressure (turgor) — occurs in living cells when water enters the vacuole, cell volume increases, and the cell wall resists stretching. This creates a pressure that “swells” the cell from within. Positive Ψₚ raises water potential (makes it less negative or even positive). It is turgor that provides rigidity to herbaceous organs and is the driving force for growth by expansion (Hopkins & Hüner, 2009; Taiz et al., 2023).

• Negative pressure (tension) — occurs in xylem vessels when transpiration creates a “vacuum” that pulls water upward. This tension lowers water potential (makes it more negative). It is the negative Ψₚ in the xylem that creates the main driving force for water ascent in the plant (Lambers & Oliveira, 2019; Medvedev, 2012).

Where is this important in the plant?

• In living cells, Ψₚ is turgor pressure, which typically ranges from 0.1 to 1.0 MPa (sometimes up to 3 MPa in some cells). It counteracts osmotic potential and determines whether water will enter or leave the cell.
• In xylem vessels, Ψₚ is negative pressure (tension), which can reach –2…–10 MPa in tall trees on a hot day. This tension is the main driving force of water flow.
• In soil, Ψₚ is usually small and often close to atmospheric pressure, so its contribution is often neglected.

Physical meaning: Pressure potential is a measure of mechanical action on water. Positive pressure “pushes” water, raising its free energy. Negative pressure “pulls” water, lowering its free energy (Lambers & Oliveira, 2019).

5.3. Matric Potential (Ψₘ) — “Surface Attraction Force”

What is this force? Water can be attracted to solid surfaces through hydrogen bonds and electrostatic interactions. These surfaces are called the matrix. In the plant, the matrix includes cell walls (cellulose, pectins, hemicelluloses), macromolecules (proteins, polysaccharides), and soil particles (clay, humus). Water molecules attracted to these surfaces lose mobility and become “bound”—their free energy decreases (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006).

How does it manifest? Matric potential is always negative (or zero when fully saturated). The thinner the water film on the surface, the stronger the attraction and the more negative Ψₘ becomes. In very small capillaries (e.g., in cell wall pores), matric potential can reach large negative values—tens or even hundreds of megapascals (Medvedev, 2012; Lambers & Oliveira, 2019).

Where is this important in the plant?

• In cell walls—water is retained in microcapillaries and on the surface of cellulose microfibrils. Matric potential is especially important in tissues with thin walls where water can be easily held by capillary forces.
• In soil—water is retained on soil particle surfaces. Matric potential (together with osmotic potential) determines how much water is available to plants. The finer the soil particles (clay), the greater the specific surface and the more strongly water is retained (more negative Ψₘ).
• In dry seeds—during imbibition, water enters dry seeds precisely because of the very negative matric potential created by proteins, starch, and other hydrophilic polymers.
• In vacuoles and cytoplasm—the contribution of matric potential is usually small compared to osmotic potential, so it is often omitted in calculations for mature cells.

Physical meaning: Matric potential is a measure of how strongly the surface “holds” water. The larger the surface area and the stronger its interaction with water, the more negative Ψₘ becomes, and the more difficult it is for water to “detach” from that surface (Hopkins & Hüner, 2009).

5.4. Gravitational Potential (Ψ₉) — “Gravity Force”

What is this force? Water has mass, so it is subject to gravity. To lift water to a certain height (e.g., from the root to the top of a tree), work must be done against gravity. This work lowers the free energy of water at the elevated height, making Ψ₉ negative (Taiz et al., 2023; Lambers & Oliveira, 2019).

How does it manifest? Gravitational potential is calculated by:

where ρ_w is the density of water (≈ 1000 kg/m³), g is the acceleration due to gravity (9.8 m/s²), h is the height above the reference point (m). For every 10 m of height, Ψ₉ is about –0.1 MPa (Hopkins & Hüner, 2009; Medvedev, 2012).

Where is this important in the plant?

• For small plants (herbs, shrubs up to 5–10 m tall), the contribution of gravitational potential is negligibly small compared to osmotic and pressure potentials. It is often omitted in water balance calculations.
• For tall trees (over 10 m), gravitational potential becomes significant. For example, lifting water to a height of 100 m requires overcoming Ψ₉ ≈ –1.0 MPa. This is comparable to the magnitude of tension in the xylem (Lambers & Oliveira, 2019).

Physical meaning: Gravitational potential is a measure of the work required against gravity to lift water. The higher the water is, the more negative Ψ₉ becomes, and the greater the “suction” (tension) needed in the xylem to raise water to that height (Taiz et al., 2023).

5.5. How Do the Four Forces Work Together?

It is important to understand that all four components act simultaneously. The water potential at any point in the system is the algebraic sum of all four components (Hopkins & Hüner, 2009; Medvedev, 2012).

Let us see how this works in different parts of the plant.

5.6. Why Is It Important to Distinguish These Four Forces?

For a physiology student and agronomist, the ability to distinguish the four components of water potential is not just academic knowledge. It allows:

1. Diagnosing causes of water stress. If a plant wilts but the soil is wet, the problem is likely in osmotic potential (salinity) or root pressure impairment (root damage).

2. Understanding fertiliser action. Application of mineral fertilisers raises the osmotic potential of the soil solution, which can temporarily hinder water uptake by roots (especially under dry conditions).

3. Explaining plasmolysis and turgor. Turgor pressure (Ψₚ) and osmotic potential (Ψₛ) are the two forces that determine whether a cell swells or shrinks.

4. Understanding the mechanism of water ascent. Tension in the xylem (negative Ψₚ) is the main driving force, but osmotic potential (salts in the root) and gravitational potential (height) also contribute.

5. Planning irrigation. Knowing soil Ψ (determined by Ψₘ and Ψₛ), one can choose the optimal irrigation schedule.

Conclusion of Section 5

Water potential is composed of four forces, each with its own physical meaning:

All four forces act simultaneously, and the water potential at any point in the system is their algebraic sum.

Important: For most calculations in plant physiology (especially at the cellular level), only two main components are considered: osmotic and pressure (Ψ ≈ Ψₛ + Ψₚ). Matric and gravitational are added in special cases (dry seeds, tall trees, soil).

Transition to the next section: Now that we know what forces constitute water potential, we can understand how a continuous Ψ gradient arises along the soil–root–stem–leaf–atmosphere system. Why is Ψ one value in the soil, another in the root, another in the leaf, and yet another in the air? This will allow us to complete the SPAC picture and answer the main question: “Why does water move through the plant without a pump?” We now move to the description of the water potential gradient.

6.1. Soil—the Start of the Path: Where Does Water Have the Highest Ψ?

In well‑watered, non‑saline soil, the water potential is close to zero. Why?

• Matric potential (Ψₘ) in moist soil is small because water fills most pores, and water films on particles are relatively thick. Surface tension in large and medium pores does not create strong negative pressure (Lambers & Oliveira, 2019; Medvedev, 2012).
• Osmotic potential (Ψₛ) of the soil solution is usually low (salt concentration of the order of 10⁻³ M), so its contribution to Ψ is insignificant (Hopkins & Hüner, 2009).
• Pressure potential (Ψₚ) in soil equals atmospheric (ignoring capillary effects), i.e., close to zero by definition.
• Gravitational potential (Ψ₉) at root level is usually taken as zero (reference point).

Thus, for moist soil, Ψ is approximately –0.01…–0.03 MPa (Taiz et al., 2023; Lambers & Oliveira, 2019). This is nearly the maximum possible value under natural conditions. This is where the water path begins.

However, as the soil dries, the picture changes. When water is removed from large pores, it remains only in thin capillaries and as thin films on particle surfaces. Matric potential becomes very negative (down to –1.5 MPa and lower). At soil Ψ = –1.5 MPa, the permanent wilting point is reached—most plants can no longer extract water (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006). Therefore, for water uptake, soil Ψ must be higher (less negative) than root Ψ.

6.2. Root—Entry into the Plant: How Is the First Drop Created?

In root cells, especially in cortex and endodermis cells, water contains dissolved salts, sugars, and organic acids, the concentration of which can reach 0.2–0.8 M (Kuznetsov & Dmitrieva, 2006). This creates a significant negative osmotic potential (Ψₛ), typically in the range –0.5 to –1.0 MPa (Hopkins & Hüner, 2009; Taiz et al., 2023).

At the same time, cells possess positive turgor pressure (Ψₚ), which counteracts osmosis. In a fully turgid cell, Ψₚ can reach 0.5–1.0 MPa, and then cell Ψ = Ψₛ + Ψₚ ≈ 0. But in a cell actively absorbing water, turgor is somewhat lower, and cell Ψ remains negative, usually in the range –0.3…–0.8 MPa (Lambers & Oliveira, 2019).

Thus, between soil (Ψ ≈ –0.01 MPa) and root cells (Ψ ≈ –0.5 MPa) there is a first drop of about 0.5 MPa. It is this drop that drives water into the root. It is important to understand: the root does not actively “suck” water—it merely creates a more negative Ψ by accumulating salts (active ion transport), and water enters passively down the gradient (Medvedev, 2012).

6.3. Xylem—the Transport Highway: How Is Tension Created?

From root cells, water enters the xylem vessels. In xylem sap, the salt concentration is relatively low (Ψₛ ≈ –0.1…–0.2 MPa), and matric and gravitational potentials are often small (the latter only at great height). The main contribution to xylem Ψ comes from pressure potential (Ψₚ). But unlike living cells, in the xylem Ψₚ is negative pressure (tension) created by transpiration in the leaves (Hopkins & Hüner, 2009; Taiz et al., 2023).

Under active transpiration, tension in the xylem can reach –0.5…–2.5 MPa, and in extreme cases up to –10 MPa (Lambers & Oliveira, 2019). This tension is transmitted down the vessels, and as a result, xylem Ψ becomes significantly more negative than root cell Ψ. Thus, between root and stem xylem there arises a second drop—now due to pressure.

As water rises up the stem, xylem Ψ becomes increasingly negative because transpirational tension intensifies toward the top (closer to the leaves). In addition, the gravitational component (Ψ₉) adds: for every 10 m of height, Ψ₉ ≈ –0.1 MPa (Taiz et al., 2023; Hopkins & Hüner, 2009). For a 30‑metre tree, this is already –0.3 MPa, which is significant.

6.4. Leaf—the Site of Evaporation: Where Does Ψ Reach Its Minimum in the Plant?

In leaves, water exits the xylem into the apoplast of mesophyll cell walls, then evaporates into the intercellular spaces. This process creates a very negative Ψ in leaf cells. Why?

• Osmotic potential (Ψₛ) in leaf cells is often lower (more negative) than in the root because leaves accumulate photosynthetic products—sugars, organic acids (Lambers & Oliveira, 2019).
• Pressure potential (Ψₚ) in a actively transpiring leaf can drop to zero (loss of turgor) or even become negative (under severe cytorrhysis), further lowering Ψ (Taiz et al., 2023).

As a result, leaf Ψ at midday can be –1.0…–3.0 MPa (in xerophytes up to –8…–10 MPa) (Hopkins & Hüner, 2009; Kuznetsov & Dmitrieva, 2006). This is significantly lower than in stem xylem, creating a third drop—from stem to leaf. This drop “pulls” water from the xylem into leaf cells and then into the intercellular spaces.

But the largest drop awaits us further—at the leaf–atmosphere boundary.

6.5. Atmosphere—the Final Sink: Why Is Ψ of Air So Low?

In leaf intercellular spaces, the air is usually saturated with water vapour (relative humidity close to 100%), and its Ψ roughly corresponds to that of the leaf (about –1…–3 MPa). But atmospheric air is generally much drier. Even at relatively high humidity (50%), its Ψ is about –94 MPa at 20 °C (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).

Why such a huge difference? Because the water potential of air is determined by the ratio of the actual partial pressure of water vapour to the saturation vapour pressure (relative humidity). The formula for calculating Ψ of air:

where V_w is the partial molar volume of water (≈ 18·10⁻⁶ m³/mol), RH is relative humidity in percent (Schopfer & Brennicke, 2016; Taiz et al., 2023). At 20 °C and RH = 50%, this gives –94 MPa. This is tens of times more negative than Ψ in any plant tissue.

Thus, between leaf intercellular spaces (Ψ ≈ –2…–3 MPa) and the atmosphere (Ψ ≈ –94 MPa) there is a giant drop—the main driver of transpiration. This drop creates the very tension in the xylem that pulls water upward.

6.6. Summary Table: A Continuous Ladder of Potentials

Now we can assemble all the values into a single table and see how Ψ monotonically decreases from soil to atmosphere (Lambers & Oliveira, 2019; Hopkins & Hüner, 2009; Taiz et al., 2023; Medvedev, 2012; Kuznetsov & Dmitrieva, 2006).

Note: Ψ monotonically decreases—each successive link has a more negative Ψ than the previous one. There are no upward jumps. This is the continuous gradient that drives water from the soil through the plant to the atmosphere.

6.7. Why Is the Gradient Continuous but Not Linear?

It is important to understand that the drop in Ψ along SPAC is not necessarily uniform. The main resistance to water flow is concentrated in certain links:

• Soil – root. Here resistance can be high if the soil is dry or compacted, or if roots have poor contact with the soil.
• Root – xylem. Radial transport through the cortex and endodermis creates significant resistance (especially because of Casparian bands, which force water to pass through membranes).
• Xylem – leaf. Resistance in vessels is relatively low but increases with embolism (air blockage of vessels).
• Leaf – atmosphere. The main resistance is concentrated in the stomata and the boundary layer of air. It is here that the plant can regulate the flow by opening or closing stomata (Lambers & Oliveira, 2019; Hopkins & Hüner, 2009).

Nevertheless, despite uneven resistances, Ψ always decreases in the direction of flow. If resistance in some link increases (e.g., stomata close), the gradient across that segment increases, and the flow rate decreases—the system self‑regulates.

6.8. What Do We Now Know About Water Movement?

Now we can give a full answer to the main question of the lecture:

Water moves through the plant without a pump because it moves passively down a continuous water potential gradient created by the difference in free energy between the soil (high Ψ) and the atmosphere (extremely low Ψ). The plant merely provides hydraulic continuity along this path, and transpiration in the leaves creates tension that is transmitted down the xylem, “pulling” water from the root and soil.

The root does not pump water—it merely creates an osmotic gradient (by accumulating salts) that helps water enter the plant. But the main work is done by the atmosphere, which, owing to its dryness, possesses a colossal water‑absorbing potential. The plant is a “hydraulic cable” connecting these two reservoirs.

Conclusion of Section 6

• In the soil–plant–atmosphere system, water potential monotonically decreases: from –0.01 MPa in moist soil to –94 MPa in air at 50% humidity.
• This continuous gradient is created by different forces at each site:
  • In soil—matric and osmotic forces.
  • In the root—osmotic potential (salts) and turgor pressure.
  • In the xylem—negative pressure (tension) from transpiration.
  • In the leaf—evaporation, which further lowers Ψ.
  • In the atmosphere—low relative humidity, creating a huge negative Ψ.
• The plant does not actively create this gradient—it merely “plugs into” it, providing a continuous water path.
• The main engine is transpiration, creating tension in the xylem. Root pressure is only an auxiliary mechanism.
• Understanding this gradient is the key to managing water regimes in agronomy: irrigation, root protection, stomatal regulation—all are interventions at different links of SPAC.

Transition to the concluding section: We now have a complete picture: we know why water is important, how it is embedded in a unified system, why it moves, what water potential is, what forces compose it, and how the gradient arises. It remains to formulate the main conclusions of the entire lecture—the key points that the student should take away and remember.

7.1. Water Is the Central Factor in Plant Life

Conclusion one. Water constitutes 70–95% of the mass of actively growing tissues and is the most abundant component of the plant (Hopkins & Hüner, 2009). Without water, no physiological function can exist: from photosynthesis, where water serves as an electron donor and a source of oxygen, to respiration, hydrolysis, and synthesis, where it acts as a reactant or product. Water provides membrane structure, macromolecule hydration, ion and assimilate transport, thermoregulation, and mechanical strength through turgor pressure (Medvedev, 2012; Kuznetsov & Dmitrieva, 2006). That is why water relations permeate all sections of plant physiology—they are the common axis around which understanding of growth, nutrition, stress tolerance, and productivity is built. Water relations are not an isolated topic but an integrating foundation of all physiology (Lambers & Oliveira, 2019).

7.2. The Plant Is Part of a Single Hydraulic System

Conclusion two. The plant does not exist in isolation—it is a link in the soil–plant–atmosphere continuum (SPAC). Water in this system moves continuously from the soil through roots, stem, and leaves to the atmosphere, and the plant merely provides hydraulic continuity of this path (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).

A disturbance in any link of SPAC—soil salinisation, root damage, vessel blockage, stomatal closure, high air temperature—immediately affects the entire water flow. Therefore, the agronomist, physiologist, and ecologist must consider the plant’s water regime not in isolation but in the context of the entire hydraulic chain. This view underpins the modern understanding of water relations (Medvedev, 2012; Taiz et al., 2023).

7.3. Water Moves Down a Free‑Energy Gradient, Not “Upwards”

Conclusion three. Water never moves “upwards” by itself. It always moves toward a decrease in free energy—from a region of higher water potential (Ψ) to a region of lower (more negative) Ψ (Schopfer & Brennicke, 2016; Taiz et al., 2023). In the SPAC system, this gradient is directed from the soil (Ψ ≈ –0.01 MPa) to the atmosphere (Ψ ≈ –94 MPa at 50% humidity). This gradient, not the work of any pump, is the driving force of the water flow.

The plant does not “pump” water or actively “suck” it (except for root pressure, which plays an auxiliary role). It merely uses the pre‑existing gradient created by differences in physicochemical properties of soil and air. Transpiration in the leaves creates tension in the xylem that “pulls” water from the root—and this process is passive with respect to the water flow, although transpiration itself (stomatal opening) requires metabolic control (Lambers & Oliveira, 2019).

7.4. Water Potential Is an Integral Measure of the Water State

Conclusion four. Water potential (Ψ) is a quantitative measure of the free energy of water, expressed in pressure units. It is defined as the difference between the chemical potential of water in a given state and that of pure water under standard conditions, divided by the partial molar volume of water (Hopkins & Hüner, 2009; Taiz et al., 2023).

The standard state—pure water at atmospheric pressure and 25 °C—is taken as zero (Ψ = 0). Any deviation from this state (dissolution of substances, pressure change, capillary retention, elevation) makes Ψ negative or (more rarely) positive. This integral parameter determines whether water will move and in which direction.

7.5. Four Forces Form the Water Potential

Conclusion five. Water potential is composed of four components, each reflecting the action of a particular physical force (Hopkins & Hüner, 2009; Medvedev, 2012; Lambers & Oliveira, 2019):

These four forces act simultaneously, and their contribution differs at each point in the system. In mature vacuolated cells, Ψₛ and Ψₚ usually predominate; in soil, Ψₘ and Ψₛ; in the xylem, Ψₚ (tension); in the atmosphere, Ψ determined by humidity. Understanding this allows not only explanation but also quantitative prediction of the plant’s water status.

7.6. A Continuous Ψ Gradient Is the Key to Understanding the Water Flow

Conclusion six. In the soil–plant–atmosphere system, water potential monotonically decreases:

• Moist soil: Ψ ≈ –0.01 … –0.03 MPa
• Root cells: Ψ ≈ –0.3 … –0.8 MPa
• Root xylem: Ψ ≈ –0.5 … –1.0 MPa
• Stem xylem (at height): Ψ ≈ –1.0 … –2.0 MPa (with Ψ₉ adding –0.1 MPa per 10 m height)
• Leaf xylem: Ψ ≈ –1.5 … –3.0 MPa
• Leaf intercellular spaces: Ψ ≈ –2.0 … –3.0 MPa
• Atmosphere (50% RH, 20 °C): Ψ ≈ –94 MPa

This continuous gradient is created by the successive action of osmotic, pressure, matric, and gravitational forces at each site. The main drop occurs at the leaf–atmosphere boundary, where Ψ falls by tens of megapascals. This drop creates the main driving force for transpiration and, consequently, for tension in the xylem (Lambers & Oliveira, 2019; Taiz et al., 2023).

7.7. The Key Answer: The Plant Does Not Pump Water

Conclusion seven, final. Now we can answer with full certainty the main question of the entire lecture:

Water moves through the plant without a pump because it moves passively down a continuous water potential gradient created by the fundamental difference in free energy between the soil and the atmosphere.

The plant does not actively create this gradient (except for a small contribution from root pressure). It merely provides:

1. Hydraulic continuity—the path from root to leaf via the xylem.

2. Regulable conductance—opening and closing of stomata, which control transpiration rate and thus the magnitude of tension in the xylem.

3. Osmotic gradient in the root (via active ion accumulation), which facilitates initial water entry.

But the main “engine” is the atmosphere, which, owing to its dryness (low partial pressure of water vapour), possesses a colossal water‑absorbing potential. It is this that “pulls” water out of the plant, and this tension is transmitted down the xylem to the roots and soil (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).

7.8. Significance for Agronomic Practice

Understanding the principles set out above has direct applied value in agriculture and crop production:

• Irrigation. The efficiency of irrigation is determined not only by the amount of water but also by soil Ψ, which depends on soil structure (Ψₘ), salinity (Ψₛ), and groundwater depth (Ψ₉).
• Fertilisers. Application of mineral salts raises Ψₛ of the soil solution, which can temporarily hinder water uptake (especially under dry conditions). Therefore, fertiliser application to dry soil can be dangerous—it creates a physiological drought effect.
• Plant protection. Damage to roots by diseases or pests increases resistance in the soil–root link, disrupting the entire water flow and causing wilting even when soil moisture is adequate.
• Transpiration regulation. Knowledge of the dependence of leaf Ψ on air humidity and temperature allows forecasting of water stress and planning protective measures (e.g., shading, mulching, use of antitranspirants).

Thus, the theory of water potential and SPAC is not an abstract science but a working tool for managing plant water regimes in field conditions.

Concluding Summary