Integration of the water regime and water use efficiency

Last updated: July 07, 2026 Русский Español

1. Plant Water Balance as a Unified System

In previous lectures, we examined individual links of water exchange one by one: from water uptake by roots to evaporation from leaves. However, a plant is not a sum of disparate processes, but a single, integral system in which all parts are interconnected. Today we take the next step and consider the water regime as an integrated system that determines not only plant survival but also its productivity.

1.1. The Water Balance Equation

The water balance of a plant is described in its simplest form by the fundamental equation:

Water uptake − Water loss = Change in tissue water content

This equation underlies the entire physiology of water exchange and helps us understand why a plant is never in an absolutely stable state. Let us examine each component:

Water uptake — mainly absorption by the root system from the soil. The amount of water taken up depends on soil water potential, root condition, soil temperature, aeration, and other factors we discussed in previous lectures.
Water loss — primarily transpiration through stomata, and, to a lesser extent, cuticular transpiration.
Change in water content — the difference between what has entered and what has been lost over a given time interval. If uptake exceeds losses, tissue water content increases; if losses exceed uptake, it decreases.

1.2. Dynamic Equilibrium of Water Exchange

The plant water balance is not a static state but a dynamic equilibrium. Over the course of a day, uptake and expenditure of water continuously change, and the plant constantly adapts to these fluctuations (Connor et al., 2011; Kuznetsov and Dmitrieva, 2006).

Observing a plant during the day, we see a characteristic pattern:

Early morning. The soil is still moist from night dew or previous irrigation. Transpiration is minimal because stomata are just beginning to open and the evaporative demand of the atmosphere is low. Water uptake exceeds losses — tissues become saturated, cell water content reaches a maximum, and turgor is maximal.
Noon. Solar radiation peaks, air temperature rises, and relative humidity falls. Stomata are wide open for photosynthesis. Transpiration increases sharply. The root system cannot always compensate for the losses — a water deficit develops. Turgor decreases, leaves may wilt slightly, and stomata partially close.
Evening. Solar radiation declines, temperature drops, and air humidity rises. Transpiration gradually decreases. Water uptake again exceeds losses — the water deficit is alleviated, tissues restore turgor, and guttation droplets appear on the stomata.

Thus, water balance is an oscillating equilibrium in which small deviations in either direction are normal. It is important to understand that the plant does not strive for absolute stability — it can withstand certain fluctuations in water status without significant damage to its vital functions.

1.3. What Happens When Water Balance Is Disrupted?

When adverse factors (drought, overheating, soil salinity, root damage) cause water losses to systematically exceed uptake, the dynamic equilibrium is disturbed. Tissue water content begins to decline, and we observe the development of water deficit (Passioura, 1994; Taiz et al., 2023).

It is important to emphasise: disruption of the balance is not an instantaneous event, but a process that goes through several stages:

1. Decrease in cell water potential — water leaves cells, and their water potential becomes more negative.

2. Reduction in turgor pressure — cells lose elasticity, which manifests as the onset of wilting.

3. Stomatal closure — as a protective response to reduced turgor.

4. Inhibition of physiological processes — slowing of growth, reduction in photosynthesis.

5. Development of water stress — if the water deficit persists for a prolonged period.

1.4. Key Takeaway

Water balance is not simply the sum of absorption and evaporation processes. It is an integral indicator reflecting the current state of the plant and its ability to adapt to changing environmental conditions. Disruption of the balance triggers a cascade of physiological reactions, which we will discuss in the next section.

Main idea of this section: The plant is never at rest with respect to water — it continuously balances between uptake and loss, and its ability to maintain this equilibrium determines its survival and productivity.

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2. Water Deficit and Water Stress

In the previous section, we considered water balance as a dynamic equilibrium between water uptake and expenditure. Now we move to a situation where this equilibrium is disrupted — not briefly, as during midday hours, but systematically and with increasing intensity. This is where stress physiology begins — one of the key topics determining crop fate under arid conditions.

2.1. What Is Water Deficit and How Does It Differ from Water Stress?

In plant physiology, it is important to distinguish two concepts that are often confused:

Water deficit — an objective state of the plant in which tissue water content is below the level observed at full saturation (full turgor). Water deficit is expressed as a percentage of the maximum tissue water content and represents a quantitative measure of water “shortage” (Kuznetsov and Dmitrieva, 2006). The deficit can be moderate and even beneficial — it serves as a signal for triggering protective mechanisms.
Water stress — a broader concept that includes not only the deficit itself but also the entire set of physiological, biochemical, and molecular responses of the plant to that deficit. Stress is the organism’s reaction, not merely a state (Schopfer & Brennicke, 2016). When we speak of water stress, we mean the entire cascade of events: from changes in cell water potential to activation of stress genes, synthesis of protective proteins, and ultimately, changes in productivity.

The distinction between deficit and stress has practical significance. Moderate water deficit that occurs in the middle of the day in well‑watered plants is a normal physiological phenomenon. Water stress, however, is an alarm signal that requires the plant to mobilise protective resources, often at the expense of productivity.

2.2. The Causal Chain: From Soil to Yield Reduction

The development of water stress follows a strict sequence of events, which we can represent as a logical chain. This is very important for understanding where and how we can intervene using agronomic or breeding methods:

Step 1. Decrease in soil water availability

The soil dries out, and its water potential becomes increasingly negative. This process depends on soil structure, its water‑holding capacity, and the intensity of evaporation. It is important to understand that the plant does not “feel” soil moisture directly — it responds to changes in its own water status and to signals coming from the root system (Passioura, 1994; Lambers & Oliveira, 2019).

Step 2. Reduction in water uptake by roots

When soil water potential becomes lower than the water potential of root cells, water uptake decreases. This process worsens as the soil dries because soil hydraulic conductivity drops exponentially (Taiz et al., 2023). The root system can adapt to these conditions — for example, by increasing rooting depth or root density — but these changes require time and assimilate costs.

Step 3. Changes in tissue water status

Reduced water uptake, while transpiration is maintained or even increased, leads to a decline in cell water potential. The sequence is as follows:

Leaf water potential decreases first, especially at midday.
Turgor pressure in cells decreases. This is a critical moment because many physiological processes — from cell expansion to stomatal opening — depend on turgor.
Water begins to flow out of cells, and they lose volume.

Under normal diurnal variation, these changes are reversible. Under increasing water deficit, they become more pronounced and may become irreversible (Taiz et al., 2023).

Step 4. Physiological responses of the plant

A complex set of protective reactions is now triggered, many of which will be discussed in detail in future lectures. The key ones are:

Stomatal closure — the most rapid response, limiting transpiration. Stomatal closure is regulated by the hormone abscisic acid (ABA), whose concentration rises sharply under water deficit.
Inhibition of cell expansion — cell elongation, as we recall, depends on turgor, so it is one of the first processes to slow down.
Reduction in photosynthesis — initially due to stomatal closure and reduced CO₂ influx, and later due to direct damage to the photosynthetic apparatus.

Step 5. Reduction in productivity

If stress continues for a long time, the most significant consequence for agricultural production occurs — yield reduction. This can happen for several reasons:

Reduced leaf area (due to growth inhibition) lowers overall photosynthesis.
Poor conditions for seed formation and filling, especially if drought coincides with the critical period of flowering and fruit set.
Premature senescence and abscission of leaves, fruits, or flowers.

It is important to emphasise: productivity declines long before visible signs of wilting appear. The first yield losses occur at the stage of growth inhibition — even before stomata are fully closed.

2.3. How Does the Plant “Know” About Drought?

This is a key question that long remained a mystery. The traditional explanation — that the plant responds to a decrease in leaf water potential — turned out to be incomplete. Today we know that there are at least two pathways for perceiving water deficit:

Pathway 1. Hydraulic signals

This is the “classical” pathway: a decrease in leaf water potential is perceived as a mechanical signal. When cells lose turgor, this affects the operation of ion channels in membranes, triggering a cascade of intracellular signals. However, this pathway operates relatively late — when the tissue is already substantially dehydrated.

Pathway 2. Root signals (chemical)

A much more important and earlier mechanism is signals coming from roots. When roots encounter drying soil, they begin to synthesise abscisic acid (ABA) — the stress hormone (Passioura, 1994; Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016). This hormone is transported via the xylem to the leaves and causes stomatal closure long before leaf water potential begins to decrease.

This was brilliantly demonstrated in classic experiments by Passioura (1994). If wheat roots are placed in a special chamber where pressure around the roots can be increased to maintain a high leaf water potential even as the soil dries, the leaves still respond to soil drying — their growth slows and stomata close. This means that the signal originates from the roots, not from the leaves.

Thus, the plant does not measure soil moisture — it “senses” it through changes in the functioning of its own root system. This is an evolutionarily sound mechanism: it allows the plant to prepare for an approaching drought in advance, rather than responding to an already developed deficit.

2.4. Stress Intensity and Tolerance

Different plants respond differently to water deficit. In physiology, three strategies of resistance, or tolerance, to drought are distinguished:

1. Drought escape — the plant completes its life cycle before the driest period arrives. This is typical of many ephemerals and short‑cycle crops.

2. Drought avoidance — the plant maintains a high tissue water potential by limiting water losses (stomatal closure, reduced leaf area, thick cuticle) or by increasing uptake (deep root system) (Taiz et al., 2023; Chapman & Huang, 2020). This is an active strategy — the plant does not allow water deficit to develop.

3. Drought tolerance — the plant can withstand low tissue water potential without losing viability. This is achieved through osmotic adjustment (accumulation of compatible osmolytes — proline, glycine betaine, sugars), increased cell wall elasticity, and membrane protection (Chapman & Huang, 2020; Schopfer & Brennicke, 2016).

It is worth noting that many crop plants combine elements of different strategies. For example, modern wheat varieties for dry regions often have a deep root system (avoidance) and the ability for osmotic adjustment (tolerance).

2.5. From Stress to Yield Reduction: The Critical Threshold

To conclude this section, let us consider an important practical question: at what degree of water deficit do irreversible productivity losses begin?

For most mesophytic crop plants, the critical threshold occurs at the loss of turgor — that is, when cells lose the ability to maintain internal hydrostatic pressure. For many plants, this occurs at a water potential of about –1.5 MPa (the conventional “wilting coefficient” used in soil science) (Kuznetsov and Dmitrieva, 2006). However, physiological processes — cell expansion, photosynthesis — begin to be inhibited at much smaller deficits, already at water potentials of –0.3 to –0.5 MPa (Taiz et al., 2023).

This leads to an important practical conclusion: we cannot wait for visible signs of wilting. By the time the plant begins to “ask for water,” part of the yield has already been lost. That is why modern monitoring systems for plant water status focus on early indicators — growth rate, leaf temperature, ABA content — rather than visual symptoms.

Key Idea of This Section

Water deficit triggers a sequential chain of events: from soil — through roots — to changes in tissue water status — to physiological responses — and finally to reduced productivity. The plant does not respond to drought passively, but actively, through a complex of signalling systems, primarily through abscisic acid. And the sooner we learn to recognise these signals, the more effectively we can manage the water regime of crops.

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3. Water Use Efficiency (WUE)

We come now to the key question that concerns every agronomist and physiologist: can we obtain more yield while using less water? In the face of growing freshwater scarcity, this question has become not just scientific but vital for global agriculture. The answer is provided by the concept of Water Use Efficiency (WUE) .

3.1. What Is Water Use Efficiency?

Water use efficiency is an indicator that describes how much product (biomass or yield) is produced per unit of water used. Depending on what we consider the “product” and which water we account for, there are different definitions of WUE (Lambers & Oliveira, 2019; Connor et al., 2011; Taiz et al., 2023).

Agronomic Water Use Efficiency

This is the most practical definition, used in agricultural production and variety evaluation. Agronomic WUE is the ratio of harvested yield (or total biomass) to total water consumption over the growing season:

WUE_{agr} = \frac{Y}{ET}$$ or $$WUE_{agr} = \frac{Biomass}{ET}

where Y is grain yield or other economically valuable product (kg/ha), and ET is total evapotranspiration over the season, expressed in mm of water (e.g., m³/ha).

For cereal crops under dryland conditions, typical agronomic WUE values range from 5 to 20 kg of grain per 1 mm of evaporated water, although under optimal conditions and with modern technologies these values can be higher (Sadras & Angus, 2006; Sadras & Calderini, 2015). For example, in wheat grown under Mediterranean conditions, a so‑called “boundary function” — the maximum attainable WUE under given conditions — was established at about 22 kg grain per 1 mm evapotranspiration, assuming that soil evaporation losses amount to a minimum of about 60 mm (Sadras & Angus, 2006).

Physiological (Intrinsic) Water Use Efficiency

This ratio is measured directly at the leaf level, per unit time. It shows how efficiently the plant uses water in photosynthesis:

WUE_{phys} = \frac{A}{E}

where A is the photosynthetic rate per unit leaf area (e.g., μmol CO₂·m⁻²·s⁻¹), and E is the transpiration rate over the same area (μmol H₂O·m⁻²·s⁻¹). This is an instantaneous characteristic, reflecting stomatal and photosynthetic performance at that specific moment (Lambers & Oliveira, 2019; Taiz et al., 2023).

Physiological WUE depends strongly on environmental conditions — light, temperature, air humidity, and CO₂ concentration. For example, under high light and adequate air humidity, photosynthesis can proceed vigorously while transpiration is not too rapid, resulting in high WUE. At high temperature and low air humidity, stomata may partially close, reducing photosynthesis, and WUE declines.

There is also an intrinsic (or internal) water use efficiency, described by the ratio A / g_w, where g_w is stomatal conductance to water vapour. This indicator does not depend on the humidity gradient between the leaf and the atmosphere and better reflects the genetic characteristics of a variety related to stomatal function (Taiz et al., 2023; Lambers & Oliveira, 2019).

3.2. C₃, C₄, and CAM Plants: Different Strategies for Water Use

It is in the context of water use efficiency that the evolutionary differences among the three main types of photosynthesis become most apparent (without delving into their biochemical mechanisms — that is a topic for a separate module, but we compare them here as a physiological outcome).

C₃ Plants

Most plants on Earth are C₃ plants (wheat, rice, barley, soybean, potato, most trees). Their main disadvantage is photorespiration, during which part of the fixed carbon is lost. From the standpoint of water relations, this means that to fix one molecule of CO₂, a C₃ plant requires on average 400–500 molecules of H₂O. This is a high “cost” of water (Lambers & Oliveira, 2019; Connor et al., 2011). Nevertheless, at moderate temperatures and sufficient humidity, C₃ plants can be quite productive, as they have a higher photosynthetic potential at low temperatures than C₄ plants.

C₄ Plants

This group includes maize, sorghum, sugarcane, millet, and many tropical grasses. They have virtually no photorespiration (or it is strongly suppressed), so they can fix CO₂ with smaller losses. As a result, C₄ plants can maintain photosynthesis with partially closed stomata, which is especially important in hot and dry conditions. Their WUE is 1.5–2 times higher than that of C₃ plants: to fix one molecule of CO₂, they need about 250–300 molecules of H₂O (Lambers & Oliveira, 2019; Connor et al., 2011). In Table 9.4 of Connor et al. (2011), average transpiration efficiency values (biomass per unit of transpired water) for C₄ cereals are about 4–5 g dry biomass per kg water (excluding soil evaporation), whereas for C₃ cereals this value is lower — about 3 g/kg.

CAM Plants

This is a special group of succulents (cacti, agaves, Crassulaceae, pineapple) that open their stomata only at night, when temperatures are low and air humidity is high. At night, CO₂ is stored as organic acids, and during the day, when stomata are closed, this CO₂ is used in photosynthesis. This mechanism allows CAM plants to achieve the highest WUE among all plants — up to 4–20 mmol CO₂ per mol H₂O (in terms of photosynthesis/transpiration ratio) (Lambers & Oliveira, 2019; Taiz et al., 2023). However, the price is very slow growth — CAM plants generally produce low biomass per unit time.

These three strategies illustrate an important physiological principle: high WUE does not always mean high productivity. C₄ plants have higher WUE than C₃, but in temperate climates C₃ plants may produce greater yields. And CAM plants have the highest WUE, but their productivity is extremely low. Therefore, when choosing a crop, we must consider not only its water‑saving ability but also its potential yield under the given climatic conditions.

3.3. What Determines Water Use Efficiency in Real Conditions?

In practice, WUE is determined by many factors, and understanding these factors opens pathways for improvement.

Losses to Non‑Productive Evaporation

Evapotranspiration (ET) consists of two components: transpiration through the plant and evaporation from the soil surface (Connor et al., 2011). Soil evaporation is a “waste” of water that does not contribute to biomass production. The faster the leaf canopy closes, the smaller the fraction of evaporation. Early in the growing season, when leaf area index is low, evaporation losses can reach 50 % or more of total ET (Connor et al., 2011). Therefore, agronomic practices that accelerate leaf canopy development (optimal sowing dates, plant density, fertilisation) indirectly increase WUE by reducing the evaporation fraction.

Seasonal Distribution of Rainfall

Water use efficiency depends strongly on the periods during the growing season when rainfall occurs. In Mediterranean climates, where most rainfall falls in autumn–winter and drought sets in during spring and summer, the key factor is the crop’s ability to use winter moisture for yield formation (Sadras & Calderini, 2015). If rainfall occurs mainly after flowering, when grain is filling, this can be even more effective — water is used directly for grain filling. In any case, the more water is used for productive transpiration rather than for evaporation or non‑productive vegetative growth, the higher the WUE.

Mineral Nutrition Level

Nitrogen nutrition has a dual effect on WUE. On the one hand, nitrogen stimulates leaf area development, which increases transpiration and, under adequate water supply, raises productivity and WUE. On the other hand, excess nitrogen under drought conditions can lead to “haying‑off” — excessive vegetative growth at the expense of yield and premature crop senescence (Sadras & Calderini, 2015). Under Mediterranean conditions, however, nitrogen fertilisers generally have a positive or neutral effect on WUE, especially when applied in a balanced manner and with consideration of weather forecasts (Sadras & Calderini, 2015). The key idea here is co‑limitation by water and nitrogen: when both resources are available in balanced amounts, they are used most efficiently.

3.4. Pathways to Improve WUE: Modern Approaches

Modern physiology offers several avenues for increasing water use efficiency without sacrificing yield.

Breeding for Physiological Traits

In recent decades, breeding for drought tolerance has increasingly used physiological markers rather than purely field‑based yield evaluations. Such markers include:

Carbon isotope signature (δ¹³C) . This method, developed by Farquhar and colleagues, is based on the fact that plants with higher WUE have a lower ratio of intercellular CO₂ concentration to atmospheric CO₂ (ci/ca), which is reflected in the isotopic composition of carbon in tissues (Farquhar et al., 1989; Passioura, 1994). δ¹³C can be used to assess integrated WUE over the entire growing season, which is convenient for breeding.
Leaf temperature. Measuring leaf temperature with infrared thermometers allows rapid assessment of stomatal opening: higher temperature often indicates partially closed stomata and, hence, more conservative water use (Taiz et al., 2023). However, caution is needed: high temperature may also result from other causes.
Proline and other osmolyte content. The capacity for osmotic adjustment correlates with drought tolerance and, in some cases, with WUE (Chapman & Huang, 2020). However, as discussed in Section 2, osmotic adjustment is a tolerance strategy, not an avoidance strategy, and its contribution to WUE is not always unambiguous.

Optimising Stomatal Behaviour

Physiological WUE = A/E, so it can be improved either by increasing photosynthesis (A) at the same transpiration, or by reducing transpiration (E) while maintaining photosynthesis. In practice, the second path often proves more achievable — for example, by developing varieties with more sensitive stomatal responses to atmospheric drought (low air humidity). Such plants more rapidly close stomata partially when evaporative demand increases, saving water without strongly reducing photosynthesis (Oren et al., 1999; Lambers & Oliveira, 2019).

Hydraulic Traits

In recent years, increasing attention has been paid to the hydraulic architecture of the plant (this will be the topic of the next section). Varieties with a more efficient water transport system — vessels that are wide but not too wide, an optimal ratio of leaf area to xylem hydraulic conductance — may have higher WUE because they can keep stomata open under lower water deficits (Passioura, 1994; Taiz et al., 2023). For example, in wheat, a positive relationship has been shown between reduced xylem vessel diameter in roots and increased yield under dry conditions (Richards & Passioura, 1989).

Agronomic Practices

Alongside genetic improvement, agronomic methods are of great importance:

Mulching and minimum tillage reduce surface evaporation and increase the transpiration fraction of evapotranspiration (Connor et al., 2011).
Optimal plant density — under dry conditions, a less dense stand is often more effective, providing each plant with a larger volume of soil moisture (Connor et al., 2011).
Sowing date — early sowing (where possible) allows the use of winter‑spring moisture reserves, reducing the risk of summer drought (Sadras & Calderini, 2015).

3.5. Why Is Maximum WUE Not Always a Synonym for Maximum Yield?

This principle is one of the most important for understanding productivity physiology. Consider a simple example. Imagine two wheat varieties:

Variety A has very sensitive stomata that close quickly at the slightest water deficit. Its WUE (A/E) is high, but total daily photosynthesis is low because stomata are often closed.
Variety B has less sensitive stomata; they remain open longer, even when water deficit begins to develop. Its WUE is lower (it spends more water per unit of fixed CO₂), but total daily photosynthesis is higher, and under favourable water supply, yield may be greater.

Under adequate moisture, variety B will produce a higher yield than variety A. Only under severe drought, when water is scarce, will variety A prove preferable. Therefore, the breeder’s task is to find a compromise between efficient water use and high productivity that maximises economic return under specific climatic conditions, taking into account the probability of droughts (Sadras & Calderini, 2015; Connor et al., 2011).

3.6. Brief Summary on WUE

Water use efficiency is not just a physiological indicator, but a practical criterion on which the sustainability and profitability of crop production in dry zones depend. It integrates:

Physiological processes at the leaf level (photosynthesis and transpiration);
Hydraulic properties of the plant (conductance, stomatal regulation);
Soil and climatic conditions (rainfall, evaporative demand, soil type);
Agronomic practices (fertilisation, tillage, sowing dates);
Genetic potential of the variety.

Improving WUE is a complex challenge with no simple solutions. But it is precisely through this multi‑level approach that we can move towards more efficient water use in agriculture — one of the major challenges of the 21st century.

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4. Hydraulic Architecture of the Plant

So far, we have considered the water regime mainly “in parts”: how water enters roots, how it moves through the stem, and how it evaporates through leaves. Now it is time to view the plant as a single hydraulic system, in which all elements are connected in series and work in a coordinated manner. This modern approach, called hydraulic architecture, allows us to understand why even small changes in one part of the pathway can strongly affect the performance of the whole plant, and why different species and varieties have different drought tolerances.

4.1. What Is Plant Hydraulic Architecture?

The term “hydraulic architecture” was proposed to describe the spatial and functional organisation of the plant’s water‑conducting system (Taiz et al., 2023; Lambers & Oliveira, 2019). The analogy with a domestic water supply system is quite apt: water comes from a source (soil), passes through pipes (roots, stem, leaf veins), then through taps and pressure regulators (stomata), and finally exits into the atmosphere. However, unlike a water supply system where pressure is created by a pump, in the plant the driving force is the water potential gradient generated by evaporation from leaves and, to a lesser extent, by active ion transport in roots (so‑called root pressure).

Hydraulic architecture describes three key parameters:

1. Conductance of individual segments along the pathway — how easily water moves through roots, stem xylem, leaf veins, and stomata.

2. Coordination — how well the operation of different segments is synchronised.

3. Safety margin — how resistant the system is to disruptions (embolism, cavitation, damage).

4.2. Conductance of Different Segments of the Hydraulic Pathway

The path of water from soil to atmosphere can be divided into four successive segments, each with its own hydraulic conductance. The total conductance of the whole system is determined by the weakest link — analogous to Ohm’s law for resistances in series.

Root System Conductance

The root system is the first barrier on the water path. Water must cross several cell layers: root hair, cortex (parenchyma), endodermis with Casparian bands, and finally enter the xylem vessels (Lambers & Oliveira, 2019; Kuznetsov and Dmitrieva, 2006). The main resistance in this segment is related to:

Hydraulic conductivity of membranes — water must cross the plasmalemma and tonoplast. This is facilitated by aquaporins — channel proteins that regulate membrane permeability to water (Taiz et al., 2023; Lambers & Oliveira, 2019). Aquaporins can open and close in response to signals (e.g., ABA or pH changes), allowing the plant to rapidly alter root conductance.
Casparian bands in the endodermis — they force water to move from the apoplast (cell walls) into the symplast (cell protoplasts), increasing resistance. This prevents backflow and ensures selective uptake.
Root‑soil contact — if the soil dries or roots lose contact with soil particles, conductance drops sharply.

Total root conductance depends on root length, root diameter, number of root hairs, and density of root tips. Drought‑tolerant species often have higher specific root conductance per unit length or greater rooting depth, allowing them to tap deep moisture reserves (Passioura, 1994; Connor et al., 2011).

Xylem Conductance (Stem and Branches)

Once water enters the xylem vessels, it moves through the stem and branches to the leaves. Xylem conductance is primarily determined by vessel diameter and their number per unit cross‑sectional area of the stem. According to the Hagen–Poiseuille law, the volume flow of water through a cylindrical vessel is proportional to the fourth power of the vessel radius (Taiz et al., 2023; Lambers & Oliveira, 2019):

J = \frac{\pi r^4}{8\eta} \cdot \frac{\Delta P}{L}

This means that even a small increase in vessel diameter sharply increases its conductance. That is why lianas, for example, have very wide vessels — they provide rapid water transport with a small stem cross‑section (Lambers & Oliveira, 2019). In trees, on the other hand, vessels are often narrower, providing greater mechanical strength and resistance to cavitation.

However, xylem conductance is not limited only by vessel diameter. An important role is played by pits in the vessel walls, through which water flows from one vessel to another. Pits are constrictions that create additional resistance (Taiz et al., 2023; Lambers & Oliveira, 2019). In many woody species, the main hydraulic resistance is concentrated precisely in the pits, not in the vessel lumina.

In addition, xylem conductance depends on its length — the longer the path, the greater the resistance. However, unlike in roots, where water must cross membranes, in the xylem water moves through a passive pathway (apoplast), and there are no membrane barriers except pits.

Leaf Vein Conductance

The path of water in the leaf is perhaps the least studied segment of the hydraulic pathway, but it is critically important. Water leaving the main vein is distributed through progressively finer veins until it reaches the mesophyll cells. Vein conductance depends on vein density and diameter. In many plants, especially those adapted to dry conditions, vein density is higher, providing more efficient water supply to cells (Lambers & Oliveira, 2019; Taiz et al., 2023).

Interestingly, it is often in the leaf veins that the first hydraulic limitation occurs under water stress. When leaf water potential drops, small veins may collapse, sharply reducing conductance (Taiz et al., 2023). This serves as an additional protective mechanism, preventing further tissue dehydration.

Stomatal Conductance

Stomata are the final “tap” on the water path before it exits into the atmosphere. Their conductance is determined by the degree of stomatal opening and, as we have already discussed, is regulated by many factors: light, humidity, CO₂, ABA, and the plant’s water status (Taiz et al., 2023; Lambers & Oliveira, 2019). Stomatal conductance is the most variable part of the hydraulic system; it can change tenfold within a day.

It is important to understand that stomata are not a passive element of the hydraulic system. They do not simply let water through when it is available and close when it is not. Stomata actively regulate the flow of water and CO₂ in response to multiple signals, integrating information about soil, atmosphere, and the plant itself. Moreover, stomata are the main regulator of leaf hydraulic status: their closure reduces the water potential gradient and thus lowers the tension in the xylem, protecting vessels from cavitation.

4.3. Coordination of the Hydraulic System

For the hydraulic system to work efficiently, all its components must be coordinated. This means that the conductances of roots, xylem, veins, and stomata must be balanced. If one segment is too narrow, it becomes a “bottleneck” limiting the entire flow, and the rest of the system operates underloaded. If one segment is too wide while another is narrow, excessive tensions arise that can lead to damage.

Root‑Shoot Coordination

One key example of coordination is the relationship between leaf area and xylem hydraulic conductance. For each unit of leaf area, a certain xylem conductance is required to supply water. Species growing in dry conditions often have a lower ratio of leaf area to xylem area, which reduces the load on the hydraulic system (Lambers & Oliveira, 2019; Connor et al., 2011). Conversely, fast‑growing species with large leaves and thin stems may have a very high ratio, making them more vulnerable to drought.

The root system must also be coordinated with the leaf surface. Under dry conditions, plants often increase the root‑to‑shoot ratio, which enhances the absorptive capacity of roots relative to the transpiring leaf surface (Passioura, 1994; Connor et al., 2011; Kuznetsov and Dmitrieva, 2006). This is an important adaptive mechanism.

Stomatal Regulation as the “Master Valve”

Stomata function as the master valve of the hydraulic system. They do not just regulate water output, but also protect downstream parts (veins, xylem) from extreme tensions. If stomata close too late during drought development, xylem tension may reach a critical level and cavitation begins. If stomata close too early, photosynthesis is severely restricted and productivity is lost. The optimal strategy is one in which stomata close partially, preventing cavitation while maintaining sufficient CO₂ influx for photosynthesis (Taiz et al., 2023; Lambers & Oliveira, 2019).

This regulation, as we have already discussed, is mediated by ABA and other signals. Moreover, there are hypotheses that stomata “sense” the state of the xylem through changes in leaf water potential and respond in a timely manner to approaching critical tensions.

4.4. Hydraulic Limitations: Cavitation and Embolism

Cavitation (from Latin cavus — hollow) is the rupture of the water column in a xylem vessel under excessive negative pressure (tension). An air bubble (embolism) forms, and the vessel ceases to conduct water (Taiz et al., 2023; Schopfer & Brennicke, 2016; Lambers & Oliveira, 2019).

Cavitation occurs when xylem tension exceeds a threshold determined by the pit structure in vessel walls. Air enters the vessel through the largest pits, and the water column breaks (the “air‑seeding” hypothesis) (Lambers & Oliveira, 2019). The larger the pits, the easier cavitation occurs. However, vessels with narrow pits themselves have lower conductance. Therefore, there is a trade‑off between hydraulic efficiency (wide vessels and large pits) and resistance to cavitation (narrow vessels and small pits) (Taiz et al., 2023; Lambers & Oliveira, 2019).

Species growing in dry conditions generally have narrower vessels and smaller pits, making them less vulnerable to cavitation, but also less conductive (Lambers & Oliveira, 2019; Connor et al., 2011). This is another example of a trade‑off characteristic of water relations.

Cavitation can occur not only from drought but also from freezing and thawing of water in vessels (winter embolism). In this case, when water freezes, dissolved gases are released as bubbles, which after thawing can grow and block the vessel (Schopfer & Brennicke, 2016; Lambers & Oliveira, 2019).

Can Embolised Vessels Be Restored?

This is a complex question. In some plants (e.g., grapevine), embolised vessels can be restored through root pressure or other mechanisms, but in most herbaceous plants and trees, recovery occurs only through the formation of new vessels in the next growing season (Taiz et al., 2023; Lambers & Oliveira, 2019). Therefore, loss of hydraulic conductance due to cavitation is a serious and often irreversible damage that can severely limit productivity in dry years.

4.5. Hydraulic Architecture and Drought Tolerance

Understanding hydraulic architecture allows us to explain why different species respond differently to drought and why some varieties are more tolerant.

Drought avoidance in many plants is based on the hydraulic system having a safety margin — they can withstand fairly low water potentials without cavitation. This is achieved through narrow vessels and pits with small apertures. Such plants can keep stomata open at lower water potentials, but their maximum conductance is limited.

Drought tolerance is often associated with hydraulic flexibility — the ability to rapidly restore conductance after cavitation (e.g., through growth of new vessels or redistribution of flow through remaining vessels).

“Risk” strategy is typical of fast‑growing species with wide vessels. They use water efficiently during wet periods, but at the first drought they may lose a large part of their conducting capacity.

Breeding for hydraulic traits is a modern direction. For example, in wheat, lines with narrower root vessels were selected, leading to higher yields under dry conditions without loss of productivity under wet conditions (Richards & Passioura, 1989; Passioura, 1994). This approach shows how managing hydraulic architecture can improve crop adaptation to specific environmental conditions.

4.6. Brief Summary on Hydraulic Architecture

Hydraulic architecture is an integral characteristic of the plant, showing how its water‑conducting system is structured and how it functions. It includes:

Resistance of individual segments (roots → xylem → veins → stomata);
Coordination among them for efficient water use;
Resistance to cavitation and ability to recover.

Understanding hydraulic architecture allows us to predict plant behaviour under drought, assess safety margins, and select optimal varieties for specific soil and climatic conditions. This is a modern and powerful approach that is actively being introduced into physiology and breeding.

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5. Water Regime as an Integrator of Physiological Processes

We have considered water balance, water stress, water use efficiency, and hydraulic architecture. Now it is time to take the most important step — to show that the water regime does not exist in isolation. It permeates virtually all spheres of plant life, acting as an integrator that links photosynthesis, growth, mineral nutrition, hormonal regulation, and stress resistance into a unified whole. Without understanding this integrative role, it is impossible to correctly interpret any physiological process under real field conditions.

5.1. Water Regime and Photosynthesis

The relationship between water regime and photosynthesis is among the closest and most studied in plant physiology. It operates through two main channels: stomatal (rapid and reversible) and metabolic (slower and often irreversible).

Stomatal Regulation of Photosynthesis

As we already know, stomata are simultaneously the gateway for CO₂ entry and the outlet valve for water vapour. When water deficit develops, stomata close, and CO₂ entry into the leaf sharply decreases. This leads to a drop in intercellular CO₂ concentration (Cᵢ) and, consequently, a reduction in the rate of carboxylation catalysed by Rubisco (Taiz et al., 2023; Lambers & Oliveira, 2019). Thus, even if the photosynthetic apparatus is not damaged, stomatal closure already limits photosynthesis.

Stomatal closure under water stress is mediated by ABA, which is synthesised in roots and transported via xylem to leaves. But ABA is not the sole regulator: changes in xylem sap pH, electrical signals, and hydraulic signals are also involved in regulating stomatal conductance (Lambers & Oliveira, 2019; Passioura, 1994). This is a complex, multi‑channel system that ensures a rapid response to changes in water status.

Non‑Stomatal (Metabolic) Inhibition of Photosynthesis

Under more severe and prolonged water stress, the inhibition of photosynthesis can no longer be explained solely by stomatal closure. The biochemical reactions themselves begin to suffer, primarily:

Reduction in Rubisco activity — the key enzyme of CO₂ fixation becomes less active at low water potential, possibly due to changes in protein structure or impaired regeneration (Taiz et al., 2023; Lambers & Oliveira, 2019).
Impairment of photosystem II (PSII) — under severe stress, electron transport in the photosynthetic chain is disrupted, leading to accumulation of reduced components and formation of reactive oxygen species (ROS) (Schopfer & Brennicke, 2016; Lambers & Oliveira, 2019).
Decreased synthesis of photosynthetic components — during prolonged drought, chloroplast biogenesis slows, and chlorophyll and photosynthetic protein content decrease (Lambers & Oliveira, 2019).
Damage to carotenoids — this leads to photo‑oxidative damage to chlorophyll, manifesting as leaf bleaching (Schopfer & Brennicke, 2016).

In addition, water deficit enhances photorespiration because CO₂ concentration in the chloroplast falls while O₂ concentration remains high. As a result, the fraction of carbon lost through photorespiration increases, further reducing photosynthetic efficiency (Taiz et al., 2023).

It is important to note that under moderate stress, the main limitation of photosynthesis is stomatal and reversible. Under severe and prolonged stress, metabolic damage dominates, and recovery of photosynthesis after stress relief is much slower (Chapman & Huang, 2020).

5.2. Water Regime and Growth

Growth is one of the most sensitive processes to water stress. As we saw in Section 2, inhibition of cell expansion is observed already at minimal turgor reduction, long before stomata begin to close (Passioura, 1994; Taiz et al., 2023). This is because cell expansion directly depends on turgor pressure, which drives cell volume increase.

Cellular Level: Impairment of Expansion

According to the Lockhart equation, the rate of cell expansion is determined by two factors: turgor pressure (P) and the mechanical properties of the cell wall (extensibility, φ, and yield threshold, Y) (Lambers & Oliveira, 2019; Taiz et al., 2023):

r = \phi (P - Y)

Under water stress, turgor decreases, but more importantly, the properties of the cell wall also change. ABA and other signals induce cell wall stiffening (increase in yield threshold Y and reduction in extensibility φ) (Lambers & Oliveira, 2019). Thus, even if turgor could be maintained (e.g., through osmotic adjustment), expansion would still be slowed due to wall changes. This shows that growth regulation under water deficit is not merely a passive response to turgor loss, but an active, regulated process that allows the plant to reallocate resources.

Organ Level: Differential Regulation

One of the most striking adaptations is the differential effect of water deficit on growth of different organs:

Shoot growth (leaves and stems) is inhibited first and most strongly. This reduces the transpiring surface, helping to conserve water (Kuznetsov and Dmitrieva, 2006; Connor et al., 2011). This response is mediated by ABA and possibly other signals from roots.
Root growth either is not slowed down or is even stimulated under moderate stress. This allows the root system to penetrate deeper into the soil in search of water (Passioura, 1994; Kuznetsov and Dmitrieva, 2006). Such reallocation of resources towards roots is a classic example of functional equilibrium, which we discussed in Section 4 (Brouwer, 1963; Lambers & Oliveira, 2019).

Thus, water deficit acts as a signal that reconfigures biomass allocation, directing resources from the shoot to the root. This enhances drought tolerance but at the cost of current productivity.

5.3. Water Regime and Mineral Nutrition

The relationship between water regime and mineral nutrition is two‑way and interdependent. On the one hand, the transport of many ions in the plant depends on the water flow in the xylem. On the other hand, mineral elements affect the osmotic potential of cells and, consequently, water uptake.

Ion Transport in the Xylem

The main flow of ions from roots to shoots occurs via the xylem, together with the transpiration stream. Ions are taken up by roots (actively, with ATP expenditure), enter the xylem, and are then carried with water to the shoots. The higher the transpiration rate, the faster this flow. When stomata close and transpiration decreases, ion transport to shoots also slows (Lambers & Oliveira, 2019; Taiz et al., 2023). This can lead to nutrient deficiencies (especially nitrogen, phosphorus, potassium, and calcium) in leaf tissues, further exacerbating stress.

Effect of Mineral Nutrition on Water Regime

Potassium (K⁺) plays a key role in water relations. K⁺ is the main osmoticum responsible for stomatal opening and maintenance of turgor in guard cells (Taiz et al., 2023). Under potassium deficiency, stomata become less sensitive to signals, and their regulatory capacity declines. In addition, potassium influences osmotic adjustment by participating in ion accumulation in vacuoles (Lambers & Oliveira, 2019).

Nitrogen is also important: it is a major component of photosynthetic enzymes (including Rubisco) and influences leaf growth. Nitrogen nutrition and water regime interact such that when both resources are supplied in a balanced manner, their use efficiency is maximised (co‑limitation, as discussed in Section 3) (Sadras & Calderini, 2015; Connor et al., 2011).

Aquaporins and Ion Transport

Aquaporins are not just water channels. Some of them, especially those of the PIP subfamily (plasma membrane aquaporins), are also involved in the transport of small neutral molecules such as CO₂, ammonia, urea, and hydrogen peroxide (Lambers & Oliveira, 2019). Under water stress, aquaporin expression and activity often change, affecting both water flow and the transport of metabolites and signalling molecules.

5.4. Water Regime and Phytohormones

Hormonal regulation is the mechanism that links the water regime to other physiological processes at the whole‑plant level. Water stress causes changes in the balance of major phytohormones, and it is this shift that triggers the cascade of adaptive responses.

Abscisic Acid (ABA) — the Major Stress Hormone

As already noted many times, ABA is synthesised in roots when soil dries and transported to shoots, where it causes stomatal closure, inhibition of cell expansion, and activation of stress genes (Passioura, 1994; Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016). But ABA also has other functions: it promotes osmotic adjustment, stimulates accumulation of protective proteins, and participates in senescence regulation. ABA also acts on roots: at moderate concentrations, it can stimulate root growth, enhancing their hydrotropic response (Lambers & Oliveira, 2019).

It is important to emphasise that ABA is not simply a “drought hormone.” It is part of a complex signalling network that integrates information about water status, light, temperature, and other factors. Its level in tissues is constantly changing, and it is the dynamics of ABA, not its absolute concentration, that determines many physiological responses (Taiz et al., 2023).

Cytokinins — Antagonists of ABA

Cytokinins are synthesised in roots (mainly in root tips) and transported via xylem to shoots. In contrast to ABA, cytokinins stimulate shoot growth, delay leaf senescence, and promote stomatal opening (Chapman & Huang, 2020; Lambers & Oliveira, 2019). Under water stress, cytokinin synthesis and transport decrease, shifting the hormonal balance towards ABA and enhancing the stress response. Restoration of cytokinin supply upon re‑watering promotes rapid growth recovery (Chapman & Huang, 2020). Thus, the ABA/cytokinin ratio is an important indicator of the plant’s physiological state.

Ethylene

Water stress is often accompanied by increased ethylene production, especially in roots. Ethylene, as we recall, inhibits shoot growth, promotes senescence, and can cause leaf abscission (Schopfer & Brennicke, 2016). In some cases, ethylene promotes aerenchyma formation in roots under flooding, but under drought its role is rather negative, as enhanced senescence accelerates leaf loss. However, in some species, ethylene may participate in adaptation by stimulating root growth under limited water supply (Lambers & Oliveira, 2019).

Gibberellins and Auxins

Gibberellins generally stimulate shoot growth, and their content decreases under stress. Auxins play a role in polar transport and growth regulation, but their involvement in drought responses is less studied. However, auxin and ABA are known to interact in regulating root growth under osmotic stress (Lambers & Oliveira, 2019).

Thus, the water regime is tightly linked to the hormonal network. Changes in water status lead to reorganisation of the hormonal balance, which in turn alters all physiological processes — from stomatal conductance to growth and senescence.

5.5. Water Regime and Stress Tolerance

Drought tolerance in plants is not a single property but a complex of protective mechanisms, many of which are activated precisely through the water regime and associated signals.

Osmotic Adjustment

As we have already mentioned, osmotic adjustment is the accumulation of compatible osmolytes (proline, glycine betaine, sugars, sugar alcohols) in the cytoplasm and vacuoles when water potential decreases (Chapman & Huang, 2020; Lambers & Oliveira, 2019). This allows turgor to be maintained at lower water potentials. Osmotic adjustment requires energy expenditure for osmolyte synthesis, but it enables continued growth and photosynthesis under moderate stress. Drought‑tolerant varieties often show more pronounced osmotic adjustment (Chapman & Huang, 2020).

Antioxidant Defence

Water stress causes excessive production of reactive oxygen species (ROS) in chloroplasts, mitochondria, and peroxisomes. ROS damage membrane lipids, proteins, and DNA (Schopfer & Brennicke, 2016; Chapman & Huang, 2020). To protect themselves, plants activate antioxidant systems: enzymatic (superoxide dismutase, catalase, peroxidases, ascorbate peroxidase) and non‑enzymatic (ascorbate, glutathione, tocopherols, carotenoids). Tolerant varieties often have higher antioxidant enzyme activity under stress (Chapman & Huang, 2020; Lambers & Oliveira, 2019).

Protective Proteins (LEA Proteins, Dehydrins, Chaperones)

Under water stress, genes encoding various protective proteins are activated, helping to maintain cell structure in a dehydrated state. These include:

LEA proteins (late embryogenesis abundant) — accumulate during seed maturation and in vegetative tissues under drought. They act as “molecular chaperones,” protecting other proteins from denaturation and aggregation (Lambers & Oliveira, 2019; Schopfer & Brennicke, 2016).
Dehydrins — a subgroup of LEA proteins particularly associated with water stress. They can bind to membranes and proteins, preventing damage during dehydration (Schopfer & Brennicke, 2016).
Chaperones (e.g., HSP70) — heat shock proteins that are also induced by drought and help refold damaged proteins (Schopfer & Brennicke, 2016).

Recovery after Stress

After stress is relieved (e.g., after rain), the plant must restore all impaired functions. This process involves:

Rehydration of tissues;
Stomatal opening (often involving cytokinins);
Reactivation of the photosynthetic apparatus (replacement of damaged proteins, chlorophyll synthesis);
Resumption of growth (by mobilising stored carbohydrates and nitrogen from old tissues) (Chapman & Huang, 2020; Kuznetsov and Dmitrieva, 2006).

The speed of recovery depends on the degree of damage, plant age, and the availability of reserves. In perennial grasses with well‑developed root systems and storage organs (tubers, rhizomes), recovery occurs faster, allowing them to withstand repeated droughts (Chapman & Huang, 2020).

5.6. Water Regime as an Integrator: From Cell to Agroecosystem

Thus, the water regime links all levels of plant organisation:

At the molecular level — through signalling molecules (ABA, ROS, Ca²⁺), gene activation, osmolyte synthesis, and protective proteins.
At the cellular level — through regulation of water potential, turgor, ion channel opening, and aquaporin function.
At the organ level — through redistribution of water and assimilates between roots, stems, and leaves.
At the whole‑plant level — through coordinated operation of the hydraulic system and hormonal network.
At the agroecosystem level — through interactions among plants for water and light, which determine the productivity and stability of the entire crop.

Understanding this integration is crucial for practice: when growing crops, we must view the water regime not as an isolated factor, but as a central element determining the efficiency of use of all other resources — light, carbon dioxide, nutrients. That is why the first module of our plant physiology course was devoted to the water regime: it serves as the foundation on which all other physiological processes are built. Without water, there is no photosynthesis, no growth, no yield.

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6. Lecture Summary

We conclude the first introductory lecture of the module on plant water relations. Over this time, we have travelled from the simple water balance equation to understanding the plant as a highly complex hydraulic and signalling system. Now it is time to assemble all the concepts we have considered into a single, logically structured picture.

6.1. A Unified Causal Chain: From Soil to Yield

All the material from our lecture can be represented as a sequential chain in which each link logically follows from the previous one and determines the next:

Soil → Root water uptake → Xylem transport → Leaf transpiration → Stomatal regulation → Water balance → Physiological processes → Productivity and tolerance

Let us examine this chain in more detail:

1. Soil — the source of water. Its water potential, structure, and water reserves determine how much water is available to the plant.

2. Root water uptake — the first active step. Depends on the root system (depth, density, hydraulic conductance), aquaporins, and the operation of ion pumps that create an osmotic gradient.

3. Xylem transport — passive movement of water through vessels driven by the transpirational gradient. The hydraulic architecture of the xylem (vessel diameter, pits) determines the efficiency and safety of this transport.

4. Leaf transpiration — evaporation of water through stomata. This is the “upper end engine” of the water stream, creating the main driving force.

5. Stomatal regulation — the main regulated valve, integrating signals about soil status (ABA from roots), atmosphere (humidity, light), and the plant itself (leaf water potential).

6. Water balance — the balance between water uptake and loss. Its disruption triggers a cascade of protective reactions.

7. Physiological processes — photosynthesis, growth, mineral nutrition, hormonal regulation — all are modulated by the plant’s water status.

8. Productivity and tolerance — the final outcome determining the plant’s success under given conditions.

This chain also has feedback: for example, productivity (leaf area size) affects transpiration, and tolerance (osmotic adjustment) affects the ability to take up water. But as a first approximation, we can consider this sequence as the basis for understanding the water regime.

6.2. Key Concepts to Remember

During the lecture, we introduced a number of fundamental concepts. Here is a brief list — a kind of “skeleton notes” for further study:

Concept	Brief Definition
Water balance	Balance between water uptake and loss; determines changes in tissue water content.
Water deficit	State in which tissue water content is below the maximum possible; a quantitative measure of water “shortage.”
Water stress	The set of physiological, biochemical, and molecular responses of the plant to water deficit.
Agronomic WUE	Ratio of yield (or biomass) to total evapotranspiration over the season; shows how much product is obtained per mm of water.
Physiological WUE	Ratio of photosynthetic rate to transpiration rate (A/E) at the leaf level; reflects instantaneous water use efficiency.
Hydraulic architecture	Spatial‑functional organisation of the plant’s water‑conducting system; includes conductance of individual segments, coordination, and safety margin.
Cavitation (embolism)	Rupture of the water column in a xylem vessel due to excessive negative pressure; leads to loss of conductance.
Osmotic adjustment	Accumulation of compatible osmolytes (proline, glycine betaine, sugars) to maintain turgor at low water potential.
ABA (abscisic acid)	The main stress hormone, synthesised in roots under soil drying, causing stomatal closure, growth inhibition, and activation of protective genes.

6.3. Main Conclusions of the Lecture

1. The water regime is a dynamic system. The plant is never in an absolutely stable state. Water balance continuously fluctuates during the day and season, and it is the ability to maintain this dynamic equilibrium that determines plant resilience.

2. Disruption of water balance triggers a cascade of protective responses. The first “target” is cell expansion, then stomata close, photosynthesis declines, and protective mechanisms are activated (osmotic adjustment, antioxidant defence, synthesis of stress proteins). All these changes occur long before visible wilting.

3. The plant responds to signals from roots, not directly to soil moisture. ABA is the main mediator transmitting information about soil drying from roots to leaves. This is an evolutionarily sound mechanism that allows preparation for drought in advance.

4. High WUE does not always mean high yield. There is a trade‑off between water saving and productivity. The agronomist’s and breeder’s task is to find the optimal balance for specific conditions, not to maximise WUE at any cost.

5. Hydraulic architecture determines drought tolerance. Plants with “safer” vessels (narrow, with small pits) suffer less from cavitation, but their conductance is lower. Plants with “efficient” vessels (wide) grow faster in wet conditions but suffer more under drought.

6. The water regime integrates all physiological processes. Changes in water status affect photosynthesis, growth, mineral nutrition, hormonal regulation, and tolerance. Therefore, the water regime is the foundation on which all other sections of plant physiology are built.

6.4. Practical Significance for Crop Production

The knowledge gained in this lecture has direct applied value:

Assessing water status. Understanding the sequence of stress development allows selection of optimal diagnostic methods (e.g., measuring leaf temperature, ABA or proline content) for early problem detection.
Irrigation management. Knowledge of critical thresholds of water deficit and sensitivity of different developmental stages helps design irrigation systems that prevent stress during the most critical periods (flowering, fruit set).
Variety selection. Understanding hydraulic architecture and stress strategies helps choose varieties best adapted to specific soil and climatic conditions.
Agronomic practices. Mulching, minimum tillage, optimal sowing dates and plant densities — all these practices aim to increase WUE by reducing evaporation losses and improving rainfall use.

6.5. Instead of a Conclusion: An Invitation to Further Study

We have completed the first module of our course — the foundation on which all subsequent topics will be built. Ahead of us are:

Plant respiration — a process closely linked to the water regime and the energy balance of the cell.
Mineral nutrition — another resource that interacts with water through osmotic and transport mechanisms.
Photosynthesis — the central process we have already mentioned several times in the context of stomatal regulation.
Growth and development — processes directly dependent on turgor and hormonal regulation.
Tolerance to adverse factors — extending the theme of stress to other abiotic (temperature, salinity, oxygen deficiency) and biotic factors.

Now that you understand how the water regime is organised and how it integrates all physiological processes, you can approach each new topic from a systemic perspective, seeing the connections between different sections of plant physiology.

References

Chapman, C., Huang, B. (2019). ‘Physiological, Biochemical and Molecular Mechanisms Regulating Post-Drought Stress Recovery in Grass Species’, in Handbook of Plant and Crop Stress, Fourth Edition. Fourth edition. | Boca Raton, FL : CRC Press, Taylor & Francis Group, 2019.: CRC Press, 41-49.
Connor, D.J., Loomis, R.S., Cassman, K.G. (2011). ‘Strategies and tactics for rainfed agriculture’, in Crop Ecology. Productivity and Management in Agricultural Systems. Cambridge, UK: Cambridge University Press, pp. 358-383.
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.
Lambers, H., Oliveira, R.S. (2019). ‘Growth and Allocation’, in Plant Physiological Ecology. Cham: Springer International Publishing, 385-449.
Lambers, H., Oliveira, R.S. (2019). ‘Plant Water Relations’, in Plant Physiological Ecology. Cham: Springer International Publishing, 187-263.
Passioura, J.B. (1994). ‘The Yield of Crops in Relation to Drought’, in Boote, K.J., Bennett, J.M., Sinclair, T.R., Paulsen, G.M. (ed.) Physiology and Determination of Crop Yield. Florida, USA: American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, pp. 343-360.
Savin, R., Slafer, G.A., Cossani, C.M., Abeledo, L.G., Sadras, V.O. (2015). ‘Cereal yield in Mediterranean-type environments: challenging the paradigms on terminal drought, the adaptability of barley vs wheat and the role of nitrogen fertilization’, in Crop Physiology. : Elsevier, 141-158.
Schopfer, P., Brennicke, A. (2010). ‘Stress und Stressresistenz’, in Pflanzenphysiologie. Berlin, Heidelberg: Springer Berlin Heidelberg, 583-616.
Taiz, L., Møller, I.Max., Murphy, A., Zeiger, E. (2023). ‘Water and Plant Cells’, in Plant Physiology and Development. New York, NY: Oxford University Press, pp. 153-168.
Кузнецов, В.В. (2006). ‘Водный обмен растений [Water exchange in plants]’, in Физиология растений [Physiology of plants]. Москва: Высшая школа, pp. 143-202.