Transpiration

Three stages of stomatal transpiration

The process of stomatal transpiration can be divided into three sequential stages (Tretyakov et al. 2000; Connor et al. 2011):

Stage one — evaporation of water from the surface of mesophyll cells into the intercellular spaces.

Water from the xylem enters the cell walls of mesophyll cells, where it evaporates into the air spaces of the leaf. Each mesophyll cell borders the intercellular space on at least one side. The surface area of all cell walls in contact with intercellular spaces is 10–30 times greater than the surface area of the leaf itself (Lambers & Oliveira 2019). This creates an enormous evaporating surface.

It is important to note that already at this stage, the plant is able to regulate transpiration. Reduction of water evaporation into the intercellular spaces is achieved through at least two mechanisms:

1. Changes in the water-holding capacity of the cytoplasm — increased osmotic and colloidal binding of water, its compartmentalization in organelles, and reduced membrane permeability.

2. Reduced hydration of cell walls — the so-called mechanism of incipient drying. When water supply from the root decreases and the water-holding capacity of mesophyll cell cytoplasm increases, the cell walls become less saturated with water. Water menisci in the capillaries between fibrils become concave, which increases surface tension forces and impedes the transition of water into vapour (Tretyakov et al. 2000).

This is an extra-stomatal way of regulating transpiration, which provides an obvious advantage to the plant: it allows reducing water expenditure without compromising CO₂ assimilation. In cotton, for example, this mode of regulation predominates. In plants of temperate zones, extra-stomatal regulation is less pronounced — transpiration can be reduced by 30% without stomata closing (Tretyakov et al. 2000).

Stage two — diffusion of water vapour through the intercellular spaces toward the stomatal pore.

Water vapour formed in the intercellular spaces diffuses down the concentration gradient toward the stomata — the exit points from the leaf. The path of vapour inside the leaf is not straight: the intercellular spaces form a complex branched network, and water molecules move through this network toward the stomatal chambers.

Stage three — exit of water vapour through the stomatal pore into the atmosphere.

This is the key regulated stage. The degree of stomatal opening is determined by the turgor of guard cells. At high turgor, stomata are open, and resistance to vapour flow is minimal. When guard cells lose turgor, they close, and resistance increases sharply (Taiz et al. 2023).

Why does up to 90% of water escape through just 1–3% of the leaf area?

This seems paradoxical: stomatal openings occupy only 1–3% of the entire leaf surface, yet through them, up to 90% of water evaporates. The explanation lies in the phenomenon of edge diffusion (Tretyakov et al. 2000; Schopfer & Brennicke 2016).

According to Stefan's law, evaporation from many small openings occurs faster than from one large opening of the same total area. This is because at the edges of openings, diffusion is enhanced — vapour molecules can spread sideways without colliding with each other. The smaller the opening, the larger the proportion of its perimeter (edge) to the total area. Therefore, many small stomata provide much higher total conductance than one large opening of the same area.

This phenomenon also has a reverse side: when stomata close partially (for example, by half), transpiration does not decrease as much as one might expect. Complete stomatal closure, on the contrary, reduces transpiration by about 90% (Taiz et al. 2023).

Stomatal transpiration — the main object of regulation

It is through the stomata that the plant can most rapidly influence water loss. Regulation is achieved by changing the osmotic potential of guard cells (this mechanism will be discussed in detail in the next lecture).

Therefore, in plant physiology, stomatal resistance (rₛ) is considered the main adjustable parameter determining the rate of transpiration. The maximum diffusive resistance of leaves (when stomata are closed) in deciduous trees is 30–45 times greater than the minimum (when stomata are open). In herbaceous dicots, this difference is 10–15 times, and in grasses, 5–15 times (Tretyakov et al. 2000).

Structure of the cuticle as a protective barrier

The cuticle is the outer protective covering of epidermal cells. It protects plants not only from water loss but also serves as a barrier against bacteria and fungi (Medvedev 2012). The cuticle consists of two main components:

Cutin — a polymer made of hydroxylated fatty acids (usually C₁₆ and C₁₈), linked by ester bonds into a strong three-dimensional network. Cutin provides the structural framework of the cuticle.

Waxes — not polymers, but a mixture of long-chain (25–35 carbon atoms) alkanes, fatty acids, alcohols, and their esters. Waxes are highly hydrophobic — they repel water and sharply reduce its permeability (Taiz et al. 2023).

The cuticle is a three-layered structure (Medvedev 2012):

• Surface layer — a thin layer of wax;
• Middle layer — "true cutin" — cutin embedded in wax;
• Lower layer — the cuticular layer, where cutin and waxes mix with cell wall components.

In some plants, wax crystals may form on the cuticle surface (e.g., in cabbage, apple), which additionally reduce wettability and enhance light reflection (Schopfer & Brennicke 2016).

Intensity of cuticular transpiration

In mature healthy leaves, cuticular transpiration typically accounts for 10–20% of total water loss. However, this value varies widely:

• In young leaves with a thin cuticle, cuticular transpiration can reach 50% of the total;
• In mature leaves with a thick cuticle — about 10% (Medvedev 2012);
• In senescing leaves, it increases again due to cuticle degradation and cracking;
• In xerophytes (plants of dry habitats), the cuticle is usually thicker than in mesophytes (Lambers & Oliveira 2019).

An important property of the cuticle is its ability to change hydraulic conductivity depending on hydration (Tretyakov et al. 2000):

• As the outer layers of epidermal walls dry out, the hydrophobic layers of the cuticle pack more tightly together — cuticular resistance can double.
• At low temperatures, resistance also increases.
• With increased hydration of the epidermis, the cuticle swells due to hydration of polar groups (carboxyl and hydroxyl), becomes looser — resistance decreases, and transpiration increases.

Thus, water loss through the cuticle is regulated by leaf hydration, although to a lesser extent than stomatal transpiration. At night, for example, with greater cuticle swelling, cuticular transpiration may be more intense than during the day. Wetted leaves can absorb water through the cuticle (a phenomenon we will discuss separately) (Schopfer & Brennicke 2016).

Physiological significance of cuticular transpiration

One might think cuticular transpiration is a "leak" that the plant should completely eliminate. Why then did evolution not create an absolutely impermeable cuticle?

There are several reasons:

1. Gas exchange through the cuticle is still possible — small amounts of O₂ and CO₂ can pass through the cuticle, which is especially important for tissues without stomata (e.g., fruits, young stems) (Schopfer & Brennicke 2016).

2. The cuticle must be elastic for leaf growth. A completely impermeable layer would be too rigid and would impede cell expansion.

3. Cuticular transpiration serves a signalling function — reduced epidermal hydration during cuticle drying can serve as an early signal of the onset of water stress, triggering abscisic acid synthesis and stomatal closure (Lambers & Oliveira 2019).

4. Cuticle wetting facilitates water absorption from dew and fog, which is particularly important for plants in arid regions.

Driving force of transpiration

The difference (cleaf - cair) is called the water vapour concentration gradient or the driving force of transpiration. It is this gradient that "pulls" water out of the leaf and into the atmosphere.

The water vapour concentration inside the leaf (c_leaf) is usually close to saturation (relative humidity 95–100%), since the intercellular spaces border the moist walls of mesophyll cells. This concentration is determined by leaf temperature: raising the temperature by 10 °C nearly doubles the saturated water vapour concentration (Hopkins & Hüner 2009).

The water vapour concentration in the atmosphere (cair) is determined by the relative humidity and temperature of the air. The drier the air (lower relative humidity), the lower cair and the greater the driving force.

How large is this driving force? The water potential of air at 20 °C and 50% relative humidity is about –93 MPa (Lambers & Oliveira 2019). This is hundreds of times greater than the water potential difference between soil and root! That is why even a small stomatal opening causes a powerful flow of water from the leaf.

Pathway conductance

The second component of the formula — conductance (g_w) — indicates how easily water vapour can leave the leaf. Conductance is determined by the sum of resistances along the path of a water molecule from the evaporating surface to the free atmosphere (Taiz et al. 2023).

These resistances act in series (like in an electrical circuit), so total resistance equals their sum:

And conductance is the reciprocal of resistance: gw = 1 / rtotal.

Stomatal resistance (rₛ)

Stomatal resistance is determined by the degree of stomatal opening. It is the main regulated component (Taiz et al. 2023).

Stomatal resistance depends on:

• Number of stomata per unit area (species-specific trait);
• Size of stomata and depth of the stomatal chamber;
• Degree of stomatal opening (dynamic parameter).

When stomata are fully open, resistance is minimal — water exits almost freely. When stomata are closed, resistance is maximal — 10–45 times higher than when open (Tretyakov et al. 2000).

Important: Stomata are not simply "holes" in the leaf. The stomatal pore is a narrow channel, and its resistance depends not only on width but also on depth (leaf thickness) and shape. In grasses, for example, stomata have a long slit-like shape, creating different resistance than the rounded stomata of dicots (Lambers & Oliveira 2019).

Boundary layer resistance (r_b)

Boundary layer resistance is caused by the still layer of air adjacent to the leaf surface (Hopkins & Hüner 2009). Water vapour molecules, having exited through the stomatal pore, must diffuse through this layer before reaching the turbulent flow of the atmosphere.

The thickness of the boundary layer depends on:

1. Leaf size — larger leaves have a thicker boundary layer than smaller leaves (due to the greater surface over which air has time to "slow down").

2. Wind speed — wind removes the boundary layer, reducing its thickness.

3. Presence of hairs (trichomes) on the leaf surface — hairs act as microscopic windbreaks, holding still air and increasing the boundary layer.

In light wind (up to 1–2 m/s):

• boundary layer is thick;
• boundary layer resistance is large;
• stomata have less influence on transpiration rate;
• even with widely open stomata, transpiration is limited by slow diffusion through the boundary layer (Taiz et al. 2023).

In strong wind (> 3–5 m/s):

• boundary layer becomes very thin;
• its resistance decreases;
• stomatal resistance becomes the main regulator;
• transpiration can increase greatly if stomata are open.

An interesting consequence: on a windless day, even wide-open stomata do not lead to maximum transpiration — the limiting factor is slow diffusion through the boundary layer. That is why ventilation in greenhouses and closed rooms can significantly increase transpiration and, accordingly, plant water consumption (Connor et al. 2011).

Internal (mesophyll) resistance

Internal resistance is associated with water vapour movement inside the leaf: from evaporation sites (mesophyll cell surfaces) through the intercellular system to the stomatal chamber (Schopfer & Brennicke 2016).

This resistance is mainly determined by leaf architecture:

• In leaves with loose spongy parenchyma (many intercellular spaces), internal resistance is lower;
• In dense, thick leaves (sclerophytes), intercellular spaces are fewer, and internal resistance is higher;
• In leaves with stomata located in pits (crypts), the internal vapour path is longer, and resistance is higher (Lambers & Oliveira 2019).

Internal resistance is usually not regulated by the plant rapidly — it is determined by leaf anatomy, which is formed during development.

Total resistance and its dynamics

All three resistances act in series:

On a sunny windy day: rb is small, rinternal is constant, r_s is regulated. Transpiration is mainly determined by stomatal resistance.

On a windless hot day: r_b is large, and even with open stomata, transpiration is limited by the boundary layer.

In closed rooms or greenhouses: r_b can be so large that stomata lose control over transpiration. This is one reason why forced ventilation is necessary in greenhouses (Connor et al. 2011).

Stomata rarely close completely

Even in the dark, in most plants, stomata retain a small residual opening (a "night slit"). Complete stomatal closure (infinite stomatal resistance) is a rare phenomenon, observed only under severe water stress or in some xerophytes (Taiz et al. 2023).

Reason: if stomata close completely, gas exchange stops, and CO₂ (a respiration product) may accumulate in the intercellular spaces, shifting pH and triggering nocturnal stomatal opening. Moreover, even at night, the plant needs a small influx of CO₂ to maintain metabolism (Lambers & Oliveira 2019).

Cuticular transpiration continues always

As we have already discussed, the cuticle is not an absolute barrier. As long as there is water in the leaf and a concentration difference persists in the atmosphere, evaporation through the cuticle will occur. This is a physical process that cannot be completely stopped (Schopfer & Brennicke 2016).

Cuticular transpiration at night can account for 5–15% of daily water loss (depending on cuticle thickness). In some cases (in plants with a thin cuticle), it may be even higher.

Night-time transpiration serves important functions

• Maintaining the upward flow — even at night, water must rise through the xylem to deliver mineral elements to growing organs and maintain the continuity of water strands (Connor et al. 2011).
• Cooling — in hot tropical regions, even at night temperatures can be high, and nocturnal transpiration helps the plant avoid overheating.
• Refilling xylem — at night, when transpiration is minimal and root pressure is active, water fills the xylem vessels, restoring water strands after daytime cavitation (Taiz et al. 2023).

Special case — CAM plants

In plants with Crassulacean Acid Metabolism (CAM), stomata are open specifically at night. These plants (cacti, Crassulaceae, agaves) accumulate CO₂ at night in the form of organic acids, and during the day, when stomata are closed, they use it for photosynthesis. Night-time transpiration in them is minimal because it is cooler at night and the water vapour concentration gradient is small (Lambers & Oliveira 2019). This allows CAM plants to have an exceptionally low transpiration coefficient (30–150 g water per 100 g biomass).

Water vapour concentration inside the leaf (c_leaf)

Inside the leaf, in the intercellular spaces, the air is almost always saturated with water vapour — relative humidity is 95–100% (Taiz et al. 2023). This is because mesophyll cells have a huge evaporating surface (10–30 times the leaf area) and continuously supply water to the intercellular spaces.

Thus, c_leaf is determined by leaf temperature. The relationship is exponential: a 10 °C increase in temperature approximately doubles the saturated water vapour pressure (Hopkins & Hüner 2009).

For example:

• At 20 °C, the saturated water vapour concentration is about 1.15 mol/m³ (or 2.34 kPa partial pressure);
• At 30 °C — already about 2.09 mol/m³ (4.24 kPa).

Therefore, even with unchanged air humidity, heating the leaf (for example, in sunlight) sharply increases the driving force for transpiration.

Important nuance: Leaf temperature is often higher than air temperature (especially on sunny days when transpiration is limited). This difference can reach 5–10 °C and significantly affect transpiration calculations (Connor et al. 2011).

Water vapour concentration in the atmosphere (c_air)

This value is determined by the relative humidity of the air and its temperature. The drier the air (lower relative humidity), the lower c_air and the greater the difference Δc.

The influence of relative humidity is illustrated in Table 2.1 from Hopkins & Hüner (2009):

Note: At 30 °C and 50% air humidity, c_air is about 2 times lower than at 20 °C and the same humidity. This means that in hot weather, the driving force for transpiration increases significantly — even if relative humidity remains unchanged.

Why is the driving force so large?

The water potential of air at 20 °C and 50% relative humidity is about –93 MPa (Lambers & Oliveira 2019). For comparison, the water potential of soil in the range of plant-available moisture is between –0.01 and –1.5 MPa. The difference between the atmosphere and the plant is hundreds of times larger than that between soil and root. That is why even a small stomatal opening causes a powerful flow of water from the leaf.

This explains why air humidity is the main environmental factor determining transpiration rate in the field.

Stomatal resistance (rₛ) — the main regulated barrier

Stomatal resistance is determined by the degree of stomatal opening. It is the main regulated component (Taiz et al. 2023; Tretyakov et al. 2000).

What rₛ depends on:

1. Number of stomata per unit area — species-specific trait determining maximum potential conductance.

2. Stomatal size and depth of stomatal chamber — longer and narrower channels have higher resistance.

3. Degree of stomatal opening — a dynamic parameter that the plant can change within minutes.

When stomata are fully open, resistance is minimal — water exits almost freely. When stomata are closed, resistance is maximal — 10–45 times higher than when open (Tretyakov et al. 2000).

Key idea: It is through changes in rₛ that the plant can rapidly regulate water loss. This is the basis of all stomatal regulation, which we will discuss in detail in the next lecture.

Boundary layer resistance (r_b) — the still-air barrier

Boundary layer resistance is caused by the layer of still air adjacent to the leaf surface (Hopkins & Hüner 2009). Water vapour molecules, having exited through the stomatal pore, must diffuse through this layer before reaching the turbulent flow of the atmosphere.

What r_b depends on:

1. Leaf size — larger leaves have a thicker boundary layer because air remains still near the surface for longer. Smaller leaves have a thinner layer.

2. Wind speed — wind "blows away" the boundary layer, reducing its thickness. At wind speeds of 5 m/s, the layer thickness can decrease 5–10 times compared to calm conditions.

3. Presence of hairs (trichomes) on the leaf surface — hairs act as microscopic windbreaks, holding still air and increasing the boundary layer. This is one of the mechanisms for reducing transpiration in xerophytes.

Practical implication: In calm weather, rb can be so large that even with widely open stomata, transpiration is limited. In greenhouses and closed rooms, ventilation significantly increases transpiration because it reduces rb (Connor et al. 2011).

Internal (mesophyll) resistance (r_internal)

Internal resistance is associated with water vapour movement inside the leaf: from evaporation sites (mesophyll cell surfaces) through the intercellular system to the stomatal chamber (Schopfer & Brennicke 2016).

What r_internal depends on:

• Leaf anatomy — in loose leaves with a large volume of intercellular spaces (e.g., many dicots), internal resistance is lower than in dense leaves (sclerophytes) with small intercellular volume.
• Leaf thickness — in thick leaves, the vapour path is longer, and resistance is higher.
• Stomatal location — in plants with stomata in pits (crypts), the internal path is lengthened, increasing resistance (Lambers & Oliveira 2019).

Important: Internal resistance is practically not regulated by the plant rapidly — it is determined by leaf anatomy, which is formed during development. However, in some plants under water stress, changes in intercellular structure (e.g., their compression) may slightly increase r_internal.

Interaction of resistances under different conditions

Since resistances act in series, total resistance equals their sum:

The contribution of different resistances changes depending on conditions:

On a sunny windy day:

• r_b is small (wind removes the boundary layer);
• r_internal is constant (leaf anatomy);
• r_s is the main regulated factor.
• Conclusion: Transpiration is mainly determined by stomatal resistance. The plant can effectively control water loss through stomata.

On a windless hot day:

• r_b is large (air is still);
• Even with stomata open (r_s small), transpiration is limited by slow diffusion through the boundary layer.
• Conclusion: Stomata lose part of their regulatory role. The plant cannot completely prevent water loss, even by closing stomata, because the main barrier — the boundary layer — still allows vapour through (albeit slowly).

In a greenhouse or closed room:

• r_b can be so large that the plant experiences a "greenhouse effect" — transpiration is slowed, leaves overheat.
• Conclusion: Forced ventilation is necessary to reduce r_b and normalize gas exchange (Connor et al. 2011).

In xerophytes with a thick cuticle and stomata in crypts:

• rs and rinternal are large even when stomata are open;
• This reduces the maximum transpiration rate, helping survival in arid conditions, but limits photosynthesis (Lambers & Oliveira 2019).

Stomata rarely close completely

Even in the dark, in most plants, stomata retain a small residual opening. This is because:

• Complete stomatal closure requires significant energy expenditure (active ion transport);
• With complete closure, CO₂ (a respiration product) may accumulate in the intercellular spaces, shifting pH and triggering nocturnal opening mechanisms;
• Some CO₂ influx is necessary even at night to maintain metabolism.

Therefore, r_s at night is large, but not infinite. Residual conductance is maintained, and through it, a small vapour flow occurs (Taiz et al. 2023).

Cuticular transpiration continues always

As we discussed, the cuticle is not an absolute barrier. Even when stomata are completely closed (r_s → ∞), cuticular resistance is finite. As long as there is water in the leaf and a concentration difference persists in the atmosphere, evaporation through the cuticle will occur (Schopfer & Brennicke 2016).

The magnitude of cuticular transpiration depends on cuticle thickness and condition:

• In young leaves — up to 50% of daytime transpiration;
• In mature leaves — 10–20%;
• In senescing leaves — increases due to damage.

Night-time transpiration can be physiologically significant

For some crops, night-time water loss can account for 5–10% of daily transpiration (Lambers & Oliveira 2019). This is not "wasteful loss" but an important physiological process:

1. Maintaining upward flow — even at night, it is necessary to deliver mineral elements to growing organs. Water rising through the xylem carries these elements.

2. Xylem refilling — at night, when transpiration is minimal and root pressure is active, water fills the xylem vessels, restoring water strands after daytime cavitation (Taiz et al. 2023).

3. Cooling — in hot tropical regions, even at night temperatures can be high, and nocturnal transpiration helps the plant avoid overheating.

4. Removal of excess mineral elements — some ions (e.g., sodium in halophytes) can be excreted through the transpiration stream.

Special case — CAM plants

In plants with Crassulacean Acid Metabolism, stomata are open specifically at night. They accumulate CO₂ at night in the form of organic acids, and during the day, with stomata closed, they use it for photosynthesis. Night-time transpiration in them is minimal because it is cooler at night and the water vapour concentration gradient is small (Lambers & Oliveira 2019).

Managing air humidity in greenhouses

In closed greenhouses, air humidity is often high (80–90%), which reduces Δc and limits transpiration. This can lead to:

• Leaf overheating;
• Reduced mineral element uptake (due to weak upward flow);
• Development of fungal diseases (high humidity).

Solution: Ventilation and air dehumidification to maintain optimal humidity (60–70%), where Δc is sufficient for normal transpiration but not excessively high (Connor et al. 2011).

Irrigation and drought protection

On dry windy days:

• Δc is large (dry air);
• r_b is small (wind removes the boundary layer);
• Transpiration can be very high, even if stomata are partially closed.
• Conclusion: On such days, irrigation should be more frequent and abundant to compensate for losses.

On windless hot days:

• r_b is large;
• Transpiration is limited by the boundary layer, even if stomata are open.
• Conclusion: Plants may suffer from overheating, even if soil water is sufficient. In such cases, fine spray irrigation (sprinkling) is useful for cooling leaves.

Breeding for drought tolerance

When developing drought-tolerant varieties, all components of the model are considered:

• Reduced stomatal conductance — fewer stomata or smaller size (but this also reduces photosynthesis);
• Increased cuticular resistance — breeding for thick cuticle and waxy bloom;
• Increased internal resistance — denser leaf anatomy (scleromorphy);
• Reduced boundary layer — small leaves or dissected leaf blades (Lambers & Oliveira 2019).

However, it is important to remember: too great a reduction in conductance limits photosynthesis. Therefore, breeders seek the optimal balance between water conservation and productivity.

Application of antitranspirants

Knowledge of resistances allows the choice of antitranspirant type:

• Metabolic (ABA, its analogues) — increase r_s (close stomata).
• Film-forming (polyacrylamide, wax emulsions) — increase rb and rcuticular (create an additional barrier on the leaf surface) (Tretyakov et al. 2000).

The choice depends on the situation: under short-term drought, metabolic antitranspirants are effective; during transplanting and transport, film-forming ones are better.

Absorption of solar radiation

The leaf is a thin, often dark-green plate, rich in pigments, primarily chlorophyll. Chlorophyll effectively absorbs light in the red and blue regions of the spectrum — those regions where solar radiation is maximal (Schopfer & Brennicke 2016).

What happens to the absorbed light energy? It has three possible fates:

1. Use in photosynthesis — light energy is stored in the chemical bonds of organic matter. But this is only a small fraction — under typical conditions about 5–10% of absorbed energy (Lambers & Oliveira 2019).

2. Re-emission — part of the energy can be re-emitted as fluorescence (this property of chlorophyll is used in photosynthesis research). But the re-emission fraction is small.

3. Conversion to heat — all remaining energy (about 80–90%) inevitably turns into thermal energy (Taiz et al. 2023).

Quantitative estimate of heat load

On a bright sunny day, about 600–1000 W/m² of solar radiation falls on a horizontal surface (depending on latitude, season, and atmospheric clarity). A horizontally oriented leaf absorbs a significant portion of this energy.

Consider a typical energy balance of a mesophytic leaf (Hopkins & Hüner 2009):

The positive radiation balance (+370 W/m²) means that the leaf constantly receives more energy than it emits as infrared radiation. This excess energy must be removed, otherwise leaf temperature will rise without limit.

Without cooling, the leaf would heat up to temperatures exceeding 50–60 °C within a few minutes. At such temperatures:

• Proteins denature (including Rubisco — the key enzyme of photosynthesis);
• Membranes lose integrity;
• Cells die.

The solution is evaporative cooling.

Physics of the process: latent heat of vaporization

Evaporation of water is an endothermic process. To convert 1 mol of water from liquid to vapour requires 44 kJ of energy (at 20 °C). This energy is called the latent heat of vaporization (Hopkins & Hüner 2009; Taiz et al. 2023).

Where does this energy come from? It is taken from the surrounding environment — in our case, from leaf tissues. When water molecules evaporate from mesophyll cell surfaces, they "carry away" this heat. The leaf cools.

Analogy: Evaporative cooling works just like sweating in humans or a wet towel on the head. As long as water evaporates, the surface cools.

Quantitative estimate of the cooling effect

Let us calculate how much heat a transpiring leaf can remove. Take a typical transpiration rate for a mesophyte — about 4 mmol water per 1 m² per second (this corresponds to a rate of about 1.5 mm/hour, which is typical for many crops on a sunny day) (Hopkins & Hüner 2009).

The latent heat of vaporization of water is 44 kJ/mol. Then the heat flux removed by transpiration is:

This is about half of the typical net radiation balance of the leaf (about 370 W/m²). The remaining heat is removed by other means:

• Convection — about 150–200 W/m² (heat transfer to moving air);
• Infrared radiation — already accounted for in the radiation balance as a negative component (Taiz et al. 2023).

Thus, transpiration and convection together remove excess heat. If transpiration decreases, the entire heat load falls on convection and radiation, which is often insufficient to maintain normal temperature.

Relationship between transpiration and leaf temperature

A 1 °C increase in leaf temperature increases the saturated water vapour concentration by about 5–6%. This, in turn, increases the driving force for transpiration (Δc). A positive feedback arises:

Leaf heating → increase in c_leaf → enhanced transpiration → enhanced evaporative cooling → temperature reduction.

It is through this feedback that the plant can maintain leaf temperature within relatively narrow limits, even under strong fluctuations in external conditions (Connor et al. 2011).

Scenario: heat and drought

Imagine a typical situation: a hot summer day, the soil begins to dry, roots cannot supply enough water. The plant closes its stomata — this is a protective response to preserve remaining moisture in tissues. But this decision has a side effect.

When stomata close:

• Stomatal transpiration drops sharply (practically to zero);
• Only cuticular transpiration remains (10–20% of maximum);
• Evaporative cooling is greatly weakened.

The heat that was previously used for evaporation now goes into heating the leaf. Leaf temperature can rise 5–10 °C above air temperature (Lambers & Oliveira 2019; Connor et al. 2011).

Consequences of overheating

An increase in leaf temperature above the optimum (which for C₃ plants is typically in the range of 20–30 °C) leads to:

1. Protein denaturation — above 45–50 °C, irreversible denaturation of enzymes, including Rubisco, begins. Photosynthesis ceases.

2. Increased photorespiration — in C₃ plants, rising temperatures enhance the oxygenase activity of Rubisco, increasing carbon losses (Lambers & Oliveira 2019).

3. Increased respiration — rising temperatures increase the rate of dark respiration (proportional to Q₁₀ ≈ 2). This increases carbohydrate consumption and reduces net productivity.

4. Membrane damage — at high temperatures, membrane lipid fluidity increases, their barrier function is disrupted, and cells lose ions and water.

5. Leaf scorch — at critical heating (above 50–55 °C), tissues die, and characteristic necrosis — "scorch marks" — appear.

Heat stress: examples from practice

In field conditions, heat stress is often observed in crops with inadequate water supply:

• Wheat in steppe regions during grain filling can experience leaf temperatures 5–8 °C above air temperature, reducing yield by 20–30%.
• Maize on a hot day with closed stomata can overheat to 40–42 °C, leading to pollen sterility and reduced grain set.
• Tomato in greenhouses without adequate ventilation and irrigation can suffer from fruit sunscald (fruit temperature in the sun can exceed 45 °C).

These are examples of how the conflict between water conservation and cooling manifests in agricultural practice.

Convection — passive cooling

Convection is the transfer of heat from the leaf to moving air. It depends on:

• The temperature difference between leaf and air (the greater the difference, the more intense the convection);
• Wind speed (wind enhances convection by blowing away heated air);
• Leaf size and shape (small leaves cool by convection more effectively).

In windy weather, convection can remove up to 50–70% of excess heat. However, on a windless hot day, convection efficiency drops sharply, and the main load falls on transpiration (Taiz et al. 2023).

Infrared radiation

The leaf, like any heated body, emits infrared (thermal) radiation. This radiation is more intense the higher the leaf temperature. On a clear night, when the sky is "cold" (effective sky temperature can be 20–30 °C below air temperature), infrared radiation can be the main cooling mechanism.

However, during the day, when the sun heats surrounding objects and the atmosphere, net infrared exchange is often negative — the leaf receives more infrared radiation than it emits (Hopkins & Hüner 2009). Therefore, during the day, infrared radiation cannot fully compensate for the heat load.

Morphological adaptations to reduce heating

Evolution has offered many ways to reduce leaf overheating without increasing transpiration (Lambers & Oliveira 2019):

• Small leaves — cool faster by convection due to a thinner boundary layer (smaller r_b).
• Dissected leaves (pinnately or palmately dissected) — increase effective surface area for convection.
• Vertical leaf orientation (e.g., in many grasses) — reduces direct sunlight exposure during midday hours.
• Light colouration or waxy bloom — reflect part of the solar radiation (increase albedo).
• Hairs (trichomes) — reflect light and create a microclimate at the leaf surface, reducing overheating (although they can simultaneously increase r_b, which reduces transpiration and cooling — a dual role).
• Leaf rolling (in grasses) or wilting/drooping (in some dicots) — reduce the irradiated area during the hottest part of the day.

These adaptations work together with transpiration, complementing it. In xerophytes (plants of dry habitats), such morphological adaptations are often more developed than in mesophytes, allowing them to survive with lower water losses (Medvedev 2012).

Arguments in favour of a useful function

1. Cooling — as we have shown, transpiration removes a significant portion of the heat load. In hot and calm conditions, it can be critically important for survival (Hopkins & Hüner 2009).

2. Maintaining upward flow — transpiration creates a "suction effect" that pulls water and mineral elements from the roots to the leaves. Without it, the flow would be weak and insufficient for tall plants (Taiz et al. 2023).

3. Hormone transport — phytohormones (e.g., cytokinins from roots, ABA from leaves) move in the transpiration stream, providing signalling communication between organs (Lambers & Oliveira 2019).

4. Removal of excess mineral elements — some ions (e.g., Na⁺ in halophytes) can be excreted from the plant with transpiration water (Schopfer & Brennicke 2016).

Arguments against

1. Enormous water losses — as we have seen, up to 99% of absorbed water is simply lost, bringing no direct benefit to the plant. This is a huge "price" for gas exchange.

2. Plants can grow without transpiration — under 100% relative humidity (e.g., in tissue culture or some greenhouses), plants can grow and develop, albeit more slowly (Hopkins & Hüner 2009). If transpiration were absolutely necessary for cooling, this would be impossible.

3. Convection can replace transpiration — at sufficient wind speed, convection can remove all excess heat without water evaporation. Example: plants in windy habitats.

4. Morphological adaptations — many desert plants with thick cuticles and few stomata survive precisely through morphological defences rather than high transpiration.

Modern view: a compromise function

Today, physiologists have reached a consensus: transpiration serves multiple functions simultaneously, and its role as a cooling system is not absolute but important.

Key idea: Transpiration is not a "necessary evil" nor a "specially designed cooling system". It is an inevitable consequence of stomatal function, which under certain conditions (especially on hot, windless days) acquires vital importance for cooling (Taiz et al. 2023).

The plant assesses the balance between water loss and overheating risk, and under different conditions chooses different strategies:

• When water is abundant — stomata are wide open, transpiration is maximal, the leaf is effectively cooled. The plant "pays" with water for maximum photosynthetic productivity.
• Under water deficit — stomata close, transpiration drops, but the leaf heats up. The plant chooses between water conservation and avoiding overheating. This is a complex decision made based on signals from roots and leaves (mechanisms will be covered in the next lecture).

Transpiration coefficient and yield

The transpiration coefficient (amount of water evaporated to produce 1 g of dry matter) is an integral indicator of water use efficiency (Connor et al. 2011).

In typical C₃ plants, it is 200–800 g water per 1 g dry matter. In C₄ plants — 200–350 g, in CAM plants — 30–150 g.

This means that:

• At a grain yield of 5 t/ha (dry mass about 4 t/ha), wheat can evaporate 800–3200 t of water per hectare per season (800–3200 mm of precipitation).
• In regions with annual precipitation below 400–500 mm, rainfed agriculture is often impossible.

Conclusion: Managing transpiration is managing yield. Water spent on transpiration is not a "loss" but an investment in biomass production (provided the plant is not experiencing water stress).

Effect of temperature on water use efficiency

With rising temperatures, the transpiration coefficient increases (Connor et al. 2011). This is because:

• At high temperatures, photosynthesis decreases (due to thermal stress on enzymes);
• The driving force for transpiration (Δc) increases exponentially.

As a result, the plant spends more water for the same unit of production. This is one reason why agriculture in hot regions is less efficient in terms of water use.

CAM plants

In plants with Crassulacean Acid Metabolism (CAM), stomata are open at night, when it is cooler and the water vapour concentration gradient is minimal. During the day, stomata are closed, and transpiration is practically absent.

How do these plants avoid overheating?

• Many CAM plants have morphological adaptations: thick cuticle, waxy bloom, light colouration, spines or hairs that reflect light.
• Night-time stomatal opening allows them to store CO₂ as organic acids, and during the day, use it with stomata closed. Photosynthesis proceeds without transpiration.
• Many CAM plants are succulents; their tissues contain a lot of water, which increases heat capacity and slows heating (Lambers & Oliveira 2019).

Thus, CAM plants solve the overheating problem not through transpiration but through morphological and physiological adaptations.

Plants under high humidity conditions

In tropical rainforests, where relative air humidity is close to 100%, transpiration is minimal — the driving force is practically absent. Nevertheless, plants there do not overheat because:

• High humidity and cloudiness reduce direct solar radiation.
• Convection is effective due to constant air movement.
• Leaves are often large, but they do not heat up strongly due to low insolation.

Under such conditions, transpiration loses its cooling value and becomes an obligatory "appendage" to gas exchange.

Aquatic and wetland plants

In hydrophytes (aquatic plants), stomata are often located only on the upper side of the leaf (or absent altogether). Transpiration in them is minimal, as the leaf is surrounded by water or moisture-saturated air. Overheating is not a threat — water removes heat more effectively than air (Taiz et al. 2023).

Conflict of two needs

The plant must simultaneously:

1. Cool itself — this requires high transpiration (open stomata).

2. Conserve water — this requires low transpiration (closed stomata).

This is a conflict that the plant cannot fully resolve. It can only optimize the balance between these two needs depending on current conditions (Connor et al. 2011).

Compromise mechanisms

How does the plant balance cooling and water conservation?

1. Hydroactive stomatal regulation — stomata close when leaf water potential decreases (ABA synthesis). This protects against dehydration.

2. Photoactive regulation — stomata open in light for photosynthesis. This ensures CO₂ influx and simultaneously cooling.

3. Morphological adaptations — leaf shape and orientation, cuticle thickness, waxy bloom — all reduce the need for transpiration for cooling.

4. Seasonal and diurnal changes — stomata may be more open in the morning and evening (when it is cool and humid) and more closed at midday (when it is hot and dry).

5. Root signals — when the soil dries, roots send signals (ABA) that cause stomatal closure before the leaf begins to lose water (Lambers & Oliveira 2019).

All of this we will discuss in detail in the next lecture, devoted to stomatal regulation.

Air humidity

This is the main environmental factor determining transpiration. The effect of air humidity follows directly from Dalton's law: the drier the air, the lower cair and the greater the difference cleaf - c_air (Hopkins & Hüner 2009).

Quantitative estimate: At 20 °C and 100% relative humidity (RH), the water potential of air is 0 MPa, and transpiration is practically absent. When RH drops to 50%, air water potential falls to –93 MPa, and the driving force becomes enormous. At RH = 30% (typical dry-wind situation), air water potential reaches –150…–200 MPa (Lambers & Oliveira 2019). This means that even with partially closed stomata, transpiration will be intense.

Practical significance:

• In arid regions (steppes, semi-deserts) with low air humidity, transpiration is very high, requiring either irrigation or the use of drought-tolerant varieties.
• In greenhouses, increasing air humidity (to 70–80%) can reduce transpiration and decrease irrigation needs, but simultaneously increases the risk of fungal diseases (Connor et al. 2011).

Important: Plants can respond to decreased air humidity by closing stomata even when soil moisture is adequate. This is a direct response to air dryness (hydropassive or signalling), which we will discuss in the next lecture.

Temperature

Temperature affects transpiration in two ways:

First, leaf temperature determines c_leaf. A 10 °C increase in temperature approximately doubles the saturated water vapour concentration (Hopkins & Hüner 2009). This sharply increases the driving force.

Second, air temperature affects relative humidity. When air is heated, its relative humidity decreases (if absolute water vapour content does not change), further reducing c_air.

Example: In the morning at 15 °C and 60% RH, the driving force is moderate. At noon at 30 °C (with the same absolute water vapour content), RH drops to about 25%, and the driving force increases several times. This explains why transpiration increases so sharply during the hot hours of the day (Taiz et al. 2023).

Feedback: Rising temperature enhances transpiration, and enhanced transpiration cools the leaf. This is a negative feedback loop that helps stabilize leaf temperature within a certain range. However, under water deficit, when stomata are closed, this feedback is disrupted, and the leaf can overheat (Schopfer & Brennicke 2016).

Practical significance:

• On hot days, irrigation should be more frequent to compensate for increased transpiration.
• High temperatures can lead to a "midday depression" of transpiration due to stomatal closure (see below).
• In regions with high temperatures, breeding is aimed at creating varieties with effective thermoregulation.

Wind

Wind affects transpiration through boundary layer resistance (r_b). Wind removes the still layer of air at the leaf surface, reducing its thickness and consequently lowering resistance (Connor et al. 2011).

Relationship between wind speed and r_b:

• In calm conditions (v = 0), the boundary layer is thick, r_b is large, and even with open stomata, transpiration is limited.
• At light wind (1–2 m/s), r_b decreases, and transpiration increases.
• At moderate wind (3–5 m/s), r_b becomes small, and stomatal resistance becomes the main regulator.
• At strong wind (> 5–8 m/s), a decrease in transpiration may be observed due to mechanical stomatal closure, tissue damage, and leaf cooling (which reduces c_leaf) (Tretyakov et al. 2000).

Leaf size effect: In large leaves, the boundary layer is thicker, so wind has a stronger effect on their transpiration than on that of small leaves (Lambers & Oliveira 2019).

Practical significance:

• In open fields, wind enhances transpiration, which must be considered in irrigation planning.
• In protected cultivation (greenhouses), ventilation is an important technique for managing transpiration and cooling.
• Windbreaks and other shelterbelts reduce wind speed in fields, decreasing transpiration and saving water (Connor et al. 2011).

Light

Light acts on transpiration indirectly, through several mechanisms:

1. Direct action on stomata — blue light activates the proton pump in guard cells, causing K⁺ influx and stomatal opening. This is photoactive opening (Taiz et al. 2023). We will discuss this mechanism in detail in the next lecture.

2. Heating the leaf — light energy heats the leaf, increasing c_leaf and hence the driving force.

3. Reduction of intercellular CO₂ — photosynthesis consumes CO₂; its intercellular concentration falls, which also stimulates stomatal opening (another regulatory mechanism).

4. Photosynthetic products — products of photosynthesis (sugars) can influence the osmotic potential of guard cells, promoting their opening (Lambers & Oliveira 2019).

Diurnal light course is the main driver of the diurnal transpiration course: at sunrise, stomata open, transpiration increases, reaches a maximum around midday, and decreases toward evening (Tretyakov et al. 2000). However, as we will see below, this course can be modified by water stress.

Practical significance:

• On cloudy days, transpiration is lower due to reduced light, and irrigation needs decrease.
• In shaded parts of the canopy (lower leaves), transpiration is lower than in illuminated upper leaves (tier differentiation phenomenon).
• In greenhouses, artificial lighting can increase transpiration, requiring adjustment of irrigation schedules.

Soil water availability

This factor determines whether the plant can maintain open stomata under high driving force (Connor et al. 2011). If soil water is insufficient, the root system cannot supply water to the leaves fast enough to compensate for transpirational losses.

Mechanism of linkage:

• As soil moisture decreases, soil water potential becomes more negative.
• Roots "sense" this (osmotic and mechanical signals) and synthesize abscisic acid (ABA).
• ABA is transported to the leaves and causes stomatal closure, even if the leaf has not yet lost turgor. This is hydroactive closure (Lambers & Oliveira 2019).
• If soil water is sufficient, stomata can remain open even under high driving force.

Critical values:

• At soil water potential above –0.05…–0.1 MPa (wet soil), water availability does not limit transpiration.
• At soil water potential –0.5…–1.0 MPa (moderate dryness), transpiration begins to be limited by stomatal closure.
• At soil water potential below –1.5 MPa (permanent wilting point), stomata are completely closed, and transpiration is determined only by cuticular conductance (Taiz et al. 2023).

Practical significance:

• Regular irrigation maintains high soil water potential, allowing plants to maximize light energy use and maintain high productivity.
• Delayed irrigation leads to stomatal closure and reduced photosynthesis, affecting yield.
• In arid regions, breeding is aimed at developing varieties with a lower stomatal closure threshold (i.e., able to keep stomata open at lower soil water potential).

Number, size, and distribution of stomata

These are species-specific traits, varying widely among taxa (Taiz et al. 2023).

Number of stomata (per 1 cm² of leaf):

• In herbaceous plants — from 1 000 to 100 000 (average 10 000–30 000).
• In woody plants — usually fewer (1 000–10 000).
• In xerophytes often fewer than in mesophytes (Lambers & Oliveira 2019).

Distribution:

• In most plants, stomata are located predominantly on the lower side of the leaf (hypostomatous leaves — apple, cherry, cabbage, tomato). This reduces water loss because the lower side is less heated and less exposed to wind.
• In some plants, stomata are present on both sides (amphistomatous — maize, oats, some dicots).
• In aquatic plants, stomata may be only on the upper side (epistomatous) or absent altogether (Taiz et al. 2023).

Stomatal size is also important: small stomata create greater resistance than large stomata of the same total area (due to edge effects). However, in most crop plants, stomata are relatively large to provide high conductance for photosynthesis.

Leaf area (LAI)

The total leaf surface determines total water loss. All else being equal, more leaves mean higher transpiration (Connor et al. 2011).

Indices:

• Leaf Area Index (LAI) — the ratio of leaf area to ground area. For field crops, LAI typically reaches 3–5 (in maize, wheat) and up to 7–8 in some vegetable crops.
• The higher the LAI, the greater the evaporating surface, but at LAI > 4–5, the increase in transpiration slows because lower leaves are shaded and have closed stomata (Lambers & Oliveira 2019).

Adaptation:

• In arid conditions, plants often have smaller leaf area (xeromorphy): small leaves, dissected leaf blades, rolled leaves (in grasses).
• This reduces transpiration but simultaneously reduces photosynthetic surface, limiting productivity.

Dynamics: Leaf area changes during the growing season: increases during vegetative growth, reaches a maximum at flowering, then decreases due to senescence and leaf fall (Tretyakov et al. 2000). Accordingly, total transpiration changes.

Cuticle thickness and composition

The cuticle is the main barrier to cuticular transpiration. The thicker it is and the more hydrophobic waxes it contains, the lower the cuticular conductance (Schopfer & Brennicke 2016).

Factors affecting cuticle thickness:

• Species identity — in xerophytes, the cuticle is usually thicker than in mesophytes and hygrophytes.
• Leaf age — young leaves have a thinner cuticle; old leaves have a thicker one, but may crack late in the season.
• Growing conditions — plants grown under high light and dry air often have a thicker cuticle (adaptation).

Cuticular resistance in mature leaves can be 100–1000 times higher than stomatal resistance (when stomata are open). Therefore, cuticular transpiration is negligible while stomata are open. But once stomata close, it is the cuticle that determines the minimum level of loss (Taiz et al. 2023).

Practical significance:

• Breeding for increased cuticle thickness — one route to improving drought tolerance.
• Some antitranspirants (film-forming type) artificially increase cuticular resistance by applying waxy or polymer coatings to the leaf (Tretyakov et al. 2000).

Hydraulic state of the plant

This is a complex indicator reflecting the ability of the root system to supply water and the conducting system to transport it without disruption (Connor et al. 2011).

Components of hydraulic state:

1. Root conductance — the ability of the root system to absorb water from the soil. Depends on:

• Volume and density of roots (especially active absorbing zones);
• Condition of root hairs and mycorrhiza;
• Soil temperature (cold soil reduces conductance);
• Soil aeration (flooding causes hypoxia and reduces conductance) (Lambers & Oliveira 2019).

2. Xylem condition — presence of cavitation (air bubbles) in vessels, disrupting the continuity of water strands. Cavitation occurs under strong negative pressures (e.g., during intense transpiration in dry weather). It can be partially reversible (night-time recovery) or irreversible (Taiz et al. 2023).

3. Tissue water potential — in the morning after night-time recovery, it is high (close to soil potential); by midday, it decreases due to transpiration. When critical values are reached (about –1.5 MPa), stomata close.

4. Osmotic regulation — accumulation of osmotically active substances in cells allows them to retain water at lower external water potential. This is an important drought tolerance mechanism (Lambers & Oliveira 2019).

Practical significance:

• Disruption of hydraulic state (e.g., due to root damage during tillage) immediately affects transpiration and photosynthesis.
• Maintaining optimal soil moisture and aeration is the basis for stable hydraulic system function.
• Breeding for deep root systems and cavitation resistance are important directions for improving drought tolerance.

Typical diurnal course

With adequate water supply and moderate weather, the diurnal course of transpiration looks like this (Fig. 80 in Tretyakov et al. 2000):

1. Dawn – 7–8 a.m.: With the appearance of light, stomata begin to open (photoactive movement). Temperature and air humidity gradually rise. Transpiration increases.

2. 8–11 a.m.: Stomata are fully open. Light and temperature rise, humidity decreases. Transpiration reaches high values.

3. Noon – 1–2 p.m.: Transpiration is maximal. If water supply is adequate, there may be a single peak (at 12–1 p.m.). If water is insufficient, a midday depression may occur — stomata partially close, transpiration decreases despite high light and temperature.

4. 2–4 p.m.: Gradual decrease in light and temperature. Stomata begin to close. Transpiration falls.

5. Evening and night: Stomata are closed; transpiration is minimal (cuticular and residual stomatal).

Midday depression: causes and significance

Midday depression is a temporary decrease in transpiration during the hottest hours of the day (Lambers & Oliveira 2019). It arises from a combination of several factors:

1. Water deficit — intense transpiration in the morning may cause roots to fail to supply water, and leaf water potential drops to the stomatal closure threshold.

2. High temperature — under strong heating, stomata may close due to thermal stress (even if water is sufficient).

3. High CO₂ content — at midday, photosynthesis is most intense, and intercellular CO₂ concentration may decrease (paradox: photosynthesis consumes CO₂; its concentration falls, causing stomatal closure — see next lecture).

4. Direct effect of air dryness — at very low humidity, stomata may close in response to high vapour pressure deficit (Taiz et al. 2023).

Significance of midday depression:

• It is a protective mechanism preventing cavitation (rupture of water strands) under excessively high negative pressures in the xylem.
• It allows the plant to "wait out" the hottest hours, reducing water loss and risk of damage.
• However, midday depression reduces photosynthesis and productivity; therefore, breeding aims to develop varieties with minimal depression (i.e., with more efficient root systems and stomatal regulation).

Effect of water stress on diurnal course

Under soil water deficit, the diurnal course changes (Tretyakov et al. 2000):

• Mild stress: midday depression becomes more pronounced; stomata close earlier and open later.
• Moderate stress: stomata may remain closed for a significant part of the day; transpiration is greatly reduced; photosynthesis is limited.
• Severe stress (drought): stomata are closed almost all day; only cuticular transpiration occurs. The plant overheats; leaves wilt.

These changes reflect the survival strategy: under water deficit, the plant sacrifices photosynthesis to conserve water and avoid death. This is one of the key compromises we discussed earlier.

Rapid weighing method (by L. A. Ivanov)

This is a classical field method developed by the outstanding Russian plant physiologist Leonid Aleksandrovich Ivanov in the early 20th century. Despite its simplicity, it is still used for educational purposes and for rapid field assessments (Tretyakov et al. 2000; Connor et al. 2011).

Method principle:

1. A leaf (or small shoot) is cut from the plant and immediately placed in a sealed container.

2. First weighing is performed (within the first 10–15 seconds after cutting).

3. The leaf is kept in natural conditions for 3–5 minutes (or another predetermined interval).

4. Second weighing is performed.

5. From the mass difference and time, the transpiration rate is calculated (mg water/(cm²·h) or g/(m²·h)).

Crucial nuance: the transpiration surge

Immediately after cutting, a "transpiration surge" occurs — a sharp, short-term increase in evaporation rate. This is because cutting the xylem vessels breaks the continuity of tensioned water strands: water that was under negative pressure is "drawn" into the tissues, and the leaf seems to "exhale" water (Taiz et al. 2023).

Therefore, one cannot determine transpiration during the first seconds after cutting. One must allow the process to "settle" — to reach a short-term plateau where the evaporation rate becomes stable and reflects the true transpiration of the intact leaf.

How to determine the appropriate interval:

• For each new object (species, age, conditions), a series of weighings is performed at 1‑minute intervals over 10 minutes.
• A graph is plotted: rate of mass loss vs. time.
• The point is determined where artefacts (the surge) end and the process reaches a short-term plateau — this is the working interval for that object.
• Typically, for most mesophytes, the working interval is 3–5 minutes after cutting (Tretyakov et al. 2000).

Improvements:

• To reduce the transpiration surge, cutting can be done under molten paraffin — it solidifies, sealing the vessels and preventing air entry.
• For large leaves, discs (punches of known area) are used — allowing work with many replicates.
• To measure transpiration of different canopy parts (upper, middle, lower leaves), corresponding leaf layers are used.

Advantages of the method:

• Simplicity and availability (only scales and a stopwatch are needed).
• Can be used in field conditions.
• Allows comparison of transpiration among different species, varieties, organs.

Disadvantages:

• Traumatic — the leaf is destroyed; measurements cannot be repeated on the same object.
• Artefacts — transpiration surge, subsequent wilting (water loss leads to gradual decrease in transpiration).
• Subjectivity and dependence on experimenter skill.
• Impossible to measure transpiration dynamics (through the day, season).

Lysimetry — the reference method

Lysimeters are special containers (usually large metal or plastic vessels) in which plants are grown and which are placed on precision electronic balances (Connor et al. 2011; Tretyakov et al. 2000).

Operating principle:

1. The plant is grown in a lysimeter filled with soil.

2. The balance continuously (or discretely, e.g., every 5–10 minutes) records the mass of the system "container + soil + plant".

3. The change in mass over time is the total water loss: transpiration (via the plant) + physical evaporation from the soil surface.

4. If the soil surface is mulched or covered with film, the mass change gives pure transpiration.

5. Water input (irrigation, rain) is accounted for separately.

Accuracy: Modern automatic lysimeters have an accuracy of 0.1 mm water column (equivalent to 0.1 kg water per 1 m² area). This allows recording transpiration over very short intervals (15–30 minutes) and constructing detailed diurnal curves (Taiz et al. 2023).

Advantages:

• Absolute values — no calibration or conversion required; direct data on evaporated water.
• Long-term monitoring — measurements can be taken throughout the season.
• Natural conditions — the plant grows in soil (not in artificial media), giving realistic data.
• Link to yield — transpiration and biomass accumulation can be measured simultaneously.

Disadvantages:

• Expensive and complex equipment (especially for large plants, e.g., trees).
• Limited number of plants — usually 2–4 lysimeters per experimental plot.
• Artefacts from soil structure disturbance — soil in the lysimeter may differ from natural (compaction, capillary disruption).
• Edge effects — plants at the edges of the lysimeter may experience different conditions than those in the centre.

Application in agriculture:

• Determining evapotranspiration of crops (total water consumption over the season).
• Developing irrigation schedules — how much water and when to apply.
• Calibrating calculation models of water consumption (e.g., for irrigation forecasting).

Porometry — measuring stomatal conductance

Porometers are portable instruments that measure stomatal conductance (gs) or resistance (rs) directly on an intact leaf (Taiz et al. 2023; Lambers & Oliveira 2019).

Operating principle:

1. A small measurement chamber (sensor head) is placed on the leaf surface, sealing tightly against it.

2. Air of known humidity (e.g., dried to 0% RH) is pumped through the chamber.

3. The rate of humidity change in the chamber is measured — it is determined by the water vapour flux through the stomata into the closed chamber volume.

4. From these data, stomatal resistance (or conductance) is calculated in units of mol/(m²·s) or mmol/(m²·s).

Modern porometers (e.g., LI-6400, LI-6800 from LI‑COR, USA):

• Measure simultaneously stomatal conductance and photosynthesis (using a built-in IR gas analyser).
• Allow control of chamber conditions: light, temperature, CO₂ concentration, humidity.
• Enable repeated measurements on the same leaf throughout the day.

Advantages of porometry:

• Rapid — measurement takes 1–3 minutes.
• Non-destructive — the leaf remains on the plant; measurements can be repeated.
• High sensitivity — allows detection of even small changes in stomatal conductance.

Disadvantages:

• Measures stomatal conductance, not total transpiration (to calculate transpiration, the driving force Δc is also needed).
• Sensitive to chamber conditions — if they differ from natural, the result may be biased.
• Requires calibration and careful handling (especially with small leaves).

Practical applications:

• Assessment of water stress — decrease in g_s is an early indicator of drought onset.
• Breeding work — comparing stomatal conductance of different varieties and hybrids.
• Studying diurnal dynamics — how stomata change during the day.
• Environmental monitoring — effect of pollutants, drought, elevated CO₂ on plant gas exchange.

Infrared gas analyser system (IRGA)

Infrared gas analysers (IRGA) are instruments that measure gas concentrations (CO₂ and H₂O) in an air stream passing through a chamber containing a leaf (Lambers & Oliveira 2019; Taiz et al. 2023).

Operating principle:

1. The leaf is placed in a transparent chamber with controlled environment (light, temperature, humidity, CO₂).

2. At the inlet of the chamber, H₂O and CO₂ concentrations are measured.

3. At the outlet of the chamber, H₂O and CO₂ concentrations are measured.

4. The concentration difference (outlet – inlet) indicates how much CO₂ has been absorbed (photosynthesis) and how much H₂O has been released (transpiration).

5. Knowing the flow rate of air through the chamber and leaf area, transpiration and photosynthesis rates are calculated in absolute units.

Modern IRGA systems (LI-6800, GFS-3000):

• Allow simultaneous measurement of transpiration, photosynthesis, stomatal conductance, intercellular CO₂, and other parameters.
• Operate both in laboratory and field conditions.
• Have automatic regulation of chamber conditions (temperature, humidity, light, CO₂).

Advantages of IRGA:

• Simultaneous measurement of transpiration and photosynthesis — ideal for studying the relationship between these processes.
• High accuracy — concentration measurements with precision of 0.1 ppm (for CO₂) and 0.01 mmol/mol (for H₂O).
• Ability to control conditions — allows studying plant responses to different factors (temperature, humidity, light, CO₂).

Disadvantages:

• Expensive — high-precision equipment costs tens of thousands of dollars.
• Complex operation — requires special training and care.
• Limited area — one leaf is measured, not the whole plant or crop.
• Sensitivity to edge effects — the leaf edge in the chamber may produce artefacts.

Research applications:

• Gas exchange physiology — studying photosynthesis and transpiration under controlled conditions.
• Stress physiology — plant responses to drought, salinity, high temperature, elevated CO₂.
• Breeding — searching for genotypes with high water-use efficiency.
• Modelling — obtaining parameters for crop production models.

Physical principle: temperature as a stress indicator

Leaf temperature is determined by the energy balance (Lambers & Oliveira 2019):

• Energy input: absorbed solar radiation + infrared radiation from the atmosphere.
• Energy output: infrared emission from the leaf + convection + transpiration (evaporative cooling).

If the plant is well watered, transpiration is intense, and the leaf cools — its temperature can be lower than air temperature (by 1–3 °C on a sunny day).

If the plant is water-deficient, stomata close, transpiration decreases, and the leaf heats up — its temperature can be higher than air temperature (by 3–10 °C, depending on conditions).

Thus, the difference between leaf temperature and air temperature serves as an indicator of water stress. The warmer the leaf relative to air, the stronger the stress (Connor et al. 2011).

Tools for measuring leaf temperature

1. Portable infrared thermometers (pyrometers)

• Pointed at the leaf (or canopy), they instantly measure its temperature.
• Simple to use, inexpensive.
• Provide point measurements, not suitable for large areas.

2. Thermal imagers (infrared cameras)

• Create a thermographic image (thermal map) of the field.
• Allow visualisation of spatial distribution of leaf temperatures.
• Can be mounted on drones, aircraft, satellites.

3. Satellite thermal sensors (e.g., Landsat, MODIS)

• Enable monitoring of water stress at regional and continental scales.
• Used for drought forecasting and assessment of agricultural land condition.

Water stress indices based on temperature

Based on leaf (or canopy) temperature, various water stress indices have been developed (Taiz et al. 2023):

1. Crop Water Stress Index (CWSI)

Where:

• T_canopy — canopy temperature;
• T_air — air temperature;
• The index ranges from 0 (no stress, full cooling) to 1 (maximum stress, stomata closed).

2. Temperature differential (Tcanopy - Tair)

A simple indicator: the larger the difference, the stronger the stress. For well-irrigated crops, the differential is usually negative (leaf cooler than air). Under water deficit — positive.

3. Thermal Time Index (TTI)

Based on the integral of leaf temperature over a given period. Used to assess cumulative stress effects.

Applications in precision agriculture

Thermography is one of the key tools of precision agriculture (Connor et al. 2011; Tretyakov et al. 2000):

1. Variable-rate irrigation: Drones with thermal cameras enable real-time identification of field zones under water stress. Automated irrigation systems can apply water only to these zones, saving water and increasing yield.

2. Crop monitoring: Regular thermographic surveys allow tracking of water stress dynamics throughout the season and taking measures (irrigation, antitranspirant application) before visible wilting symptoms appear.

3. Breeding work: Thermography allows rapid assessment of drought tolerance of a large number of varieties and hybrids in field conditions — by their ability to maintain low leaf temperature under water deficit.

4. Yield forecasting: Canopy temperature data correlate with yield and can be used to build predictive models.

5. Irrigation efficiency assessment: Comparing temperature maps before and after irrigation allows evaluation of water distribution effectiveness.

Determining irrigation timing

The classical approach is to irrigate when the plant is already showing water stress (visible signs: darkening leaves, loss of turgor, wilting). But this is a delayed strategy — by the time visible symptoms appear, part of the yield has already been lost.

Physiological approach: Using porometers, IRGA, or thermometers to detect early signs of stress:

• Decrease in stomatal conductance (g_s) by 20–30% from maximum.
• Leaf temperature increase of 1–2 °C relative to a non-stressed reference.
• Increase in CWSI above 0.2–0.3.

In this case, irrigation is applied before visible symptoms appear, minimising yield losses.

Estimating water consumption and irrigation planning

Based on lysimetric and calculated data, the total water consumption (evapotranspiration) of the crop over the season is determined (Tretyakov et al. 2000). Then the irrigation rate is calculated:

Knowledge of daily transpiration dynamics (from lysimeters or calculation models) allows planning frequency and amounts of irrigation with consideration of weather forecasts.

Assessing water use efficiency

The transpiration coefficient (or its reciprocal — water use efficiency, WUE) is an important indicator for breeding and evaluating agronomic practices.

The higher the WUE, the more efficiently the plant or crop uses water. Breeders aim to develop varieties with high WUE, especially for arid regions.

Integration with digital technologies

Modern precision agriculture systems integrate transpiration data (from lysimeters, porometers, thermography) with:

• Weather data (forecasts of temperature, rainfall, humidity, wind);
• Soil data (moisture, temperature, electrical conductivity);
• Satellite imagery (vegetation indices, NDVI, thermal maps);
• Crop production models (e.g., DSSAT, APSIM).

This enables optimal decisions on water management: when to irrigate, how much water to apply, which zones, and with what frequency.

Transpiration is inevitable

The plant cannot exist on land without gas exchange with the atmosphere. For photosynthesis, it needs carbon dioxide (CO₂), which enters through stomata — microscopic pores in the leaf epidermis. But through the same stomata, water vapour diffuses outward because the air inside the leaf is saturated with moisture, while outside it is usually dry (Taiz et al. 2023).

This fundamental contradiction is the inevitable price of life on land. The plant cannot have a surface that allows CO₂ to pass through but completely blocks water vapour. Therefore, transpiration is not a "design flaw" but an evolutionary compromise between the need for carbon nutrition and the risk of dehydration (Connor et al. 2011).

Two pathways of water loss

Water leaves the plant by two main pathways (Hopkins & Hüner 2009; Schopfer & Brennicke 2016):

• Stomatal transpiration (80–95% of losses) — the main regulated pathway. It is through stomata that the plant can rapidly change water loss rate by opening or closing them.

• Cuticular transpiration (10–20% of losses) — passive evaporation through the waxy cuticle layer. This pathway is not operationally regulated, but its intensity depends on cuticle thickness and condition.

Cuticular transpiration continues even when stomata are fully closed, so the plant can never reduce water loss to zero.

Dalton's law — the basis for understanding transpiration rate

Transpiration rate is described by a simple but powerful equation (Connor et al. 2011; Taiz et al. 2023):

This means that the rate is determined by the product of two independent factors:

1. Driving force (Δc) — the difference in water vapour concentration between leaf and atmosphere. It is determined by leaf temperature (warmer means higher cleaf) and air humidity (drier means lower cair).

2. Pathway conductance (g_w) — how easily vapour can leave the leaf. Conductance is determined by the sum of resistances: stomatal (regulated), boundary layer (depends on wind and leaf size), and internal (anatomical).

Key conclusion: transpiration can be high only when both driving force and conductance are high. If either factor is low, transpiration is limited. This rule explains all diurnal and seasonal changes in water regime.

Transpiration — a cooling system

Why does the plant "agree" to such huge water losses? Because water evaporation removes excess heat (Lambers & Oliveira 2019; Hopkins & Hüner 2009).

• The leaf absorbs more solar energy than it can use in photosynthesis. The excess is converted to heat.
• Evaporation of 1 mol of water requires 44 kJ of energy. Transpiration can remove up to 50% of the leaf's heat load (about 176 W/m² out of a typical 370 W/m²).
• When stomata close, transpiration drops, the leaf heats up by 5–10 °C, which can lead to protein denaturation, increased respiration, scorching, and tissue death.

Thus, transpiration acts as a natural "air conditioner", without which the plant could not exist in sunny habitats.

Multiple factors govern transpiration

Transpiration rate results from the interaction of external (environmental) and internal (physiological and morphological) factors (Tretyakov et al. 2000; Medvedev 2012):

Diurnal transpiration course — the integrated result of these factors: morning rise, midday maximum, possible depression under water stress, evening decline. Seasonal dynamics correspond to leaf area increase and changes in weather conditions.

Measuring transpiration — from classical to digital technologies

Methods for measuring transpiration vary in complexity and accuracy (Tretyakov et al. 2000; Connor et al. 2011):

• Classical: rapid weighing (Ivanov method) — simple but traumatic; lysimetry — the reference method giving absolute total transpiration values.
• Modern: porometry (stomatal conductance) and IRGA (simultaneous transpiration and photosynthesis measurement) — accurate, non‑destructive, but expensive.
• Remote sensing: thermography (leaf/canopy temperature measurement) — allows monitoring water stress over large areas using thermal cameras, drones, and satellites.

Integration with precision agriculture: transpiration data are used for variable-rate irrigation, drought tolerance screening, water consumption calculation, and yield forecasting.