The plant as an integral hydraulic-metabolic system

1.1. The Concept of Water Transport Continuity

Imagine that water in a plant is not just a liquid passively rising through vessels. It is part of a vast continuous system that permeates the entire plant from the tips of root hairs to the stomatal pores in the leaves. This system is called the Soil-Plant-Atmosphere Continuum (SPAC).

What is the essence of this concept, which you already know from your botany course? Water moves along a gradient of water potential — from locations with high (less negative) water potential to locations with low (more negative). Along this path, water sequentially passes through:

• the soil (where water potential typically ranges from 0 to –0.1 MPa),
• root hairs and root tissues,
• xylem vessels,
• leaf tissues,
• and, finally, evaporates into the atmosphere, where water potential can reach –14 MPa even at 90% relative humidity (Schopfer & Brennicke, 2016).

This movement is similar to water flowing through a pipe under the action of a pressure difference. But there is an important distinction: the plant is not just a passive pipe. It actively regulates resistance at each point along this path.

1.2. The Hydraulic Circuit: An Analogy with an Electrical Circuit

To understand the integration of processes, it is convenient to use an analogy with an electrical circuit (Schopfer & Brennicke, 2016). In this analogy:

• Water potential (ψ) is analogous to electrical voltage.
• Water flow is analogous to electrical current.
• Hydraulic resistance is analogous to electrical resistance.

The total water flow through the plant can be described by a simple equation:

Here, the total resistance is the sum of the series resistances: soil, root, stem, leaf, and stomatal apparatus (Brodribb et al., 2015).

The key idea of integration: any change in resistance at any point in the circuit alters the water flow through the entire system (Sinclair, 1994). Imagine you turn off a tap in the middle of a water pipe — the pressure drops throughout the entire system, not just at the point of closure.

1.3. Typical Scenarios: How Changes in One Part of the System Affect the Whole

Let's consider a few typical situations to see how integration works in practice.

1.4. Why the Hydraulic Circuit is Key to Understanding Integration

So, we have seen that any change at any point in the "soil-plant-atmosphere" system leads to changes throughout the entire system. This is the manifestation of the integration of physiological processes at the most fundamental level.

Crucial conclusion: we cannot consider water relations, photosynthesis, growth, and mineral nutrition as separate, independent processes. They are all linked through a unified hydraulic system. This is why, for example, when there is a lack of water in the soil, we observe not only wilting but also leaf yellowing (disruption of mineral nutrition), slowed growth, and reduced yield.

1.5. A Few Words on the Driving Forces of Water Flow

In the "soil-plant-atmosphere" system, two main drivers operate (Кузнецов & Дмитриева, 2006):

The lower-end driver — root pressure. Root cells actively absorb ions from the soil, creating a more negative osmotic potential in root cells compared to the soil solution. Water enters the root osmotically. This mechanism creates positive pressure in xylem vessels, which can reach 0.1–0.15 MPa. Root pressure is particularly important at night and under high humidity conditions when transpiration is minimal.

The upper-end driver — the transpiration pump. Evaporation of water from leaves creates negative pressure (tension) in xylem vessels, which "pulls" water from the roots. This mechanism provides the main water flow during the daytime.

The two drivers work in concert, complementing each other (Sinclair, 1994). When transpiration is strong, the upper driver works. When it is weak, root pressure comes to the fore. This coordination is another example of integration.

1.6. What Have We Learned from Examining the SPAC?

From this section, we have derived the main principle of integration of physiological processes:

The Principle of Continuity: Water, moving along the water potential gradient from the soil through the plant to the atmosphere, links all parts of the plant into a single system. Any change in hydraulic resistance at any point along this path immediately affects the operation of the entire system — from water and ion uptake by roots to photosynthesis in the leaves.

We will use this principle as a basis for understanding all subsequent topics. In the next section, we will see how this hydraulic network connects to carbon exchange through the stomata — the plant's main physiological switch.

2.1. The Stoma: Not Just a Hole, but a Regulatory Center

A stoma is a microscopic structure in the leaf epidermis, consisting of two guard cells and the stomatal pore between them. But its physiological significance extends far beyond being a simple pore. In essence, the stoma serves as the plant's main physiological switch, because it simultaneously regulates several critical processes (Taiz et al., 2023):

1. The uptake of CO₂ from the atmosphere into the leaf for photosynthesis.

2. The loss of water as water vapor (transpiration).

3. Leaf temperature through evaporative cooling.

4. The overall rate of photosynthesis.

5. Water use efficiency (the ratio of photosynthesis to transpiration).

Why does one and the same organ manage such different processes? Because the diffusion of CO₂ and H₂O occurs through the same opening and along the same path. CO₂ and H₂O molecules move in opposite directions, but the diffusion resistance is shared — and this resistance is determined by the width of the stomatal pore (Schopfer & Brennicke, 2016).

2.2. The Diffusion Compromise: The Inevitable Cost of Photosynthesis

Photosynthesis in the atmosphere has an inevitable consequence: in exchange for CO₂, leaves lose enormous amounts of water (Brodribb et al., 2015). Why does this happen?

The CO₂ concentration in the atmosphere is about 417 ppm (0.0417% by volume), whereas the water vapor concentration inside the leaf is close to saturation. The concentration gradient for H₂O between the leaf and the atmosphere is roughly 50 times greater than that for CO₂ (Taiz et al., 2023). Therefore, every time stomata open to let CO₂ in, much more water exits through them.

This compromise can be expressed by a simple relationship. The rate of photosynthesis (A) is related to stomatal conductance (gₛ) and the difference in CO₂ concentrations:

And the transpiration rate (E) is determined by the same stomatal conductance but by the difference in water vapor concentrations:

The factor 1.6 appears because water vapor molecules diffuse 1.6 times faster than CO₂ molecules (Brodribb et al., 2015). This means that for the same stomatal opening, water loss will always be greater than CO₂ uptake, in a ratio determined by external conditions.

2.3. Stomatal Movement: An Osmotic Mechanism with Hydraulic Consequences

How do stomata change the width of the pore? The mechanism of stomatal movements is a classic example of the integration of osmotic, ionic, and hydraulic processes within a single cell (Кузнецов & Дмитриева, 2006; Schopfer & Brennicke, 2016).

Stomatal opening occurs as follows:

1. Under the influence of light (especially blue light), a proton pump (H⁺-ATPase) is activated in the plasma membrane of guard cells.

2. Protons are pumped out, creating an electrochemical gradient.

3. Along this gradient, potassium ions (K⁺) enter the cells through channels.

4. To balance the charge, anions enter the cells — chloride (Cl^-) or organic anions (malate).

5. The increased salt concentration lowers the osmotic potential of the cells.

6. Water enters osmotically from neighboring cells into the guard cells.

7. The cell volume increases, and because their walls are unevenly thickened, they bend, and the stomatal pore opens.

Stomatal closure is the reverse process, but it can be triggered not only by the cessation of light but also by the action of the hormone abscisic acid (ABA), which is synthesized in roots during water deficit and transported to the leaves (Taiz et al., 2023). ABA activates the efflux of ions from guard cells, water exits, and the stomata close.

What is important for our integration topic: stomatal movement is an osmotic process dependent on ion uptake (mineral nutrition) and water (water status). Thus, even at the level of an individual stoma, we see the connection of three flows: ionic, water, and gaseous.

2.4. Why Stomata Are the Main Switch of Integration

Now we can understand why the stoma is a central node in the integration of physiological processes.

2.5. What Happens When Stomata Don't Work Optimally?

Consider the situation of midday depression of photosynthesis. On a clear, sunny day, especially under high temperature and low humidity, stomata may close in the middle of the day, even if the soil is still moist. Why?

Stomata close when leaf water potential drops below a certain threshold. This occurs because transpiration exceeds the capacity of the water-conducting system to supply water. As a result:

• CO₂ uptake ceases.
• Photosynthesis declines (seen as a "midday slump" on the diurnal curve of photosynthesis).
• Leaf temperature begins to rise, potentially causing additional damage.
• The plant loses potentially productive daytime hours (Hirasawa & Hsiao, 1999; cited in Brodribb et al., 2015).

Calculations show that midday depression can reduce the daily carbon balance by 20–70% depending on the species and conditions (Brodribb et al., 2015). This is direct evidence that hydraulic limitations, acting through stomata, constrain photosynthesis.

2.6. Stomata as a Meeting Point for Signals

Stomata are regulated not only by hydraulic signals (water potential) but also by hormonal, light, and metabolic signals. Here, the following converge:

• Hydraulic signals: leaf water potential, transpiration rate.
• Hormonal signals: ABA from roots (signal of soil water deficit), cytokinins from roots (signal of favorable conditions).
• Light signals: blue light activates proton pumps, red light influences through photosynthesis and changes in CO₂ concentration.
• Metabolic signals: intercellular CO₂ concentration, sugar levels (feedback from photosynthesis).

All these signals are integrated by the guard cells, which make the decision: open or close the stomata. This decision determines not only the current gas exchange but also the plant's long-term strategy — for example, resource allocation between roots and shoots (Taiz et al., 2023).

2.7. Stomata and Evolution: From C₃ to C₄ and CAM

Interestingly, the importance of stomatal control for the integration of water and carbon exchange has led to the evolution of different strategies in plants.

• C₃ plants (most species) open their stomata during the day but lose a lot of water due to photorespiration.
• C₄ plants (maize, sugarcane, sorghum) have a CO₂ concentrating mechanism that allows them to maintain a high CO₂ concentration around Rubisco with more closed stomata, enhancing water use efficiency (Taiz et al., 2023; Connor et al., 2011).
• CAM plants (succulents, cacti) open their stomata at night when evaporation is minimal, fix CO₂ as organic acids, and close their stomata during the day, using the stored CO₂ for photosynthesis. This is an extreme form of water conservation, but with limitations on overall productivity (Taiz et al., 2023).

These evolutionary solutions are vivid examples of how the plant "seeks" an optimum between carbon nutrition and water balance.

2.8. What Have We Learned About the Role of Stomata in Integration?

From this section, we have derived the second important principle:

The Principle of Stomatal Switching: Stomata are the main regulatory node connecting water and carbon exchange. They simultaneously manage CO₂ uptake for photosynthesis, water loss through transpiration, leaf temperature, and water use efficiency. Stomatal movement integrates hydraulic, hormonal, light, and metabolic signals, making a decision that determines the current physiological activity and the plant's long-term strategy.

Now we understand that stomata are not just a "door" for gases. They are an active regulatory center that links water relations, photosynthesis, mineral nutrition, and thermoregulation. It is through stomata that the plant responds to environmental changes as a unified whole.

In the next section, we will examine where the solar energy absorbed by the leaf goes and why water evaporation (transpiration) is a vital necessity for maintaining leaf temperature within acceptable limits.

3.1. Solar Energy: Input and Inevitable Excess

A leaf illuminated by direct sunlight receives a colossal energy flux — about 1000 Watts per square meter of surface (in terms of solar radiation). However, photosynthesis uses only 1–2% of this energy under typical conditions, and even in the most favorable cases, no more than 5% (Taiz et al., 2023; Connor et al., 2011). The remaining 95–99% of absorbed energy must be dissipated somewhere; otherwise, the leaf would inevitably heat up to temperatures lethal for all biochemical processes.

Here we encounter a fundamental principle: the energy balance of the leaf must sum to zero — as much energy as comes in, the same amount must be dissipated. If the balance is disrupted, the leaf temperature changes.

3.2. Main Pathways of Energy Dissipation

Where does the absorbed solar energy go? Three main dissipation channels can be identified (Taiz et al., 2023; Schopfer & Brennicke, 2016):

3.3. The Role of Stomata in Thermoregulation

Notice: of the three dissipation pathways, two (radiation and convection) are physical processes that the plant cannot actively regulate. However, the third — the evaporative pathway — directly depends on the degree of stomatal opening. It is the stomata, by regulating transpiration, that control the most effective mechanism for cooling the leaf.

When stomata are open, water evaporates intensely, and the leaf cools. When they close (for example, in response to water deficit or high temperature), evaporative cooling sharply decreases. In this case, the leaf must rely only on radiation and convection, which are often insufficient to dissipate all the absorbed energy. Leaf temperature begins to rise.

The relationship between stomatal conductance and leaf temperature can be expressed through the Bowen ratio — the ratio of sensible (convective) heat transfer to evaporative (latent) heat transfer (Taiz et al., 2023):

In a well-watered plant, transpiration is high, so evaporative heat transfer is large, and the Bowen ratio is small (e.g., 0.2–0.5). In a plant suffering from water deficit, stomata are closed, transpiration is minimal, and the Bowen ratio may exceed 1, meaning cooling occurs mainly through convection, but it is insufficient, and the leaf overheats.

3.4. What Happens During Overheating?

If the leaf overheats above the optimal temperature (which for most C₃ plants is in the range of 25–35 °C), serious disruptions begin:

1. Damage to the photosynthetic apparatus. High temperature denatures proteins, especially proteins of photosystem II and Rubisco activase (Taiz et al., 2023). Thylakoid membranes are particularly sensitive to overheating.

2. Increased photorespiration. In C₃ plants, as temperature rises, the solubility of CO₂ decreases, and that of O₂ increases, shifting the Rubisco balance toward the oxygenase reaction, enhancing photorespiration and reducing photosynthetic efficiency.

3. Accelerated respiration. Dark respiration increases with temperature, consuming additional carbon and reducing net productivity.

4. Oxidative stress. During overheating and stomatal closure, CO₂ uptake decreases, leading to electron transfer to oxygen, forming reactive oxygen species (ROS) — superoxide anion, hydrogen peroxide, and singlet oxygen. This causes photooxidative damage.

All these effects are interconnected: overheating reduces photosynthesis, which further reduces the utilization of light energy, which intensifies ROS formation and damage — a classic vicious cycle (Hopkins & Hüner, 2009).

3.5. Why is Transpiration Vital? An Integrated Perspective

From the above, an important conclusion follows: transpiration is not an "unavoidable evil" or a "cost" of photosynthesis. It performs a critical thermoregulatory function, without which photosynthesis would be impossible under most natural conditions. Essentially, by opening for CO₂, stomata simultaneously trigger evaporative cooling, which helps maintain the temperature optimum for photosynthesis.

But here arises the key paradox of integration: to cool down, you need to open stomata and lose water; to avoid water loss, you need to close stomata — but then the leaf overheats. Balancing between these two extremes is the essence of the plant's physiological regulation throughout the day (Sinclair, 1994; Brodribb et al., 2015).

We see this in the midday depression of photosynthesis: when transpiration is too high and leaf water potential drops, stomata close, saving from dehydration, but this leads to overheating and an additional drop in photosynthesis. The plant faces a choice: either lose water or overheat — and it must seek a compromise.

3.6. Adaptations Helping to Avoid Overheating

Throughout evolution, plants have developed numerous adaptations that help cope with excess solar energy and reduce the need for evaporative cooling. These are also examples of integration, but at the morphological level:

• Shiny cuticle and waxy bloom increase light reflection (albedo), reducing energy absorption.
• Pubescence (hairs) reflects light and creates an additional boundary layer, although it may also reduce convection.
• Narrow and finely dissected leaves improve convective heat exchange (reduce boundary layer resistance) — characteristic of many xerophytes.
• Leaf movement (paraheliotropism) allows avoiding direct sunlight in the middle of the day, reducing absorption.
• Changes in pigmentation (e.g., the appearance of anthocyanins) can absorb some light energy, although the role of anthocyanins in protection against excess light is not fully understood.

All these adaptations work within one system: they reduce the burden on evaporative cooling, allowing the plant to save water while not overheating. But under conditions where water is sufficient, transpiration remains the most effective method — and the plant actively uses it.

3.7. What Have We Learned About Energy Balance and Integration?

From this section, we have derived the third important principle:

The Principle of Energy Balance: Absorbed solar energy must be dissipated. Since photosynthesis uses only a small part of it, the main dissipation mechanism is the evaporation of water through stomata (transpiration). Closing stomata, while saving water, simultaneously reduces evaporative cooling, which can lead to overheating and damage to the photosynthetic apparatus. Thus, thermoregulation is inextricably linked to water relations and carbon exchange through the single stomatal regulatory node.

Now we see that the stoma links not two, but three flows: carbon, water, and heat. Transpiration is not a side effect but performs a critical cooling function, without which photosynthesis would be impossible under most natural conditions.

In the next section, we will move to the fourth level of integration: how water aids nutrition, and nutrition aids water movement. We will see that ion transport and osmotic pressure create a positive feedback loop that enhances water and element uptake into the plant.

4.1. The Transpiration Stream — The Main Pathway for Ion Delivery

We already know that the bulk of water moves from roots to leaves through the xylem thanks to the transpiration pump. This flow not only transports water — it also serves as a conveyor belt for ions of mineral elements absorbed from the soil into the roots (Schopfer & Brennicke, 2016; Marschner, 2012).

Ions absorbed by root cells enter the xylem vessels (either by passive leakage or active secretion) and are carried with the water flow to the shoots. The concentration of ions in xylem sap ranges from 1 to 10 mM for major cations (K⁺, Ca²⁺, Mg²⁺) and anions (NO₃^-, H₂PO₄⁻, SO₄^2-) (Marschner, 2012). Although these are relatively low concentrations, given the enormous volumes of water passing through the plant per day, the total ion flow is quite significant.

Thus, the intensity of transpiration directly influences the rate of ion delivery to the leaves — all else being equal, the more water evaporates, the more ions are brought to the shoots. However, it is important to emphasize that the relationship is not linear: the plant can regulate the concentration of ions in the xylem sap independently of the flow rate (Schopfer & Brennicke, 2016). In other words, the plant can "dose" the amount of ions sent to the shoot, even if transpiration changes abruptly.

4.2. Active Ion Uptake: Creating the Osmotic Gradient

Now let's look at how nutrition aids water movement. The key mechanism here is active ion transport into root cells.

Cells of the epiblema (rhizodermis) and root cortex actively absorb ions from the soil solution against the concentration gradient, using ATP energy obtained from respiration (Marschner, 2012; Schopfer & Brennicke, 2016). This creates a high ion concentration inside root cells compared to the soil solution. As a result, the osmotic potential of root cells becomes more negative than that of the soil solution. Water enters root cells osmotically — this is the first step on the water's path into the plant.

But the process does not end there. Ions accumulated in root cells are then actively secreted into the xylem vessels — into the apoplast of the central cylinder. This also requires energy. As a result, a high ion concentration is created in the xylem vessels, lowering the water potential in the xylem (making it more negative). Water moves from root cells into the xylem vessels, following the osmotic gradient. This creates positive hydrostatic pressure in the xylem — root pressure, which pushes water upward even during weak transpiration (at night or under high humidity) (Кузнецов & Дмитриева, 2006).

Thus, active ion uptake is the driving force of root pressure, and root pressure, in turn, promotes water movement upward in the plant, especially during periods of low transpiration. This is a classic example of how nutrition (ion transport) aids water movement.

4.3. Ions in the Xylem: Creating a Water Potential Gradient Along the Entire Path

When ions enter the xylem flow, they lower the water potential of the xylem sap along the entire route from roots to leaves. This helps maintain the water potential gradient from roots to leaves: water potential in the root xylem is lower than in the soil, and even lower in the leaves due to transpiration. Ions present in the xylem sap reduce its water potential, facilitating water movement upward.

Furthermore, some ions, especially potassium (K⁺), play a critical role in the osmoregulation of stomatal guard cells. We already discussed this mechanism in Section 2. The entry of K⁺ into guard cells lowers their osmotic potential, water enters, and stomata open. When K⁺ exits guard cells, stomata close. Thus, the availability of potassium and other ions directly affects stomatal conductance, and through it, transpiration and photosynthesis. This is another feedback cycle: nutrition (K⁺) regulates water flow (transpiration), and water flow delivers new supplies of K⁺ to the leaves.

4.4. Water Flow as a Factor Determining Ion Availability in the Rhizosphere

Water not only delivers ions to the leaves but also increases the availability of ions in the soil. The water flow moving from the soil to the roots creates convective ion transport. Ions dissolved in soil moisture are drawn toward the root surface along with water. This mass flow (convection) is particularly important for mobile ions such as nitrate NO₃^-, sulfate SO₄^2-, calcium Ca²⁺, and magnesium Mg²⁺ (Marschner, 2012). The higher the transpiration, the more intense the mass flow and the more ions are delivered to the root.

However, not all ions are equally mobile in the soil. Phosphate (H₂PO₄⁻), potassium (K⁺), and micronutrients (Fe, Zn, Mn) are often strongly adsorbed onto soil particles and move mainly by diffusion — a slow process dependent on the concentration gradient. In this case, intense water flow does not increase their delivery; on the contrary, it can create a depletion zone around the root. This is why plants have developed additional strategies to mobilize poorly soluble elements — exudation of organic acids, enzymes, changes in rhizosphere pH, symbiosis with mycorrhizae (Marschner, 2012, Chapter 14). These strategies are also part of integration: they are activated under element deficiency and alter the chemistry of the rhizosphere, ultimately affecting the plant's water and ion balance.

4.5. Ions in the Phloem: Influence on Assimilate Transport

Integration of water and ion exchange is not limited to the xylem. Ions, especially potassium, play an important role in phloem transport of organic substances from source leaves to sink organs (Marschner, 2012, Chapter 5; Engels et al., 2012).

Phloem sap, in addition to sugars (mainly sucrose), contains significant concentrations of K⁺, often up to 50–100 mM (Marschner, 2012). Potassium facilitates sucrose loading into the phloem by creating an electrochemical gradient and maintaining osmotic pressure in the sieve tubes. Potassium is also involved in phloem unloading in sink organs. Thus, the presence of ions in the phloem ensures efficient transport of assimilates from sources to consumers (roots, fruits, seeds). This links mineral nutrition not only to water relations but also to carbon distribution within the plant.

Moreover, potassium and other ions constantly recirculate between xylem and phloem. For example, potassium delivered to leaves with the xylem stream can be exported back to the roots via the phloem and then secreted again into the xylem. This cycling allows flexible redistribution of ions according to the needs of different organs (Schopfer & Brennicke, 2016). K⁺ recirculation is particularly important for maintaining osmotic potential in different parts of the plant and for regulating water and assimilate flows.

4.6. Osmotic Integration: A Unified System of Water and Ion Homeostasis

All of the above leads us to understand that water and ion exchange are unified into a single osmotic system of the plant (Кузнецов & Дмитриева, 2006). The osmotic potential of each cell, each tissue compartment is determined by the combined action of:

• ion concentrations (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, NO₃^-, etc.);
• concentrations of organic osmolytes (sugars, amino acids, organic acids, proline, glycine betaine, etc.);
• pressure on the cell wall (turgor pressure).

A change in the concentration of any of these components immediately alters the osmotic potential and, consequently, the direction and speed of water flows.

Here is what the positive feedback cycle looks like:

1. Water delivers ions to the roots (mass flow) and then to the shoots (xylem flow).

2. Ions are actively taken up by cells, lowering their osmotic potential.

3. The lowered osmotic potential enhances water uptake into these cells (osmosis).

4. Water uptake increases turgor, cell volume, and promotes cell expansion (growth) and stomatal opening (enhanced transpiration).

5. Enhanced transpiration increases the mass flow of water and the delivery of new ions — the cycle closes.

This cycle operates continuously: in the morning, when stomata open, transpiration increases, bringing more ions to the leaves; ions are used for osmoregulation and maintaining stomatal conductance; the longer stomata stay open, the more water evaporates and the more ions are drawn in, creating positive feedback. However, on a hot afternoon, negative feedbacks come into play: dehydration triggers stomatal closure, transpiration decreases, ion uptake slows down, and the plant switches to conservation mode.

4.7. What Have We Learned About the Interrelationship of Water and Nutrition?

From this section, we have derived the fourth important principle:

The Principle of Osmotic Reciprocity: Ion uptake into cells lowers their osmotic potential, enhancing osmotic water influx. Water, in turn, serves as a medium and a transport agent for ion delivery. This mutual cycle (positive feedback) underpins water relations, growth, stomatal movement, and long-distance transport. Ion and water exchange are inseparable and form a unified osmotic system that governs the physiological state of the plant.

Now we can see that all four flows — water, carbon, energy (heat), and ions — are linked in a single network. Stomata are the node where these flows intersect. The osmotic mechanism is the engine that drives water and ion movement. The energy balance determines how much heat needs to be dissipated through transpiration, and transpiration controls ion delivery and cooling.

In the concluding section, we will trace how all these integration mechanisms operate over the course of a single summer day, to solidify the holistic picture of physiological integration.

5.1. Before Dawn: Preparing for a New Day

Night. The plant is in a state of minimal activity. Stomata are closed (in most species), transpiration is practically absent. However, the root system continues to work: it actively absorbs ions from the soil, using respiratory energy, and secretes them into the xylem vessels (Schopfer & Brennicke, 2016; Кузнецов & Дмитриева, 2006).

As a result, ions accumulate in the xylem, lowering the osmotic potential, and water enters the vessels osmotically. Root pressure is generated, which can reach 0.1–0.15 MPa. This pressure pushes water up the stem, and sometimes we can observe this phenomenon as guttation — the exudation of water droplets at the leaf margins through hydathodes (Schopfer & Brennicke, 2016). Guttation is particularly noticeable in cereals, strawberries, and tomatoes.

Root pressure plays an important role: it fills the xylem vessels with water, restores the plant's water status after daytime losses, and prepares the system for the coming day. At night, restoration of hydraulic conductivity also occurs — repair of air embolisms that may have formed in the xylem during the day due to strong water tension (Brodribb et al., 2015). In many herbaceous plants, this process is active, and by morning, the xylem is completely water-filled.

5.2. Dawn: Waking Up the System

With the first rays of sunlight, a radical reorganization of the entire physiological system begins. Light, especially the blue part of the spectrum, activates proton pumps in the stomatal guard cells (Taiz et al., 2023). Potassium uptake into guard cells begins, the osmotic potential falls, water enters, and stomata open.

Here is what happens simultaneously:

• CO₂ uptake into the leaf begins.
• Photosynthesis is initiated — light reactions proceed immediately; dark reactions (Calvin cycle) require some enzyme activation.
• Transpiration begins — water vapor exits through the open stomata.
• Evaporative cooling starts, and leaf temperature begins to stabilize.
• Leaf water potential decreases — creating a "demand" for water, and the xylem flow accelerates.
• Acceleration of the xylem flow delivers ions from the root, accumulated overnight, to the leaves.
• Root respiration is activated — enhancing uptake of new ions to maintain the osmotic gradient.

It is important to note that stomata do not open instantaneously or completely: the process takes from 30 minutes to several hours depending on the species, weather, and water status (Taiz et al., 2023). Stomatal conductance initially increases slowly, then accelerates, reaching a maximum by mid-morning.

Integration moment: at dawn, we see how osmotic reciprocity works — the overnight accumulation of ions in the roots creates the osmotic gradient that facilitates the rapid onset of water flow as soon as stomata open and transpiration creates the "pull."

5.3. Morning: Rising Activity, Peak Efficiency

In the morning hours, when the sun is rising but the temperature is not yet too high and air humidity is relatively high, the plant operates most efficiently:

• Photosynthesis increases in proportion to light, reaching high values.
• Transpiration increases, but has not yet reached critical levels.
• Water balance is positive: water uptake from the roots keeps pace with evaporation.
• Leaf water potential remains in the zone favorable for open stomata.
• Evaporative cooling is effective, and leaf temperature is close to the optimum for enzymes.

Morning photosynthesis often provides the greatest contribution to the daily carbon balance because conditions are close to optimal: light is present, temperatures are moderate, stomata are open, water is sufficient, and photorespiration is not yet too high (Taiz et al., 2023).

During this time, the plant actively consumes water but also absorbs it from the soil at full capacity. The root system, receiving the signal of increased transpiration (through the decrease in water potential in the xylem), intensifies water and ion uptake. Some researchers call this morning the plant's "golden hour" — a time of maximum productivity with minimum stress (Hirasawa & Hsiao, 1999, cited in Brodribb et al., 2015).

5.4. Midday: A Test of Resilience, The Appearance of Limitations

As midday approaches, the situation begins to change. The sun is at its zenith, illumination is maximal, but air temperature rises, and relative humidity decreases. The vapor pressure deficit (VPD) increases, sharply enhancing the driving force for transpiration.

Now one of the scenarios we considered in the first section unfolds:

1. Transpiration intensifies due to high VPD.

2. Leaf water potential begins to drop faster than roots can supply water.

3. If the drop in water potential reaches a critical threshold (for many species, about –1.5…–2.0 MPa), stomata begin to close (Brodribb et al., 2015; Schopfer & Brennicke, 2016).

4. CO₂ uptake decreases, photosynthesis declines — midday depression sets in.

5. Transpiration decreases, but along with it, evaporative cooling decreases.

6. Leaf temperature begins to rise, potentially causing additional enzyme inhibition.

However, not all species and not all conditions lead to complete stomatal closure. If the soil is sufficiently moist and the plant's hydraulic system is efficient, stomata may remain partially open, sustaining some photosynthesis, albeit at a lower intensity than in the morning. In this case, we observe a "plateau" of photosynthesis in the middle of the day rather than a sharp decline.

Importantly, at midday, protective mechanisms are engaged:

• The xanthophyll cycle — conversion of violaxanthin to zeaxanthin in thylakoid membranes, allowing the dissipation of excess light energy as heat (Taiz et al., 2023; Hopkins & Hüner, 2009).
• Chloroplast movement — they can move within cells to avoid light overload.
• Leaf orientation changes (paraheliotropism) in some species — leaves turn to reduce light absorption.

All these mechanisms are part of integration at the level of photosynthesis, thermoregulation, and water relations. They allow the plant to survive the harshest conditions of midday without irreversible damage.

Integration moment: midday demonstrates the conflict between the need for cooling (transpiration) and the need to conserve water. The plant must balance, and this balance is regulated by hydraulic, hormonal, and light signals integrated at the stomatal apparatus.

5.5. Afternoon: Gradual Recovery

In the afternoon, solar radiation weakens, temperature begins to drop, and VPD decreases. The transpirational load decreases, and the plant has an opportunity to restore its water status:

• Stomata may reopen wider if leaf water potential increases.
• Photosynthesis may partially recover — sometimes a "second peak" of photosynthesis is observed late in the day.
• Leaf temperature decreases, and enzymatic processes ease.

However, if the daytime water deficit was significant, recovery may be incomplete. Some plants remain with partially closed stomata until the evening, especially if the soil is dry.

During this period, phloem transport also intensifies — assimilates accumulated during the day begin to be actively exported from leaves to sink organs (growing shoots, roots, fruits, seeds). This process depends on the availability of ions, particularly potassium, which supports phloem loading (Marschner, 2012; Engels et al., 2012).

Integration moment: in the afternoon, integration is manifested in the shift from carbon "storage" (photosynthesis) to its "distribution" (assimilate transport). This requires the coordinated work of the xylem (delivery of water and ions) and phloem (transport of organics).

5.6. Evening and Night: Completing the Cycle

Towards evening, the sun sets, illumination decreases, and stomata gradually close. Photosynthesis declines, transpiration decreases to a minimum. The plant switches to nighttime mode.

At night, two important processes occur:

1. Restoration of water balance. Root pressure ensures water uptake into the xylem, filling vessels and restoring hydraulic continuity. This is especially important if air embolisms formed during the day. In many species, active xylem repair occurs at night (Brodribb et al., 2015).

2. Ion "charging" of the roots. Roots continue to actively absorb ions from the soil, creating the osmotic gradient that will be used the next morning. This is preparation for the new day — accumulating "fuel" for the osmotic pump.

Additionally, respiration occurs at night — organic substances accumulated during the day are partially oxidized, providing energy for all maintenance processes, including ion uptake.

5.7. Generalized Picture of the Diurnal Course (Table)

For clarity, let's summarize the main parameters of the diurnal course in a table (Sinclair, 1994; Taiz et al., 2023; Кузнецов & Дмитриева, 2006):

This table shows how all parameters are interconnected and change in a coordinated manner.

5.8. What Have We Learned from the Diurnal Dynamics?

From this section, we have derived the fifth and final principle:

The Principle of Diurnal Integration: The plant operates as a single dynamic system, whose diurnal course is governed by coordinated changes in light, temperature, and humidity. From the morning opening of stomata to the nighttime recovery, all processes — transport of water, ions, carbon dioxide, assimilates, and heat — are interconnected and regulated as a whole. Midday depression and nocturnal repair are natural stages of this cycle, ensuring survival and productivity in a changing environment.