Stress as a physiological category

The Physiological Meaning of Stress

From a biological perspective, stress is a state that occurs when external conditions exceed the limits within which the plant can maintain homeostasis – the dynamic constancy of its internal environment.

Let us recall that a plant, like any living organism, is an open thermodynamic system that maintains a stable non-equilibrium state through a constant influx of energy. This state – homeostasis – is ensured by the continuous operation of metabolic processes (Hopkins & Hüner, 2009).

Any environmental change strong enough to disrupt this homeostasis triggers a stress response. In this sense:

Stress is a disruption of homeostasis caused by an external factor.

Developing Selye's ideas, Russian plant physiologists (Kuznetsov and Dmitrieva, 2006) characterize stress as "an integral non-specific response of the organism to the action of unfavorable factors, aimed at removing the organism from the damaging influence and mobilizing protective systems."

Distinctive Features of Stress as a Physiological State

1. Non-specificity. Regardless of what caused the stress – drought, salinity, heat, or cold – similar mechanisms are activated in the plant: synthesis of stress proteins, accumulation of osmolytes, changes in hormonal balance.

2. Systemicity. Stress is a reaction of the whole organism, although it is often initiated at the cellular level.

3. Dynamics. The stress response develops over time and has specific phases that follow one another.

In the definition by Kuznetsov and Dmitrieva (2006), we also find an important addition: stress is viewed as an adaptation syndrome – a complex of reactions aimed at adapting the organism to changed conditions. It is this aspect that distinguishes the physiological understanding of stress from the everyday one, where stress is perceived as something exclusively harmful.

Strategy 1. Stress Avoidance

Essence: the plant does not allow the stressor to disrupt homeostasis.

The plant does not "tolerate" stress – it actively counteracts it so that the external factor does not reach critical values inside the organism. Avoidance is protection at the input.

Strategy 2. Stress Tolerance

Essence: the plant allows the disruption of homeostasis but reorganizes its metabolism to function under the new conditions.

Tolerance is protection at the output. The plant does not prevent the stressor's action, but it is capable of "accepting" the new reality and adapting to it (Schopfer & Brennicke, 2016; Kuznetsov and Dmitrieva, 2006).

Avoidance and Tolerance in the Real Life of a Plant

In reality, these two strategies are not mutually exclusive. Most plants use a combination of avoidance and tolerance, with the ratio depending on the stress and its intensity.

For example, during mild drought, the plant first avoids stress by closing its stomata. If the drought intensifies, it switches to tolerance, accumulating proline and synthesizing LEA proteins.

It is important for the agronomist to understand this hierarchy. A breeder developing a drought-tolerant variety must decide what to focus on: avoidance (ability to close stomata quickly, developed root system) or tolerance (ability to accumulate osmolytes effectively). Each strategy has its agronomic pros and cons.

Plants using avoidance often demonstrate a higher productivity potential in favorable years because their defense mechanisms do not divert resources. However, they may be more vulnerable during sudden and severe stresses.

Plants using tolerance are more stable under unfavorable conditions. But the price of this stability is reduced productivity even in good years, because some resources are constantly "tied up" in protective systems.

We will discuss this compromise in more detail in Section 5, when we talk about the price of adaptation.

Summary:

Thus, plant resistance is not a single property, but a complex of strategies that it chooses and combines depending on its genetic program and the current situation.

Question for reflection: Which strategy – avoidance or tolerance – would you choose in breeding for a region with frequent but short droughts? And for a region with one long drought per season?

In the next section, we will move on to an unexpected aspect of stress – its potential benefit – and discuss what eustress is and how it relates to the phenomenon of hardening.

Eustress and Distress: Two Faces of Stress

In 1996, the German plant physiologist Hartmut Lichtenthaler (Lichtenthaler, 1996) proposed distinguishing two qualitatively different forms of stress:

In nature, the same impact can cause eustress or distress depending on its intensity, duration, and the physiological state of the plant.

This distinction is extremely important for agronomy. We cannot and should not completely protect plants from all stress effects. On the contrary, proper management of moderate stresses can become a powerful agronomic tool.

How Does Eustress Help the Plant?

When Does Eustress Turn into Distress?

It is important to understand: the line between beneficial and harmful stress is very thin. The same factor can be both, depending on:

1. Intensity. Mild drying – eustress (stimulation of root growth). Severe drought – distress (death).

2. Duration. Short-term cooling – eustress (hardening). Prolonged cooling – distress (membrane damage).

3. Developmental phase. Drought at the beginning of the growing season can stimulate root system development (eustress). The same drought during flowering can lead to flower sterility and yield loss (distress).

4. Plant condition. A healthy, well-supplied plant tolerates stress more easily than a weakened one affected by disease or starvation.

A classic sign of the transition from eustress to distress is the appearance of irreversible damage. If the plant fully recovers after the stress is removed, it was eustress. If necrosis, chlorosis, or reduced yield remain, it is distress.

What Does This Mean for the Agronomist?

Understanding the difference between eustress and distress gives us practical tools for managing stress in the field.

1. Managed stresses as a technique

We can deliberately create moderate stresses to achieve the desired effect:

• Hardening seedlings before planting.
• Water deficit to improve product quality (e.g., in grapes).
• Nitrogen deficit to stimulate flowering in some crops (switching from vegetative growth to generative development).

2. Preventing transition to distress

We must understand the critical thresholds for each crop:

• What minimum soil moisture can be allowed without yield loss?
• What temperatures are safe for hardening, and which ones already cause damage?
• In which developmental phases is stress particularly dangerous?

3. Recovery after stress

If distress has occurred, it is important to help the plant recover. Often, after stress is relieved, the following are needed:

• Fertilization (especially nitrogen, as the synthesis of protective proteins depletes nitrogen reserves).
• Irrigation (replenishing water deficit).
• Protection from diseases (weakened plants are more vulnerable).

Phases of the Stress Response

To better understand the dynamics of stress, it is useful to know that any stress response passes through certain phases (Kuznetsov and Dmitrieva, 2006; Medvedev, 2012).

1. Alarm phase

• Rapid, reflexive reaction.
• Recognition of the stressor.
• Primary signals (calcium, ROS).
• Lasts minutes – hours.

2. Acclimation phase

• Specific defense mechanisms are activated.
• Stress-induced proteins are synthesized, osmolytes accumulate.
• Metabolism is restructured.
• Lasts hours – days.

3. Resistance phase

• The plant functions under new conditions.
• Resistance is maximal.

4. Exhaustion phase

• If the stress is too strong or prolonged, defense systems are depleted.
• Irreversible damage begins.
• Death or sharp decline in productivity occurs.

5. Recovery phase

• Occurs after the stress is relieved.
• The plant returns to normal metabolism.
• Damaged structures are restored.

It is during the alarm phase that eustress often works. Short-term exposure triggers the first phase but does not proceed to the second and third. Defense mechanisms are "switched on" but not depleted. And when a real strong stress occurs later, the plant goes through the first phase much faster – the defense is already ready.

Summary:

• Stress can be beneficial (eustress) or harmful (distress). The difference lies in intensity and duration.
• Eustress causes hardening – an increase in resistance to future, stronger stresses.
• The mechanism of eustress is preadaptation. The plant synthesizes protective proteins and substances in advance to be ready for future loads.
• Eustress can stimulate secondary metabolism, improving product quality (wine, essential oils).
• For the agronomist, it is important to know the critical thresholds of stress for each crop, to use eustress as a managed technique and to prevent transition to distress.
• Any stress response goes through phases: alarm → acclimation → resistance → exhaustion → recovery.

Question for reflection: If you are an agronomist in an arid region and you have a choice between a variety that grows well only with irrigation and a variety that gives a stable but modest yield under any conditions – which variety would you choose and why? How does the concept of eustress help you make a decision?

In the next section, we will approach another surprising property of the stress response – its non-specificity. Why does the plant respond similarly to drought, heat, and salinity? What common nodes do all stress responses share?

Which Reactions Exactly Are Non-Specific?

We can identify four universal physiological nodes through which the response to almost any stress passes. These nodes are not just separate phenomena but interconnected links in a single chain.

Node 1. Reactive Oxygen Species (ROS) – Universal Danger Signal

This is perhaps the fastest and most widespread element of the stress response. Regardless of what is affecting the plant – drought, salinity, cold, heat, excess light, heavy metals, or pathogens – the formation of reactive oxygen species (ROS) increases in the cells.

What are ROS?

ROS are highly reactive forms of oxygen formed during incomplete reduction of molecular oxygen (O₂). They include (Kordrostami et al., 2020; Schopfer & Brennicke, 2016):

• Superoxide anion (O₂⁻•) – a radical formed by one-electron reduction of oxygen.
• Hydrogen peroxide (H₂O₂) – a stable molecule capable of penetrating membranes.
• Hydroxyl radical (•OH) – an extremely active, short-lived radical.
• Singlet oxygen (¹O₂) – an electronically excited form of oxygen formed in chloroplasts.

Where do ROS come from under stress?

Under normal conditions, ROS are formed in small amounts as by-products of metabolism, especially in:

• Mitochondria – in the respiratory electron transport chain.
• Chloroplasts – in the photosynthetic electron transport chain.
• Peroxisomes – during photorespiration and β-oxidation of fatty acids.

However, in a healthy cell, ROS are effectively removed by the antioxidant system.

Under stress, the balance is disrupted. Due to damage or overloading of electron transport chains, the probability of "leakage" of electrons to oxygen increases, giving rise to ROS.

The Dual Role of ROS:

Depending on concentration, ROS perform two fundamentally different functions:

1. Signaling (low concentrations). In small doses, ROS act as secondary messengers, activating signaling cascades and triggering the expression of defense genes. This is part of eustress (Hopkins & Hüner, 2009; Lambers & Oliveira, 2019).

2. Damaging (high concentrations). In excess, ROS begin to destroy cellular structures: cause lipid peroxidation (membranes), oxidize proteins, and damage DNA. This is distress (Kordrostami et al., 2020).

ROS are simultaneously a distress signal and a weapon of destruction. The amount decides everything.

This is why one of the key indicators of stress is the content of lipid peroxidation products, for example, malondialdehyde (MDA) (Kordrostami et al., 2020).

Node 2. Calcium Signaling – Universal Secondary Messenger

The second universal element of the stress response is the change in the concentration of calcium ions (Ca²⁺) in the cytoplasm.

Almost any stress causes a short-term increase in the level of Ca²⁺ in the cytosol. Where does it come from? Calcium enters the cytoplasm from:

• The apoplast (external environment) through plasmalemma Ca²⁺ channels.
• Intracellular stores – vacuole, endoplasmic reticulum, mitochondria.

These calcium "spikes" are recognized by calcium-binding proteins – for example, calmodulin. The change in calmodulin conformation upon Ca²⁺ binding activates protein kinases, which in turn phosphorylate other proteins and change their activity.

Example: during water stress, the concentration of Ca²⁺ in the roots increases, activating the calcium-dependent protein kinase SOS2, which then phosphorylates and activates the Na⁺/H⁺-antiporter SOS1, pumping sodium out of cells during salinity (Medvedev, 2012).

Proof of universality: If a calcium chelator (a substance that binds Ca²⁺) is introduced into the cell or Ca²⁺ channels are blocked, many protective reactions to stress are not triggered. This proves that calcium is an obligatory element of the signaling pathway for a wide variety of stressors (Knight et al., 1992; Schopfer & Brennicke, 2016).

Calcium is like the "red alert" button. Whatever stress occurs, it is pressed first.

Node 3. Growth Inhibition – Universal Economic Reaction

Any serious stress causes arrest or sharp slowdown of growth processes. Moreover, this occurs in a very characteristic sequence:

1. Cell elongation stops first. This is the fastest reaction because elongation depends on turgor, and turgor decreases under water deficit.

2. Then cell division slows down. This requires time and metabolic reorganization.

3. Later, accelerated aging and abscission of organs (leaves, ovaries) may begin – this is already an emergency measure.

Why does this happen?

From a physiological perspective, growth inhibition under stress is an adaptive strategy, not just a breakdown. The plant redistributes resources:

• Energy and substances that could have gone to growth are directed to defense (synthesis of stress proteins, osmolytes, antioxidants).
• Growth arrest reduces the need for water and nutrients, helping to survive the deficit.
• Water that could have been used to create new biomass is conserved for maintaining vital functions.

Hormonal mechanism of growth inhibition:

A key role is played by abscisic acid (ABA) – the main stress hormone of plants. Under stress, ABA is synthesized in roots and leaves and causes:

• Inhibition of cell elongation.
• Stomatal closure (which further inhibits photosynthesis, and therefore growth).
• Suppression of protein synthesis (resource conservation).

Growth arrest is not a defeat for the plant, but its strategic decision. The plant says: "No time for growth now. The main thing is to survive."

This is precisely why, as we discussed in Section 5, adaptation almost always leads to reduced yield. Growth inhibition is the most direct path to this reduction.

Node 4. Hormonal Reorganization – Systemic Coordination of the Response

Stress causes profound changes in the hormonal balance of the whole plant. Moreover, this reorganization is also non-specific for different stresses (Medvedev, 2012; Tretyakov et al., 2000).

Main changes:

It is important to note that the hormonal response depends not only on the stressor but also on its intensity. Under mild stress, changes may be reversible and beneficial (eustress). Under severe stress, they may be irreversible (distress) (Schopfer & Brennicke, 2016).

Example of complex regulation:

Under water deficit, ABA is synthesized in the roots. It is transported via the xylem to the leaves and causes:

1. Stomatal closure (fast reaction, minutes).

2. Inhibition of cell division and elongation (medium-speed reaction, hours-days).

3. Activation of genes encoding LEA proteins and dehydrins (slow reaction, hours-days) (Connor et al., 2011).

Simultaneously, cytokinin levels decrease, which promotes accelerated aging of lower leaves and redistribution of nutrients to younger organs.

Why is this non-specific?

Changes in the content of ABA, ethylene, and auxins are observed during drought, salinity, heat, cold, and mechanical damage. Qualitatively, the shift is the same, but quantitatively – different. This is precisely why we speak of the non-specificity of the stress response: the hormonal reaction is "uniform" in form but "variable" in strength depending on the stressor.

What is Cross-Tolerance (Cross-Adaptation)?

From the non-specificity of the stress response, a practically important consequence follows: if a plant has adapted to one stressor, it often becomes more resistant to other stressors as well. This phenomenon is called cross-tolerance (or cross-adaptation) (Medvedev, 2012).

Examples:

• Plants hardened to cold often tolerate drought better. Conversely, hardening to drought increases cold resistance.
• Treating plants with a weak salt concentration (salinity) can increase their resistance to subsequent drought.
• Prior heat shock (eustress) can increase resistance to oxidative stress.

Mechanism of cross-adaptation:

Since different stresses activate similar defense systems (ROS signaling, calcium, synthesis of protective proteins, accumulation of osmolytes), preliminary "training" of one of these systems prepares the plant for the operation of others.

For example:

• Drought causes the accumulation of proline and LEA proteins. The same proline and LEA proteins also help during salinity and cooling.
• Heat shock induces the synthesis of HSP70. These same proteins protect cells during salinity and water deficit as well (Schopfer & Brennicke, 2016).

Thus, the plant does not build a unique defense "from scratch" for each stress. It uses a common arsenal of tools, but in different combinations and with different intensities.

Summary: Why Do Different Stresses Cause Similar Reactions?

1. Limited set of sensors and signaling pathways. The plant has only a few ways to "sense" danger: changes in membrane potential, membrane deformation, increased ROS. All these signals converge on calcium and protein kinase cascades.

2. Common defense mechanisms. The plant cannot create a unique defense for each stress. Evolutionarily, it is more advantageous to have a set of universal "tools" (osmolytes, chaperones, antioxidants) and activate them in different combinations.

3. Resource economy. If a unique defense had to be built for each stress, it would be too expensive. The universality of the response is an adaptive economy.

4. Cross-adaptation. Precisely because of non-specificity, a plant adapted to one stress can gain protection from others. This provides a powerful evolutionary advantage.

What Does This Mean for the Agronomist?

1. Hardening techniques work broadly. Hardening to drought helps with salinity as well. Hardening to cold helps with drought as well. This allows using one technique to increase resistance to a complex of factors.

2. Resistance assessment must be comprehensive. A plant's resistance cannot be judged by just one trait. A set of indicators is needed: antioxidant enzyme activity, proline content, ABA levels, growth changes.

3. Non-specificity does not mean identity. Although the common nodes are the same, specific mechanisms (e.g., synthesis of antifreeze proteins in cold or Na⁺/H⁺-antiporters in salinity) are activated only under certain stresses. Therefore, hardening to drought cannot be considered fully protective against frost, but it will provide some advantage.

4. Using cross-adaptation in practice. For example, mild salinity (eustress) before planting can increase seedling resistance to subsequent drought. Or slight drying before planting can improve survival in hot summer conditions.

Question for reflection: Imagine you have a field where both drought and salinity are expected simultaneously (a typical situation for many arid regions with irrigated agriculture). What agronomic techniques, based on the principle of non-specificity, could you propose for preparing the plants? How could you use eustress to protect against distress?

In the next section, we will move to one of the most important questions for the agronomist: why does adaptation reduce yield? We will explore the fundamental trade-off between defense and productivity and how understanding this trade-off helps in making agronomic decisions.

Where Does the Price of Adaptation Come From?

The price of adaptation consists of several components.

1. Energy costs for synthesizing protective compounds.

Plant defense is not a free service. The synthesis of any protective compounds requires energy (ATP) and building materials (carbon, nitrogen).

Numbers for understanding:

• During drought, a plant can spend up to 20–30% of its carbon budget on the synthesis of osmolytes and protective proteins.
• Under heat stress, synthesis of heat shock proteins can consume up to 10% of cellular energy.
• Proline synthesis under stress costs about 2 times more (per mole) than the synthesis of an equivalent amount of biomass.

Every molecule of a protective protein is a protein molecule not built into a new leaf or grain.

2. Energy costs for operating defense systems.

Many defense mechanisms require not only costs for building structures but also for their constant maintenance.

Stomatal closure. Although stomatal closure itself is a passive process (loss of turgor), maintaining stomata in a closed state requires the work of ion pumps, which costs ATP. But more importantly, closed stomata drastically reduce photosynthesis, and hence the supply of new energy.

Antioxidant defense. Continuous destruction of ROS requires the work of enzymes (SOD, CAT, APX), and their synthesis and maintenance are constant costs. Reduction of ascorbate and glutathione (key antioxidants) requires NADPH and ATP.

Operation of ion pumps. During salinity, the plant actively pumps excess Na⁺ from the cytoplasm into the vacuole (vacuolar Na⁺/H⁺-antiporter) or outside (plasmalemma Na⁺/H⁺-antiporter). This work requires ATP. Energy costs for maintaining ion balance during salinity are estimated at 10–20% of the root's total energy budget.

3. Direct inhibition of metabolism related to growth.

Many defense mechanisms are biologically incompatible with active growth.

Closed stomata mean that CO₂ does not enter the leaf. Photosynthesis decreases, sometimes by 50–80% (Connor et al., 2011). This means that carbon for building new biomass is not supplied.

Arrest of cell division. Stress-induced inhibition of mitosis (cell division) directly reduces the rate of formation of new organs.

Decreased synthesis of ribosomes and proteins. Under stress, the synthesis of "ordinary" proteins (enzymes, structural proteins) is often suppressed in favor of stress protein synthesis. This leads to degradation of enzymatic systems necessary for growth.

4. Hormonal rearrangements aimed at survival.

Stress causes a shift in hormonal balance from "growth hormones" (auxins, gibberellins, cytokinins) to "defense hormones" (ABA, ethylene, jasmonate). This is a direct redistribution of flows:

• ABA suppresses cell elongation, directing resources to osmolyte and protective protein synthesis.
• Ethylene accelerates aging and abscission of organs (leaves, ovaries), freeing resources for survival but destroying potential yield elements.
• Decreased cytokinin levels weakens meristem activity, slowing growth.

The plant switches metabolism from "growth mode" to "survival mode." And survival is not yield.

The Price of Adaptation in Numbers: Examples

To imagine the scale of costs, here are some specific data (Connor et al., 2011; Medvedev, 2012).

1. Drought:

• According to data from Australia, wheat yields during drought can decrease by 40–60% even in drought-tolerant varieties.
• Proline accumulation during drought can reach 10–100 mg/g dry weight – this is up to 10% of the total dry biomass that is diverted to defense instead of growth.
• Photosynthesis reduction due to closed stomata – up to 70–80% of the control.

2. Salinity:

• Under salinity (100–200 mM NaCl), yields of sensitive crops (beans, corn) can decrease by 80–90% even with defense mechanisms present.
• In salt-tolerant crops (barley, beet), the costs of maintaining ion balance can reduce yield by 20–40%.
• Maintaining Na⁺/H⁺-antiporter operation requires up to 20% of root respiration energy.

3. Cold stress:

• In thermophilic crops (cucumber, tomato) at 10 °C, growth can completely stop. Even in hardened plants, production output is delayed by 7–14 days.
• Synthesis of antifreeze proteins in winter rye can divert up to 5–10% of the leaf nitrogen pool.

4. Heat stress:

• At 40 °C, photosynthesis drops by 50–70% in most C₃ plants.
• Synthesis of heat shock proteins (HSP) in the first hours of stress can account for up to 1% of all newly synthesized RNA and occupy up to 30% of ribosomes.

Three Strategies from a Yield Perspective

Based on the growth-defense trade-off, three fundamental strategies of plants under stress can be distinguished:

1. Strategy "Defend but don't grow" (tolerance).

• The plant actively activates defense systems (synthesizes osmolytes, chaperone proteins, enhances antioxidant defense).
• Growth is strongly slowed down, but the plant survives.
• Yield decreases, but there is a guaranteed minimum.
• Examples: varieties of winter rye and winter wheat that successfully overwinter even in severe winters.

2. Strategy "Grow but risk" (vulnerability).

• The plant continues to grow, not spending resources on defense.
• Under short-term or mild stress, yield can be high.
• Under severe or prolonged stress, the plant dies or loses almost the entire crop.
• Examples: varieties of thermophilic crops that give high yields only in favorable years.

3. Strategy "Escape".

• The plant goes through its life cycle very quickly, aiming to produce seeds before the onset of stress.
• This is characteristic of ephemeral desert plants.
• In agronomy, the analogue is early-maturing varieties in regions with a short growing season or with predictable stresses at the end of the season.

What this means for the breeder:

• There are no universally resistant varieties with maximum productivity under all conditions.
• The choice of strategy depends on the agroclimatic conditions of the region.
• In regions with predictable but moderate stresses, one can bet on avoidance (rapid stomatal closure, powerful root system) – this gives smaller losses in bad years and high yields in good years.
• In regions with severe, frequent, or unpredictable stresses, the focus is on tolerance – a stable but not maximum yield.

Agronomic Conclusions:

1. Stress management is managing the trade-off. We cannot completely eliminate yield losses under stress, but we can minimize them by helping the plant distribute resources optimally.

2. Creating optimal conditions is key to realizing potential. If we create ideal conditions for the plant (water, nutrition, protection from diseases), it can "not spend" on defense and direct all resources to growth and yield. This is the basis of intensive farming.

3. Fertilizers can reduce adaptation costs. Improved nutrition (especially nitrogen, phosphorus, potassium) gives the plant more resources that can be shared between growth and defense. Therefore, yield losses from stress are often smaller on a good agronomic background.

4. Knowledge of critical developmental phases. In some phases, stress is particularly dangerous (flowering, fruit set); in others, the plant tolerates losses more easily. For example, if drought occurs during the tillering phase, the plant may recover; if during flowering, the yield loss can be catastrophic.

5. Stress protection must be timely. If the plant is already in distress, any fertilizing and irrigation will be of little effect. Prevention and early diagnosis are the basis of stress management.

Summary of Key Points:

• Adaptation requires expenditure of energy, carbon, nitrogen, and other resources.
• The plant takes these resources from growth and productivity.
• This is a fundamental growth-defense trade-off.
• The stronger the stress, the higher the price of adaptation and the greater the yield loss.
• The agronomist's task is not to eliminate stress entirely but to minimize its negative consequences and help the plant survive with the least losses.
• Understanding the price of adaptation allows choosing the right varieties and agronomic techniques for specific conditions.

Question for reflection: You are an agronomist in a region with frequent droughts at the end of the growing season. You have two wheat varieties: one is very productive but early-maturing (manages to ripen before the drought), the other is stable but later-maturing. Which variety would you choose and why? Assess the risk of yield loss for each variety depending on the nature of the drought.

In the next section, we will move to a practical question: how to measure stress? We will discuss physiological, biochemical, and molecular criteria for stress, and also analyze how these criteria can be used for diagnosing plant conditions in the field and in the laboratory.

What Are We Measuring When We Measure Stress?

Essentially, we are measuring the deviation of physiological and biochemical parameters from the norm. The stronger the deviation, the stronger the stress. However, there is one important nuance:

The same deviation can mean a different degree of stress for different plants.

For example, a 10-fold increase in proline content for one wheat variety is a sign of strong adaptation (eustress), while for another it is a sign of already developing distress. Therefore, when assessing stress, one must always consider the species and variety specificity, as well as the dynamics of changes.

Classification of Stress Criteria

All criteria of stress can be divided into several groups.

Key Principles for Interpreting Indicators

What Does This Mean for the Agronomist?

1. Early diagnosis of water deficit.

Instead of waiting for the plant to wilt, one can measure leaf water potential (Scholander pressure chamber) or relative water content. If the water potential has dropped below a critical value (e.g., for wheat – –1.5 MPa), it is time to water.

2. Assessing varieties for resistance.

In breeding, biochemical markers (proline, SOD activity) are used for rapid screening of a large number of samples. Varieties with high background proline content and rapid accumulation under stress are candidates for resistance.

3. Monitoring the effectiveness of agronomic practices.

If you apply fertilizers, growth stimulants, or hardening, you can evaluate their effectiveness using physiological indicators. For example, treated plants should show less reduction in water potential and less MDA accumulation under stress.

4. Predicting yield losses.

Based on the degree of growth inhibition (slowing of gain) at the onset of stress, one can estimate how seriously the yield will suffer. If growth stops during a critical phase (flowering), losses can be catastrophic.

Important Caveat

No indicator of stress is absolutely specific.

• Proline increases during drought, salinity, and cold.
• ABA increases under water deficit but can also increase due to mechanical damage.
• MDA is a marker of membrane damage but can also increase with aging.

Therefore:

• A diagnosis cannot be made based on a single indicator.
• A comprehensive approach is required.
• The context must be considered: the type of stress, the plant's developmental phase, weather conditions.

Summary:

• Stress is measured by the deviation of physiological and biochemical parameters from the norm.
• Main groups of criteria: physical (water potential, RWC), biochemical (proline, ABA, MDA, antioxidants), molecular (gene expression, stress proteins), and integral (growth, yield).
• Indicator dynamics are more important than their absolute values.
• Compare with a healthy control and consider the background level.
• One indicator does not provide a complete picture – a comprehensive approach is required.
• For the agronomist, knowledge of physiological criteria allows earlier diagnosis of stress, evaluation of varieties, and monitoring the effectiveness of agronomic practices.

Question for reflection: You are irrigating a wheat field. Which indicator – leaf water potential or the appearance of the plants – will allow you to make a decision about irrigation a day earlier? Why? What would you measure first?