Stress physiology

Last updated: July 10, 2026 Русский Español

1. Why Does a Plant Almost Never Live in Ideal Conditions?

When we study plant physiology in laboratory conditions, we create an optimally comfortable environment for them: optimal temperature, sufficient lighting, regular watering, balanced mineral nutrition. Under such conditions, we observe how the mechanisms of photosynthesis, respiration, growth, and development operate. However, in nature, the picture is fundamentally different.

The Natural Environment Is Constant Variability

Imagine a field. This morning—clear sunshine and a light breeze. An hour later, clouds roll in, and the temperature drops by several degrees. By noon, a strong wind picks up, drying out leaves and causing plants to intensively lose moisture. After lunch, a downpour begins, and the roots find themselves in water. By evening, the sun is out again.

This is not an exception—it is the norm of life for any plant. Air temperature can fluctuate by 20–30°C over the course of a single day (Crawford, 2012). Soil moisture changes from complete saturation to critical deficit. Illumination varies from bright sunlight to deep shade. Winds, frosts, droughts, floods, salinization—these are not random catastrophes but regular elements of plant habitats (Medvedev, 2012).

Adaptation Is Not an Exception but the Norm

Plants, unlike animals, cannot escape unfavorable conditions. They are attached to their place of growth. Therefore, through evolution, they have developed remarkable abilities not just to survive environmental changes but to actively adapt to them. This process is called adaptation (from Lat. adaptatio — adjustment).

As Vl.V. Kuznetsov and G.A. Dmitrieva (2006) note, adaptation is "the process of forming protective systems that ensure increased resistance and protection from unfavorable conditions for plants." It is important to understand that adaptation is not a single event but a continuous process that accompanies the plant throughout its entire life.

Even under conditions we consider optimal, the plant constantly adjusts its physiology: it regulates stomatal opening depending on air humidity, changes leaf orientation following the sun, and redistributes assimilates among organs. This is continuous adaptation.

2. What Is Physiological Stress?

In the previous section, we established that a plant never lives in ideal conditions but constantly adapts to environmental changes. Now we need to understand what physiological stress is, how to define it, measure it, and describe it. These are fundamental concepts without which we cannot move forward.

2.1. From General Concept to Physiological Definition

The term "stress" came into physiology from physics and mechanics, where it denotes a force acting on a system and the resulting deformation. This concept was introduced into biology by Walter Cannon in 1932 and then developed by Hans Selye in 1936. Selye defined stress as "the totality of all non-specific changes that arise in the body under the influence of any unfavorable and damaging factors" (cited in Medvedev, 2012). This definition contains two key words: non-specific and damaging.

However, in plant physiology, we must refine this definition. Why? Because a plant, unlike an animal, cannot actively avoid the stressor, but it is capable of remarkably diverse and complex responses.

Physiological stress in plants is a state of the organism that arises when external factors exceed the optimal range and cause disturbances in homeostasis, accompanied by the activation of protective mechanisms at all levels of organization—from molecular to organismal.

This definition includes three critical components:

1. Exceeding the optimal range — not every environmental change constitutes stress. As long as the change remains within the species' norm of reaction, it is merely regulation.

2. Homeostasis disruption — stress is always accompanied by deviations of physiological parameters from normal values.

3. Activation of protective mechanisms — stress is not passive suffering but an active process of mobilizing defense systems.

2.2. Basic Concepts: Stressor, Stress, Adaptation, Resistance

Before we go further, let's clearly define the key terms we will use throughout this course. This is important because there is sometimes some terminological confusion in scientific and educational literature.

Stressor (Stress Factor)

A stressor is an environmental factor that exerts an adverse effect on a plant, pushing its physiological parameters beyond the optimal range (Medvedev, 2012).

Stressors can be:

1. Abiotic — physical and chemical factors of non-living nature:

Temperature: heat, cold, frost
Water: drought, flooding, waterlogging
Salinity: soil salinization
Light: excess light, light deficiency, ultraviolet
Mechanical: wind, damage, soil compaction
Chemical: heavy metals, pollutants, ozone stress

2. Biotic — factors of living nature:

Pathogens: fungi, bacteria, viruses
Phytophages: insects, mites, nematodes
Competition with other plants
Parasitic plants

It is important to understand that the same factor can be a stressor for one plant species and an optimal condition for another. For example, for halophytes (salt-tolerant plants), increased salt content is not a stressor but a necessary condition for existence. For glycophytes (plants of non-saline soils), even moderate salinization is a strong stressor. As Kuznetsov and Dmitrieva (2006) note, "the nature of the stress effect depends both on the plant species (variety) and on the stress factor."

Stress (as a State of the Organism)

Stress is a complex of physiological, biochemical, and molecular changes that arise in a plant in response to the action of a stressor. It is not the stressor itself but the reaction to it.

For example, drought is a stressor. Stomatal closure, ABA synthesis, proline accumulation, and other changes—that is stress (the response to the stressor).

In plant physiology, several stages of stress development are recognized:

1. Alarm phase — the initial response to the stressor. Mobilization of defense systems occurs, often through "emergency" mechanisms (e.g., ROS synthesis as signaling molecules).

2. Acclimation phase — formation of resistance to the stressor, synthesis of protective proteins and metabolites, reorganization of physiological processes.

3. Resistance phase — maximum resistance to the stressor.

4. Exhaustion phase — if the stress is too strong or prolonged, defense systems become depleted, and irreversible damage occurs.

5. Recovery phase (repair) — after the stressor ceases, a return to normal functioning occurs (Kosová et al., 2020).

Adaptation

Adaptation (from Lat. adaptatio — adjustment) is the process of forming protective mechanisms that allow a plant to survive and develop under altered environmental conditions. As Kuznetsov and Dmitrieva (2006) write, adaptation is "the process of forming protective systems that ensure increased resistance and protection from unfavorable conditions for plants."

It is important to distinguish between adaptation in the broad sense (evolutionary) and in the narrow sense (ontogenetic):

Type of Adaptation	Characteristics	Example
Evolutionary (phylogenetic)	Formed through evolution, inherited, genetically fixed	Thick cuticle in xerophytes, CAM photosynthesis in succulents
Ontogenetic	Formed during the life of an individual plant in response to specific conditions	Cold hardening of plants, osmotic adaptation under salinity

Ontogenetic adaptation manifests in two forms (Kuznetsov and Dmitrieva, 2006):

Active adaptation — formation of new protective mechanisms, synthesis of new proteins and metabolites. Requires time and energy.
Passive adaptation — utilization of already existing mechanisms (e.g., stomatal closure during drought).

Resistance (Resilience)

Resistance (resilience) is the ability of a plant to withstand the action of stressors without significant disruption of physiological processes or with rapid recovery after stress.

Resistance can be:

Constitutive — innate, genetically fixed, manifested always, regardless of prior stress.
Induced — acquired, formed as a result of prior exposure to a stressor (hardening).

As S.S. Medvedev (2012) notes, "acquiring resistance under the influence of one unfavorable factor can cause an increase in the plant organism's resistance to other stress effects. This phenomenon is called cross-resistance, or cross-adaptation."

For example, cold hardening can increase resistance to drought as well, since both stresses trigger similar responses (synthesis of LEA proteins, osmotic adaptation).

2.3. Three Strategies of Resistance: Avoidance, Tolerance, Escape

Plants can withstand stress in three fundamentally different ways (Kuznetsov and Dmitrieva, 2006; Schopfer and Brennicke, 2016):

1. Stress Avoidance

The plant does not encounter the stress but avoids it. This is achieved through different means:

Avoidance in time: ephemerals and ephemeroids complete their life cycle before the unfavorable period begins. For example, desert ephemerals germinate after rain, flower, and produce seeds within 2–3 weeks, then survive drought as seeds (Kuznetsov and Dmitrieva, 2006).
Avoidance in space: a powerful root system allows water extraction from deep soil layers inaccessible to other plants.
Morphological adaptations: thick cuticle, pubescence, waxy coating, narrow or rolled leaves, absence of leaves (in saxaul)—all reduce water loss and prevent overheating (Medvedev, 2012).

2. Stress Tolerance

The plant encounters the stress but is able to withstand it without critical damage. This is achieved through:

Osmotic adaptation — accumulation of compatible osmolytes (proline, glycine betaine, sugars) that lower cell water potential and protect macromolecules (Kuznetsov and Dmitrieva, 2006).
Synthesis of protective proteins — LEA proteins, heat shock proteins, antifreeze proteins (Schopfer and Brennicke, 2016).
Antioxidant defense — enzymes (superoxide dismutase, catalase, peroxidases) and low-molecular-weight antioxidants (ascorbate, glutathione, carotenoids) (Arias et al., 2020).
Damage repair — restoration of DNA, membranes, proteins.

3. Stress Escape

This term is sometimes used as a synonym for avoidance, but some authors distinguish it as a separate strategy where the plant "escapes" from stress by entering a dormant state (seeds, spores, tubers, bulbs, buds). In dormancy, metabolism is minimal, and the plant can survive the harshest conditions (Schopfer and Brennicke, 2016).

2.4. Non-Specific and Specific Mechanisms of Resistance

In plant stress responses, the following are distinguished (Medvedev, 2012):

1. Non-specific mechanisms — activated under any stress. These include:

Synthesis of heat shock proteins (HSPs)
Synthesis of polyamines
Activation of antioxidant systems
Production of reactive oxygen species as signaling molecules

Non-specific mechanisms operate very quickly (minutes to hours).

2. Specific mechanisms — activated only under a particular type of stress. These include:

Synthesis of antifreeze proteins (under cold)
Synthesis of phytochelatins (under heavy metals)
Switch to CAM photosynthesis (under drought and salinity)
Synthesis of aerenchyma (under flooding)

Specific mechanisms require more time (hours to days) for their formation.

2.5. Why Is Stress Not Always Bad?

It is important to understand the key idea of modern stress physiology: moderate stress can be beneficial.

As Hopkins and Hüner (2009) note, plants in natural conditions constantly experience fluctuations in various factors, and "stress" in the laboratory sense is more the exception than the rule. An organism that has never experienced stress turns out to be less prepared for its sudden occurrence.

Examples of the positive effects of moderate stress:

Hardening — prior exposure to sublethal stress increases resistance to stronger stress.
Stimulation of defense systems — moderate ROS production activates antioxidant systems (Arias et al., 2020).
Improved product quality — in some crops, moderate stress increases the content of beneficial secondary metabolites (Lambers and Oliveira, 2019).

However, it is important to remember that severe or prolonged stress is always harmful and leads to irreversible damage.

2.6. From Concepts to Practice: How to Measure Stress?

To study stress, one must be able to measure it. In plant physiology, different approaches are used:

At the whole-organism level:

Visual assessment of damage (necrosis, chlorosis, wilting)
Growth measurement (biomass increment, height)
Yield determination

At the physiological level:

Measurement of leaf water potential
Assessment of photosynthetic intensity (gas exchange)
Determination of transpiration intensity
Measurement of pigment content

At the biochemical level:

Content of proline, sugars, and other osmolytes
Activity of antioxidant enzymes
ABA content
Level of lipid peroxidation (malondialdehyde)

At the molecular level:

Expression of stress-inducible genes
Synthesis of stress proteins
Proteomic analysis (Kosová et al., 2020)

In modern stress physiology, proteomics methods are actively used—studying the entire set of proteins synthesized in response to stress. As Kosová et al. (2020) show, proteomic analysis allows not only quantitative changes to be identified but also the appearance of new protein isoforms, post-translational modifications, and changes in protein localization.

Key point of the second section: Stress is not simply "bad" or "good." It is a complex, multi-stage process involving all systems of the plant. Understanding this process is the key to managing plant resistance.

Summary of the Second Section

We have introduced and discussed key concepts:

Stressor — an external factor causing stress.
Stress — the complex of the organism's responses to a stressor.
Adaptation — the process of forming resistance.
Resistance — the ability to withstand a stressor.
Three strategies of resistance: avoidance, tolerance, escape.
Non-specific and specific mechanisms of defense.
Moderate stress can be beneficial (hardening).

In the next section, we will move on to the main point: how exactly plant functioning changes under stress. We will see that stress is a systemic response affecting all physiological processes: water relations, photosynthesis, respiration, hormonal regulation, growth, and resource allocation.

3. How Does Plant Functioning Change Under Stress?

We already know that stress is not simply the plant's "poor health" but a complex, multi-level response affecting all physiological systems. Now let's examine exactly what changes occur and why they have adaptive significance.

Imagine the plant as a complex factory where water supply lines, the energy department (photosynthesis), the internal transport system, the signaling network, and production sites (growth) all operate simultaneously. Under stress, the dispatcher (signaling system) switches the factory to "emergency mode": some lines are shut down, others work at overload, and resources are redirected to the most critical areas—defense and survival.

3.1. Water Relations: The First Line of Defense

Water deficit is the most common stressor in nature (Schopfer and Brennicke, 2016). This is why the response to dehydration is one of the fastest and most powerful.

What Happens to Water?

When a plant loses water faster than it absorbs it, the water potential (ψ) in tissues drops. For most mesophytes, the critical value is ψ ≈ –1.5 MPa, at which permanent wilting occurs—cells lose turgor and can no longer restore it (Schopfer and Brennicke, 2016).

During drought, water potential can drop even further. For example, in some desert shrubs, it falls to –8.5 MPa (Crawford, 2020). However, for most cultivated plants, the loss of just 15–20% of water is fatal.

Reducing Losses: Stomatal Closure

The fastest protective response is stomatal closure. This occurs through two pathways:

1. Hydropassive closure — when the guard cells themselves lose water, they lose turgor and the stomata close (Schopfer and Brennicke, 2016).

2. Hydroactive closure — upon leaf dehydration, abscisic acid (ABA) is synthesized in cells, which reaches the guard cells and triggers potassium ion efflux, leading to turgor loss and stomatal closure (Hopkins and Hüner, 2009; Schopfer and Brennicke, 2016).

As Westgate (1994) notes, in maize, even a slight decrease in water potential (just 0.3 MPa) can completely arrest the growth of floral organs—so sensitive are reproductive structures to dehydration.

Osmotic Adaptation: Retaining Water in Cells

If drought develops gradually, plants can adapt through osmotic regulation (osmotic adaptation). They accumulate compatible osmolytes in the cytoplasm—low-molecular-weight organic compounds that do not interfere with metabolism but lower the water potential of cells (Kuznetsov and Dmitrieva, 2006).

Major osmolytes:

Proline — one of the most universal osmoprotective amino acids. Its content under drought can increase tens to hundreds of times (Medvedev, 2012).
Glycine betaine — a quaternary ammonium compound that effectively protects enzymes from denaturation.
Sugars — sucrose, raffinose, trehalose.
Sugar alcohols — sorbitol, mannitol.

Thanks to osmolyte accumulation, the plant can maintain positive turgor even at low environmental water potential (Schopfer and Brennicke, 2016).

The Signaling Role of ABA

ABA is the main hormone of water stress. It not only triggers stomatal closure but also activates the expression of hundreds of genes encoding protective proteins, osmolyte synthesis enzymes, and growth regulators (Hopkins and Hüner, 2009).

Interestingly, the drought signal can come not only from leaves but also from roots. When roots encounter dry soil, ABA is synthesized in them and transported via xylem to leaves, causing stomatal closure even before the leaves themselves experience water deficit (Schopfer and Brennicke, 2016). This is a preventive strategy: the plant prepares for drought in advance.

3.2. Photosynthesis: Arrest and Protection

Photosynthesis is one of the processes most sensitive to stress. The reason is simple: photosynthesis requires fine coordination between the light (electron transport) and dark (carbon assimilation) phases. Any imbalance leads to damage.

Why Does Stress Disrupt Photosynthesis?

During drought, stomata close, and CO₂ stops entering the leaf. However, light continues to be absorbed, and the electron transport chain remains active. An imbalance arises: energy comes in, but the substrate for its use (CO₂) is absent. As a result, excess electrons are transferred to oxygen, generating reactive oxygen species (ROS) (Hopkins and Hüner, 2009; Arias et al., 2020).

Similar disturbances occur under heat (damage to photosystem II), salinity (disruption of thylakoid membrane structure), and cold (slowing of enzymatic reactions). Essentially, any stress that disrupts normal metabolism leads to photoinhibition—a reduction in photosynthetic efficiency under light (Schopfer and Brennicke, 2016).

What Is Photoinhibition?

Photoinhibition is a light-dependent reduction in photosynthetic activity that occurs under excess light during stress. Most commonly affected is photosystem II (PSII), particularly its reaction center (the D1 protein) (Schopfer and Brennicke, 2016).

In healthy plants, there is a D1 repair cycle: the damaged D1 protein is rapidly replaced by a new one. But under stress, protein synthesis slows down, and repair cannot keep pace with damage. As a result, photosynthetic efficiency decreases, which can be measured by the drop in the Fv/Fm ratio—an indicator of the maximum quantum efficiency of PSII (Hopkins and Hüner, 2009).

Antioxidant Defense

To prevent damage from ROS, plants activate a powerful antioxidant system (Arias et al., 2020). It includes:

1. Antioxidant enzymes:

Superoxide dismutase (SOD) — converts superoxide anion into hydrogen peroxide.
Catalase (CAT) — breaks down hydrogen peroxide into water and oxygen.
Ascorbate peroxidase (APX) — uses ascorbate to neutralize hydrogen peroxide.
Peroxidases and glutathione reductase — participate in the regeneration of damaged molecules.

2. Low-molecular-weight antioxidants:

Ascorbate (vitamin C) — water-soluble antioxidant.
Glutathione — a tripeptide involved in protection against oxidative stress.
Tocopherols (vitamin E) — fat-soluble antioxidants that protect membranes.
Carotenoids — not only participate in light harvesting but also quench excess energy (Schopfer and Brennicke, 2016).

Interestingly, ROS at moderate levels act not only as damaging factors but also as signaling molecules that activate defense systems (Arias et al., 2020). This is an example of how a "dangerous" substance becomes part of a protective signaling network.

3.3. Respiration: Metabolic Switching

Under stress, the pattern of respiration changes. In some cases, it intensifies (to provide energy for protective processes), and in others, it weakens (under severe tissue damage).

Heat Stress

At high temperatures, the respiratory quotient (RQ = volume of CO₂ released / volume of O₂ absorbed) can increase to 1.5–1.7 (Kuznetsov and Dmitrieva, 2006). This indicates that not only carbohydrates are being oxidized but also more reduced compounds (e.g., lipids), as well as uncoupling of oxidation and phosphorylation. Energy is dissipated as heat rather than stored in ATP.

Water Stress

Under drought, respiration may initially increase and then, under severe dehydration, become inhibited (Kuznetsov and Dmitrieva, 2006). This is associated with disruption of mitochondrial structure and decreased activity of respiratory enzymes.

Flooding (Hypoxia and Anoxia)

Under oxygen deficiency, the plant switches from aerobic respiration to anaerobic fermentation (Medvedev, 2012). This is a much less efficient way of generating energy (only 2 ATP molecules per glucose molecule instead of 36), but it allows survival under oxygen-free conditions.

Under anoxia, glycolytic and fermentative enzymes are activated: alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), pyruvate decarboxylase. Anaerobic stress proteins are synthesized, many of which are isoenzymes of these enzymes (Kuznetsov and Dmitrieva, 2006).

However, the accumulation of fermentation products (ethanol, lactic acid) leads to acidosis—acidification of the cytoplasm, which can cause irreversible damage. Flood-tolerant plants, such as rice, have mechanisms for the rapid removal or neutralization of these products (Medvedev, 2012).

3.4. Hormonal Balance: Restructuring of Signaling Systems

Stress causes profound changes in phytohormone content. This restructures the entire metabolism and directs plant development along a defensive pathway.

Hormone	Change under Stress	Primary Function
ABA	Sharply increases	Stomatal closure, protective protein synthesis, growth inhibition
Ethylene	Increases under mechanical damage, flooding	Accelerates senescence, leaf abscission, aerenchyma formation
Jasmonic acid	Increases upon damage, pathogen attack	Activation of defense responses against insects and diseases (Lambers and Oliveira, 2019)
Salicylic acid	Increases upon infection	Induction of systemic acquired resistance (Hopkins and Hüner, 2009)
Cytokinins	Decrease under drought, senescence	Inhibition of senescence, stimulation of growth
Auxins	Often decrease	Growth inhibition, assimilate redistribution
Gibberellins	Decrease under stress	Growth inhibition, reduced water demand

Special attention deserves ethylene. This gaseous hormone not only accelerates senescence and leaf abscission during drought but also plays a key role in adaptation to flooding. Under hypoxia, roots synthesize the ethylene precursor ACC (1-aminocyclopropane-1-carboxylic acid), which is transported via xylem to shoots, where it is converted into ethylene. Ethylene causes epinasty (downward bending of leaves) and the formation of aerenchyma—air spaces in roots and stems through which oxygen can be delivered to roots from the aboveground part (Medvedev, 2012; Schopfer and Brennicke, 2016).

3.5. Growth and Resource Allocation: Reorientation Toward Survival

Under stress, the plant radically changes its strategy for resource allocation. Instead of growth—defense. Instead of producing new leaves—preserving existing ones.

Inhibition of Shoot Growth

The fastest response to any stress is cessation of cell expansion growth. This occurs because growth depends on turgor, which falls under water deficit. In addition, the hormonal background changes: auxin and gibberellin content decreases, while ABA content increases (Kuznetsov and Dmitrieva, 2006).

Inhibition of shoot growth has adaptive significance: the area of transpiring surface decreases, and the plant loses less water. Leaves often become smaller, thicker, covered with a waxy coating or pubescence—these are xeromorphic changes (Medvedev, 2012).

Activation of Root Growth

Unlike shoots, roots under drought may even enhance growth if available moisture still exists in the soil. Roots grow deeper, following the decreasing moisture level (hydrotropism) (Kuznetsov and Dmitrieva, 2006).

The root-to-shoot mass ratio increases. This allows the plant to extract water from deeper soil layers, but because part of the assimilates are directed to roots rather than fruits and seeds, yield decreases.

Redistribution of Assimilates

Under stress, assimilates (photosynthetic products) are redistributed in favor of protective mechanisms. Synthesis of osmolytes, antioxidants, and stress proteins intensifies. This requires additional energy, so carbohydrate reserves (starch) are mobilized (Schopfer and Brennicke, 2016).

In many plants under drought, accelerated senescence and abscission of lower leaves are observed. Nutrients (nitrogen, phosphorus, potassium) are extracted from them and transported to young organs (Kuznetsov and Dmitrieva, 2006). This is a strategy of "self-sacrifice": old leaves give up resources to support the viability of the whole individual.

3.6. Synthesis of Protective Proteins: The Molecular Arsenal

Stress activates the expression of hundreds of genes encoding proteins that directly protect the cell from damage or help repair it. This is one of the most striking molecular phenomena in plant physiology.

Heat Shock Proteins (HSPs)

First discovered in Drosophila, heat shock proteins (HSPs) are synthesized under any stress that raises temperature, but also under drought, salinity, and heavy metals (Schopfer and Brennicke, 2016).

HSPs function as molecular chaperones—they help other proteins fold correctly, prevent aggregation of denatured proteins, and participate in their repair (Medvedev, 2012; Schopfer and Brennicke, 2016).

There are several HSP families:

HSP100 — participate in the disaggregation of protein aggregates.
HSP90 — regulate the activity of receptors and signaling proteins.
HSP70 — major chaperones, participate in the folding of newly synthesized and damaged proteins.
HSP60 — chaperones of mitochondria and chloroplasts.
Small HSPs (15–30 kDa) — protect against thermal denaturation, characteristic of plants (Kuznetsov and Dmitrieva, 2006).

In plants, HSPs are synthesized within minutes after temperature increase (Schopfer and Brennicke, 2016). After HSP synthesis, the plant acquires thermotolerance—the ability to withstand temperatures that were previously lethal.

LEA Proteins (Late Embryogenesis Abundant)

LEA proteins were first discovered in maturing seeds, where they protect tissues from dehydration. Later, it was found that they are synthesized under drought, salinity, and cold in all vegetative organs (Kuznetsov and Dmitrieva, 2006; Medvedev, 2012).

LEA proteins are extremely hydrophilic, containing many repeated polar amino acids (glycine, alanine, proline). They are capable of:

Retaining large amounts of water, preventing complete cell drying.
Stabilizing membranes and other proteins.
Acting as "molecular sponges," binding ions and preventing their toxic effects (Kuznetsov and Dmitrieva, 2006).

Dehydrins — are a subfamily of LEA proteins (group LEA II) that are particularly actively synthesized under water deficit and low temperatures (Medvedev, 2012).

Cold-Responsive Proteins (COR Proteins)

At low positive temperatures, many plants synthesize COR proteins (cold-responsive). Their synthesis is regulated through CBF/DREB transcription factors, which activate more than 100 genes (Kuznetsov and Dmitrieva, 2006).

COR proteins increase membrane cryostability, prevent ice formation in the protoplast, and stabilize protein structure.

Antifreeze Proteins (AFP)

Antifreeze proteins (AFPs) were first discovered in polar fish, then in insects and bacteria, and later in plants (Schopfer and Brennicke, 2016). They bind to ice crystals in the apoplast and inhibit their growth, thereby preventing mechanical cell damage.

Remarkably, in plants, AFPs are often modified forms of chitinases and β-1,3-glucanases—enzymes that also participate in pathogen defense (Schopfer and Brennicke, 2016). This is a brilliant example of cross-functionality and evolutionary economy.

3.7. Synthesis of Secondary Metabolites: Protection from UV and Phytophages

Under stress, the synthesis of secondary metabolites intensifies—compounds that do not participate directly in primary metabolism but play a key role in defense (Lambers and Oliveira, 2019).

Flavonoids — accumulate in the leaf epidermis and act as UV-B radiation filters, protecting chloroplasts from photo-oxidation. Their synthesis is induced by UV light and regulated by photoreceptors (Kuznetsov and Dmitrieva, 2006).

Alkaloids, glucosinolates, cyanogenic glycosides — protect against insect phytophages and pathogens (Lambers and Oliveira, 2019).

Phenolic compounds, tannins — accumulate in response to damage and slow down tissue digestion by animals.

Enhanced synthesis of secondary metabolites under stress is further evidence of how the plant redistributes resources: from growth to defense.

3.8. All Systems Work Together

We have examined each system separately, but in a real plant, they all act in concert. Here is an example of the chain of events during drought:

1. Roots perceive a decrease in soil moisture → synthesize ABA.

2. ABA is transported to leaves → triggers stomatal closure → transpiration and CO₂ uptake decrease.

3. In chloroplasts, an imbalance between light and dark phases develops → ROS are generated.

4. ROS activate antioxidant enzymes and signaling cascades → synthesis of HSPs, LEA proteins, and other protective proteins is triggered.

5. Simultaneously, levels of auxins and cytokinins decrease → shoot growth slows down.

6. Roots receive more assimilates → their growth is activated.

7. In leaves, osmolytes (proline, sugars) are synthesized → cell water potential decreases, facilitating water uptake.

8. Old leaves senesce, releasing nutrients to young ones.

9. If drought passes, ABA is degraded, and recovery of all processes begins.

As you can see, the stress response is not an isolated reaction of one system but a comprehensive reorganization of the whole organism aimed at survival.

Key point: Under stress, the plant reorganizes the functioning of all physiological systems: water relations, photosynthesis, respiration, hormonal balance, growth, and resource allocation—all are subordinated to one goal: to survive and, if possible, to leave offspring. This coordination is achieved through a complex signaling system, with phytohormones and reactive oxygen species playing leading roles.

4. What Lies Ahead? Key Questions of Stress Physiology

We have explored what stress is, how it manifests, and which plant systems are involved in the response. Now it is time to formulate the main questions we will seek answers to in subsequent lectures. These questions are arranged in a logical chain: from signal perception to recovery after stress. Each is a separate large topic, but all are closely interconnected.

4.1. How Does a Plant Know That Drought Has Arrived? (Perception and Primary Signaling)

Imagine this: the soil becomes dry. Plant roots have no eyes, no nervous system. How do they "sense" water deficiency? How does a plant perceive environmental changes at all?

This is the question of sensory mechanisms—how a physical or chemical signal from the external environment is converted into a biological signal within the cell.

What Do We Already Know?

We know that under drought, abscisic acid (ABA) is synthesized in roots and then transmitted to leaves. But how do roots "know" it is time to synthesize ABA? Which molecules serve as sensors?

What Lies Ahead?

Mechanosensors — membrane proteins that respond to changes in turgor and mechanical tension of the cell wall. When water potential decreases, the cell loses volume, and this can activate ion channels (Schopfer and Brennicke, 2016).

Osmotic sensors — proteins sensitive to changes in osmotic pressure or ion concentration in the cytoplasm.

ABA receptors — PYR/PYL/RCAR proteins that bind ABA and trigger the signaling cascade leading to stomatal closure and activation of defense genes (Hopkins and Hüner, 2009).

The signaling role of calcium — an increase in cytosolic Ca²⁺ concentration is one of the earliest and most universal responses to stress. How exactly does this "calcium signal" work?

Receptor kinases — proteins that phosphorylate other proteins, transmitting the signal further along the chain.

As S.S. Medvedev (2012) notes, "plants are capable of responding very quickly to signals from the environment and, by adapting to them, adjusting their development program." Understanding exactly how this happens at the molecular level is one of the most exciting areas of modern plant physiology.

Key question of this topic: What molecular "antennas" allow a plant to sense environmental changes and initiate protective programs?

4.2. Why Does Oxidative Stress Occur? (Intracellular Damage and Defense)

We have repeatedly mentioned reactive oxygen species (ROS) — superoxide anion, hydrogen peroxide, hydroxyl radical, singlet oxygen. This is perhaps the most universal link in the stress response. Almost any stress—heat, cold, drought, salinity, UV irradiation, pathogen attack—leads to ROS formation (Arias et al., 2020; Hopkins and Hüner, 2009).

Why Are ROS Formed Under Stress?

The reason is that stress disrupts the normal course of energy processes—photosynthesis and respiration. Electrons that should flow along the electron transport chain and ultimately reduce NADP⁺ or O₂ to water "escape" and are partially transferred to oxygen, forming ROS. This occurs when:

Stomata are closed, and chloroplasts lack CO₂ for reduction (drought)—electrons overload the electron transport chain and are transferred to O₂.
Thylakoid membranes are damaged (heat, cold)—the normal functioning of photosystems is disrupted.
Mitochondrial function is impaired (hypoxia, aging)—electrons leak to O₂ in the respiratory chain.

Why Are ROS Dangerous?

ROS are extremely reactive molecules. They can:

Damage membranes — trigger lipid peroxidation, leading to loss of membrane integrity and ion leakage.
Oxidize proteins — modify amino acids (especially cysteine, methionine, tryptophan), leading to loss of enzymatic activity and denaturation.
Damage DNA — cause mutations, strand breaks, formation of 8-hydroxyguanine.
Trigger programmed cell death (apoptosis).

How Does the Plant Defend Itself?

Plants possess a powerful antioxidant system, including enzymes and low-molecular-weight compounds (Arias et al., 2020). However, it is important to understand: ROS are not only enemies. At moderate concentrations, they act as signaling molecules, activating defense genes. This is an example of how the same molecule can be both a damaging agent and an alarm signal.

What Lies Ahead?

Mechanisms of ROS generation under different types of stress.
Main types of ROS and their properties.
Pathways of cell damage by ROS (lipids, proteins, DNA).
The antioxidant system: enzymes (SOD, catalase, peroxidases) and low-molecular-weight antioxidants (ascorbate, glutathione, tocopherols, carotenoids, flavonoids).
How ROS can act as signaling molecules.
The concept of the "oxidative burst" and its role in pathogen defense.

Key question of this topic: How does a plant balance the harmful and beneficial effects of reactive oxygen species, using them simultaneously as damaging agents and alarm signals?

4.3. How Does the Signal Spread Throughout the Organism? (Systemic Signaling)

Stress, even if it begins in one place (e.g., roots encountering dry soil), must trigger a response in all parts of the plant. How is information transmitted from roots to leaves, from one leaf to another?

What Mechanisms of Systemic Signaling Exist?

1. Hormonal pathway — ABA synthesized in roots is transported via xylem to leaves and triggers stomatal closure. This is a classic example of systemic hormonal signaling (Schopfer and Brennicke, 2016).

2. Electrical signals — plants, like animals, possess action potentials that propagate through tissues and can transmit information about damage (Lambers and Oliveira, 2019). They play an important role, for example, in mechanical injury.

3. Hydraulic signals — pressure changes in xylem vessels can propagate at speeds of up to several meters per second and serve as rapid signals of drought (Schopfer and Brennicke, 2016).

4. Volatile organic compounds — when damaged by insects, plants release volatile substances (terpenes, green leaf alcohols) that can serve as signals to neighboring plants (Lambers and Oliveira, 2019). This phenomenon is called the "eavesdropping effect"—neighboring plants "hear" the alarm signal and prepare for defense.

5. Calcium waves — an increase in cytosolic Ca²⁺ concentration can spread from cell to cell through plasmodesmata, transmitting the signal over distance (Medvedev, 2012).

What Lies Ahead?

How different types of systemic signals work and how they differ in speed and range of action.
How signals are integrated into a unified network (e.g., ABA and calcium signals).
How stress type is recognized—why the signal for drought differs from the signal for tissue damage.
How the plant distinguishes local from systemic signals (e.g., local leaf damage triggers jasmonate synthesis, while the systemic response involves ABA synthesis).

Key question: How does a plant integrate information from different parts of the body into a unified picture and coordinate a systemic stress response?

4.4. Why Do Different Plants Differ in Resistance? (Genetics and Physiology of Resistance)

This is perhaps the most practical question. We know that some wheat varieties tolerate drought well while others do not. Some plant species live on saline soils (halophytes), while others die there (glycophytes). What is the reason?

Resistance as a Complex Trait

Stress resistance is not a single gene or trait. It is a complex set of morphological, anatomical, physiological, and biochemical features, each with its own genetic basis (Kuznetsov and Dmitrieva, 2006).

For example, drought resistance may be determined by:

The ability to rapidly close stomata at the first signs of water deficit.
The efficiency of osmotic regulation (ability to synthesize proline and other osmolytes).
The depth and branching of the root system.
The ability to synthesize LEA proteins and other protective proteins.
The efficiency of the antioxidant system.
The speed of repair of damaged structures (Schopfer and Brennicke, 2016).

Differences at the Molecular Level

Comparative studies of resistant and sensitive genotypes show that in resistant plants:

Protective proteins are synthesized constitutively (always) or more rapidly in response to stress (Kosová et al., 2020).
The antioxidant system operates more efficiently (Arias et al., 2020).
Membranes contain more unsaturated fatty acids, providing fluidity at low temperatures and stability under heat (Schopfer and Brennicke, 2016).
Ion transport systems function more efficiently (e.g., the SOS system in Arabidopsis under salinity) (Medvedev, 2012).

What Lies Ahead?

How genetic differences determine differences in resistance.
Which physiological parameters can be used for screening resistant genotypes (marker-assisted selection).
How the SOS system of salinity tolerance works.
Which genes and proteins are responsible for resistance to drought, cold, heat, and hypoxia.
What cross-adaptation is and why it is important for breeding.

Key question of this topic: Which molecular and physiological mechanisms determine whether a given variety will be stress-resistant, and how can we use this knowledge in breeding?

4.5. How Does a Plant Recover After Stress? (Repair and Return to Normal)

Stress ends sooner or later. Rain follows drought. Thaw follows frost. The plant survived, but it is damaged. How does it recover?

The Repair Phase

After the stressor ceases, the recovery phase (repair) begins. This is not simply a "return to the original state" but an active process requiring significant energy expenditure (Kosová et al., 2020).

At the molecular level, the repair phase is characterized by:

Enhanced synthesis of proteins and enzymes (including energy metabolism) to restore damaged structures.
Degradation of damaged proteins and their replacement with new ones.
Restoration of membrane integrity.
DNA repair (Hopkins and Hüner, 2009).
Return of hormonal balance to normal (decrease in ABA, increase in cytokinins and auxins).

Not Always Complete Recovery

It is important to understand that recovery is not always complete. If the stress was too strong or prolonged, irreversible damage may remain:

Some chloroplasts may have been destroyed irreversibly.
Some cells may have died.
Mutations may have occurred in DNA.
Carbohydrate and other resource reserves may have been depleted (Schopfer and Brennicke, 2016).

Therefore, the yield of plants that have experienced severe stress is often lower, even if they appear healthy externally.

What Lies Ahead?

Mechanisms of DNA repair (photoreactivation, excision repair, post-replication repair) (Kuznetsov and Dmitrieva, 2006).
Restoration of membranes and organelles.
Dynamics of protein synthesis during recovery.
How stress can alter the plant's developmental program even after it has ended.
What determines whether a plant will fully recover or whether consequences will remain.

Key question of this topic: How does a plant "remember" the stress it has experienced, and what mechanisms allow it to return to normal functioning after a stress episode?

4.6. All Questions Are Linked in a Unified Process

All five questions are stages of one continuous process:

Perception → Intracellular reaction (ROS) → Systemic signaling → Formation of resistance (or death) → Recovery

Understanding each link in this chain is the key to practical management of plant stress tolerance.

5. Why Is Stress Physiology Important to the Agronomist?

We have understood that a plant never lives in ideal conditions, that stress is a complex and multi-stage process affecting all physiological systems, and that ahead lies the study of many questions—from signal perception to recovery after damage.

But the most important question remains: why does an agronomist need all this? Why study complex molecular mechanisms if you can simply water, fertilize, and treat with pesticides?

The answer is simple: because modern crop production is stress management. And the better we understand these mechanisms, the more effectively we can manage.

5.1. Crop Production Is Stress Management

When an agronomist goes out into the field, they do not see laboratory greenhouse conditions. They see:

Soil that may be too dry or too wet.
Temperature that may drop sharply at night or rise to critical levels during the day.
Wind that dries leaves and breaks stems.
Weeds competing for water and nutrients.
Pests and diseases attacking weakened plants.

The agronomist's task is not to create ideal conditions (that is impossible) but to minimize stress and help the plant cope with what cannot be changed. And for this, one needs to understand how the plant responds to stress.

As Savin et al. (2015) note, in Mediterranean regions, "water stress is considered the dominant factor limiting yield." But understanding stress physiology shows that the problem is not only the amount of water but also the timing of its availability, the plant's ability to use water efficiently, and the balance between water and nitrogen nutrition.

5.2. Yield Losses from Stress: Numbers That Make You Think

How serious is the problem of stress for agriculture? The data presented by Hopkins and Hüner (2009) are striking:

Crop	Record Yield, kg/ha	Average Yield, kg/ha	Losses, %
Maize	19,300	4,600	76
Wheat	14,500	1,880	87
Sorghum	20,100	2,830	86
Potato	94,100	28,300	70

Source: Bray et al., 2000, cited in Hopkins and Hüner, 2009.

Between 70 and 87% of potential yield is lost due to stress! These are not just numbers. They represent the food security of millions of people. They represent farm economics. They represent the survival of farmers in arid regions.

And importantly: a significant portion of these losses can be prevented or reduced through proper stress management—choosing resistant varieties, optimizing agricultural practices, and applying growth regulators. But for this, one needs to understand the physiology.

5.3. What Does Knowledge of Stress Physiology Give the Agronomist?

Informed Selection of Varieties and Hybrids

Varieties of the same species can differ dramatically in stress resistance. Knowledge of physiological mechanisms allows:

Selecting varieties with the desired traits for specific soil and climatic conditions.
Understanding why one variety is drought-resistant and another is not (e.g., due to more efficient osmotic regulation or a deeper root system).
Using molecular markers for accelerated breeding of resistant varieties (Kosová et al., 2020).

For example, studies have shown that maize drought resistance is closely linked to the ability to maintain photosystem II activity and efficiently operate the antioxidant system (Westgate, 1994). These traits can be used as markers in breeding.

Optimization of Sowing Dates and Methods

Understanding critical periods of stress sensitivity allows:

Shifting sowing dates to avoid the coincidence of sensitive phases (flowering, grain set) with the most unfavorable periods.
Selecting varieties with different growing season durations (early, medium, late).

A classic example is the revision of ideas about "terminal drought." Previously, it was thought that the most critical period was grain filling. However, research by Savin et al. (2015) shows that the pre-flowering period (formation of generative organs) is no less, and often more, important for yield formation. It is at this time that the number of grains is determined, which ultimately determines potential yield.

This knowledge changes agronomic practice: efforts are now directed at providing plants with water and nutrients precisely during the pre-flowering period, not only during grain filling.

Water Regime Management

Understanding how a plant regulates water relations under stress allows:

Optimizing irrigation regimes (when to water, how much water to give to avoid waterlogging stress).
Using drip irrigation methods to save water and reduce soil salinization (Kuznetsov and Dmitrieva, 2006).
Applying mulching to retain soil moisture.
Using drought-resistant rootstocks for fruit crops.

As Schopfer and Brennicke (2016) note, even short-term water deficit can cause irreversible damage to reproductive organs in maize. Therefore, timely irrigation during the critical period can save the crop.

Application of Growth Regulators

Knowledge of hormonal regulation of stress allows the use of exogenous growth regulators:

ABA — can be used to close stomata and reduce transpiration during drought (although its use is limited due to high cost and rapid degradation).
Cytokinins — slow down leaf senescence under stress, maintain photosynthesis.
Brassinosteroids — increase resistance to drought, cold, and salinity.
Salicylic acid — increases resistance to diseases and abiotic stresses.
Polyamines — protect membranes from oxidative stress (Kuznetsov and Dmitrieva, 2006).

As Lambers and Oliveira (2019) note, "knowledge of the chemical compounds that protect plants, preferably with identification of the genes encoding the traits, will allow us to develop crop plants that are better protected against phytophages." The same applies to abiotic stresses.

Mineral Nutrition Management

Stress physiology helps understand how stress affects nutrient uptake and utilization. This allows:

Adjusting fertilizer rates and timing depending on stress conditions.
Using micronutrients to increase resistance (e.g., silicon increases resistance to drought and salinity).
Understanding why fertilizer efficiency decreases under stress and how to compensate for this.

An important conclusion: under stress, the plant reallocates resources from growth to defense. Therefore, the optimal nitrogen dose for yield is smaller than for biomass (Savin et al., 2015). Excess nitrogen under dry conditions can even be harmful, stimulating excessive vegetative growth and exacerbating water stress ("hay-off"—a phenomenon where excess nitrogen leads to premature plant drying).

Pest and Disease Management

Stress weakens plants and makes them more vulnerable to pathogens and pests. Understanding this allows:

Timely implementation of protective measures when plants are weakened by stress.
Using induction of systemic resistance (e.g., with salicylic acid or jasmonates) to enhance immunity (Hopkins and Hüner, 2009; Lambers and Oliveira, 2019).
Selecting varieties resistant to both stress and diseases (cross-adaptation).

For example, in some plants, hardening to cold or drought increases resistance to pathogens. This phenomenon is associated with the activation of common defense systems, including the synthesis of PR proteins (pathogenesis-related proteins) (Schopfer and Brennicke, 2016).

5.4. Practical Example: Rethinking Strategy in Mediterranean Regions

Let's consider a concrete example of how knowledge of stress physiology changes agronomic practice.

In Mediterranean-climate regions (Southern Europe, North Africa, the Middle East, parts of Australia), it was traditionally believed that the main problem was terminal drought, i.e., water deficit during grain filling. Accordingly, breeding and agronomic practices were aimed at "stretching" water use until the end of the growing season.

However, recent research (Savin et al., 2015) has shown that this view is incomplete. It turned out that:

1. Yield is primarily determined by the number of grains, not their size. And the number of grains is set during the period from the beginning of stem elongation to flowering.

2. Stress during this critical period (pre-flowering) has a much greater impact on yield than stress during grain filling.

3. Therefore, strategies aimed only at protecting grain filling are ineffective. The period of generative organ formation must also be protected.

This has led to a revision of breeding programs: attention is now paid not only to drought resistance at the end of the growing season but also to the ability to form a large number of flowers and grains even under moderate pre-flowering stress.

In addition, the view that barley is more drought-tolerant than wheat has been reconsidered. Comparative studies have shown that under the same conditions, the yields of barley and wheat often do not differ (Savin et al., 2015). This calls into question the traditional practice of sowing the driest lands with barley rather than wheat.

5.5. The Future: From Stress Management to Resilience Management

Modern stress physiology goes beyond simple "how the plant responds to stress." We are moving toward understanding how resilience can be managed:

Genetic engineering — transfer of resistance genes from resistant species to crop plants. Transgenic plants resistant to drought (with LEA protein genes, osmolytes) and salinity (with ion transporter genes) have already been created (Kuznetsov and Dmitrieva, 2006).

Marker-assisted selection — using molecular markers for accelerated breeding of resistant varieties (Kosová et al., 2020).

Priming (preconditioning) — treating plants with low doses of a stressor to activate defense systems and increase resistance to stronger stress. This method is already used for hardening seedlings to cold and drought.

Signaling molecules — using ABA, salicylic acid, jasmonates to "switch on" protective programs without the stressor itself (Lambers and Oliveira, 2019).

5.6. Concluding Thought: The Agronomist as a "Stress Manager"

We have become acquainted with the basic concepts, seen how stress affects all systems of the plant, and understood why this knowledge is critically important for the agronomist.

An agronomist is not just a person who "waters and fertilizes." An agronomist is a manager of the complex "plant–environment" system, constantly balancing between optimal and stressful conditions, helping the plant use its genetic potential as efficiently as possible, despite constant environmental changes.

Understanding stress physiology gives the agronomist:

1. Diagnostic tools — the ability to recognize signs of stress at early stages.

2. Forecasting tools — anticipating critical periods and preparing for them in time.

3. Management tools — selecting varieties, agronomic practices, and growth regulators to minimize losses.

4. Understanding of causes — why the plant is "behaving poorly" and what can be done about it.

Brief Summary of the Entire Lecture

1. A plant never lives in ideal conditions — the environment is constantly changing, and adaptation is the norm, not the exception.

2. Stress is a state of the organism arising from the action of stressors (unfavorable factors) and accompanied by homeostasis disruption and activation of protective mechanisms.

3. Adaptation is the process of forming resistance. It can be evolutionary (genetically fixed) and ontogenetic (formed during life).

4. Under stress, all physiological systems change: water relations (stomatal closure, osmotic adaptation), photosynthesis (inhibition, ROS formation), respiration (metabolic switching), hormonal balance (increase in ABA, ethylene; decrease in cytokinins), growth (shoot inhibition, root activation), and synthesis of protective proteins (HSPs, LEA, COR, antifreeze proteins).

5. Five key questions lie ahead: how the plant learns about stress, why oxidative stress occurs, how the signal spreads throughout the organism, why different plants differ in resistance, and how recovery occurs.

6. Stress physiology is important to the agronomist because crop production is stress management. Knowledge of stress mechanisms allows informed selection of varieties, optimization of agronomic practices, application of growth regulators, and minimization of yield losses.

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