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Acclimation, hardening and stress memory

Last updated: July 10, 2026 Русский Español

1. What Is Acclimation?

In plant physiology, a key concept that serves as our starting point for discussing stress responses is acclimation. However, to understand its place within the body of knowledge, we must clearly distinguish three fundamentally different mechanisms that are often confused in everyday usage: evolutionary adaptation, physiological acclimation, and stress memory. Let us examine them in order.

Evolutionary Adaptation: What Is Fixed in the Genes

Evolutionary adaptation (phylogenetic adaptation) refers to changes that occur at the population level through natural selection over many generations (Kuznetsov and Dmitrieva, 2006). Such adaptations have a genetic basis, are heritable, and become established as species-specific traits.

Classic examples include the anatomical and morphological features of xerophytes: thick cuticles, leaf pubescence, reduced leaf blades, and extensive root systems. These traits evolved in plants from arid habitats and are expressed even when they are grown under favourable conditions. This type of adaptation results from long-term evolutionary selection; it is constitutive and reliable, but... inflexible.

The key distinction: evolutionary adaptation is a property of a species or population that is inherited; acclimation is a property of a particular organism, acquired during its lifetime and not passed to offspring (Hopkins and Hüner, 2009).

Physiological Acclimation: A Flexible Response to Environmental Change

Unlike evolutionary adaptation, acclimation is a reversible physiological reorganisation that occurs during the lifetime of a single plant in response to changing environmental conditions. It does not involve genetic mutations and is not inherited. Acclimation is the realisation of a genetic programme already present in the plant, but which is only "switched on" under certain conditions (Kuznetsov and Dmitrieva, 2006).

I. I. Tumanov (one of the founders of the theory of hardening) emphasised that acclimation is not a passive suffering of the organism under a stressor, but an active process of metabolic reorganisation aimed at achieving a new stable state. This is precisely why acclimated plants can survive conditions that would be lethal for non-acclimated ones.

Forms of acclimation can be divided by timescale:

1. Short-term acclimation — rapid responses developing within minutes or hours (e.g., stomatal closure under water deficit, changes in membrane transport systems). This is essentially a stress response that operates "here and now". Sometimes such responses are called stress response in the narrow sense.

2. Long-term acclimation — slower but more fundamental rearrangements that require synthesis of new proteins, changes in enzyme systems, and even morphological characteristics. This is where we speak of hardening. Such acclimation takes days to weeks (Hopkins and Hüner, 2009).

It is important to understand: acclimation is not merely "getting used to" in the everyday sense. It is an energy-consuming, programmed process aimed at restoring homeostasis under new, altered conditions. Schematically, this process can be represented as follows:

Stressor → Homeostasis disruption → Stress signal → Metabolic reorganisation → New stable state (acclimation)

It is worth noting that acclimation is based on phenotypic plasticity — the ability of a single genotype to produce different phenotypes depending on environmental conditions. It is precisely this plasticity that allows a plant to adapt to a changing world without waiting for evolutionary changes (Hopkins and Hüner, 2009).

Stress Memory: The Ability to Remember Troubles

The third concept we must introduce is stress memory. This is the plant's ability to retain information about a stress it has experienced and to respond more rapidly to its recurrence. Unlike acclimation, stress memory can persist long after the stressor itself has disappeared and acclimatory adjustments have subsided (Taiz et al., 2023).

For example, a plant that has once experienced drought may retain a "memory" of that event in the form of altered metabolic status, modified proteins, or changes in the epigenetic landscape. Upon a second drought, such a plant activates its defence mechanisms much faster (a phenomenon called "priming").

Thus, we have a system of three levels of protection:

Level	Timescale	Heritable	Example
Evolutionary adaptation	Eons	Yes	Xeromorphic leaf structure
Acclimation (hardening)	Days–weeks	No	Accumulation of compatible osmolytes
Stress memory	Hours–months	Partially	Accelerated response to repeated stress

Hardening as the Central Mechanism of Acclimation

So, acclimation is a broad concept encompassing all adaptive reorganisations. But when we speak of hardening, we mean precisely long-term acclimation that develops in response to prolonged action of a stressor and leads to the formation of resistance to that factor (Schopfer and Brennicke, 2016).

Classical plant physiology distinguishes different types of hardening depending on the nature of the stressor. Here, it is important to understand the general logic: hardening is not merely a response to a specific factor, but a systemic reorganisation of the organism affecting membranes, water relations, carbohydrate metabolism, and the synthesis of protective proteins.

In the next part of the lecture, we will examine how hardening occurs in response to different types of stress and see that behind the outward diversity of responses lie strikingly similar mechanisms. It is this similarity that underpins cross-adaptation — the ability of one stress to increase resistance to another, completely different in its physical nature. And this is perhaps the most remarkable property of the plant organism.

2. Hardening

Now that we have clarified the concept of acclimation and its place within the system of defence mechanisms, we can turn to the central topic of our lecture — hardening. In plant physiology, hardening is understood as long-term acclimation that develops in response to prolonged action of a stressor and leads to the formation of resistance to that factor (Schopfer and Brennicke, 2016).

It is important to emphasise straight away: hardening is not passive "getting used to" or a gradual weakening of sensitivity. It is an active, energy-consuming reorganisation of the organism, involving synthesis of new proteins, changes in membrane composition, switching of metabolic pathways, and even morphological changes. This is why hardening takes time — from several days to several weeks (Hopkins and Hüner, 2009).

We will examine three classical types of hardening: cold, heat, and osmotic. But beneath the external differences, we will look for commonalities — those universal mechanisms that make cross-adaptation possible.

2.1. Cold Hardening

Let us start with cold hardening, as it is the most thoroughly studied and serves as a classical model for understanding acclimation processes.

From Stress to Resistance: Sequential Stages

When a plant from a temperate zone (e.g., winter wheat or rye) is exposed to low positive temperatures (around +2…+5 °C), a cascade of sequential rearrangements is triggered that ultimately increases its frost resistance. These rearrangements affect all levels of organisation — from membranes to gene expression (Schopfer and Brennicke, 2016; Kuznetsov and Dmitrieva, 2006).

Stage one: Membrane remodelling

When temperature drops, membrane lipids transition from a liquid-crystalline to a gel state, sharply reducing the activity of membrane-bound enzymes and disrupting solute transport. Cold hardening is aimed at preventing this phase transition. How is this achieved?

The plant increases the proportion of unsaturated fatty acids in membrane phospholipids. Double bonds in fatty acid chains create "kinks" in the structure, preventing tight packing of molecules. As a result, the membrane retains fluidity at lower temperatures (Schopfer and Brennicke, 2016; Lambers, 2019).

Key players here are desaturases — enzymes that introduce double bonds into fatty acid chains. Their activity is induced by cold, and this is precisely why cold-tolerant varieties have a higher proportion of unsaturated lipids in their membranes. Membrane remodelling is the foundation for everything that follows, because only functional membranes can maintain transport, energy metabolism, and signalling (Hopkins and Hüner, 2009).

Stage two: Osmotic adaptation

Cold hardening, paradoxically, has much in common with drought tolerance. At low temperatures, root water uptake decreases, and the plant faces the threat of dehydration. Moreover, when frost occurs, water leaves the cells into the intercellular spaces, where ice crystals form, further exacerbating protoplast dehydration (Schopfer and Brennicke, 2016).

In response, the plant accumulates osmotically active substances — soluble sugars (primarily sucrose), amino acids (especially proline), and other compatible osmolytes. These are not merely "nutrient reserves". By accumulating in the cell sap, these compounds lower the freezing point and, more importantly, reduce the protoplast water potential, lessening the gradient by which water leaves cells during extracellular ice formation (Kuznetsov and Dmitrieva, 2006; Pessarakli, 2020).

Interestingly, in winter cereals, starch accumulated in autumn is hydrolysed to sucrose, which performs this protective function. Upon warming, the reverse process occurs — conversion of sucrose back to starch. This dynamic is a vivid example of the reversibility of acclimation processes.

Stage three: Synthesis of protective proteins

Cold acclimation is accompanied by activation of cold-inducible genes (COR genes) . These genes encode proteins that directly protect cellular structures from damage caused by freezing and dehydration (Schopfer and Brennicke, 2016; Taiz et al., 2023).

Among COR proteins, dehydrins deserve special attention. These are small, extremely hydrophilic proteins belonging to the LEA protein family. They act, in essence, as "molecular sponges" — binding water that might otherwise form ice crystals, and stabilising membranes and other proteins under dehydrating conditions. In addition to dehydrins, other proteins are synthesised that protect enzymes from denaturation and maintain chloroplast structure (Hopkins and Hüner, 2009; Pessarakli, 2020).

Stage four: Antifreeze proteins

In some plants, especially winter rye, cold hardening induces synthesis of antifreeze proteins (AFPs) . Their function is fundamentally different: they do not affect the freezing point of the solution but physically bind to ice crystals in the apoplast, preventing their further growth and recrystallisation. This prevents small crystals from turning into large ones that could mechanically damage the plasmalemma (Schopfer and Brennicke, 2016; Taiz et al., 2023).

Remarkably, many AFPs are structurally related to pathogen defence enzymes (e.g., chitinases and β-1,3-glucanases). This is a brilliant example of evolutionary economy: the same protein can be used to solve different tasks.

The Regulatory Network of Cold Hardening

The key signal for triggering cold hardening is a drop in temperature. However, signal perception and transduction is a complex multistep process involving:

1. Changes in membrane fluidity as the primary physical signal.

2. Increase in cytosolic Ca²⁺ — a secondary messenger.

3. Activation of CBF/DREB transcription factors — key regulators of COR genes. These switch on hundreds of genes responsible for protective protein synthesis and metabolic rearrangements (Schopfer and Brennicke, 2016; Taiz et al., 2023).

Important: the CBF pathway is not the only one, but it is the most studied and likely the main pathway for cold acclimation in temperate-climate plants.

2.2. Heat Hardening

Now let us turn to heat hardening. Unlike cold hardening, the problem here is different: proteins begin to denature, membranes become overly fluid, and metabolic pathways become uncoupled. However, as we shall see, the plant's response is built on similar principles.

Heat Shock Proteins: Emergency Protection

If a plant is suddenly transferred from a comfortable temperature (e.g., +25 °C) to a critically high one (say, +42 °C), its response is immediate: membranes are damaged, electron transport in photosystem II is disrupted, normal protein synthesis is suppressed — synthesis of "housekeeping" proteins stops, and the cell switches to synthesising heat shock proteins (HSPs) (Schopfer and Brennicke, 2016; Kuznetsov and Dmitrieva, 2006).

This is an astonishing phenomenon: in a critical situation, the cell urgently synthesises a new set of proteins that it did not actively use under normal conditions. HSPs belong to the class of chaperones — proteins that help other proteins fold correctly, prevent their aggregation, and aid in restoring the structure of denatured molecules (Taiz et al., 2023).

The mechanism is simple and elegant. Under normal conditions, HSPs are bound to other proteins or regulatory elements, in particular to heat shock transcription factors (HSFs). When temperature rises, denatured proteins with exposed hydrophobic patches appear. These patches act as "traps" for HSPs — HSP molecules switch from HSFs to denatured proteins, releasing HSFs, which immediately activate synthesis of new HSPs. Once the denatured proteins are refolded, HSPs rebind HSFs, and HSP synthesis ceases (Hopkins and Hüner, 2009; Schopfer and Brennicke, 2016).

Hardening to Heat: Pre-emptive Protection

But HSPs are an emergency measure. What if the temperature rises gradually? In that case, the plant has time to harden to heat. It synthesises HSPs and other protective proteins before critical overheating occurs. As a result, when a sudden temperature spike occurs, the hardened plant already has these proteins present; they are already protecting the cell. This is why plants grown at slightly elevated temperatures withstand sudden heat shocks significantly better (Schopfer and Brennicke, 2016).

Here we see a fundamental principle of hardening: the organism does not merely respond to ongoing damage — it prepares for possible future stress using incoming signals.

In the case of heat stress, the signals are the elevated temperature itself, as well as the accumulation of misfolded proteins and reactive oxygen species. The plant "pre-emptively" activates a defence system that under normal conditions is "idle" or operates at a minimal level.

2.3. Osmotic Hardening

The third type of hardening we will examine is osmotic hardening. It develops in response to water deficit and salinity. And here we come close to the realisation that all stresses, in essence, often boil down to a single problem — loss of water by the cell.

What Is Osmotic Hardening?

Under water shortage, the plant faces two threats: a decrease in soil water potential makes water less available, and excessive ion accumulation (under salinity) worsens the situation. The classical plant response to water deficit is osmotic adaptation: active accumulation of soluble substances in the cell to lower the water potential (Lambers, 2019; Schopfer and Brennicke, 2016).

Here it is important to understand the fundamental difference between a passive and an active osmotic response. When water is lost, the concentration of cell sap increases passively — simply because cell volume decreases. But osmotic hardening is an active process requiring energy and synthesis of new metabolites.

Key osmolytes include:

1. Proline — a universal amino acid that accumulates in almost all plants under water deficit. Besides its osmotic function, proline acts as an antioxidant and protein stabiliser (Pessarakli, 2020; Taiz et al., 2023).

2. Glycine betaine — a quaternary ammonium compound, particularly characteristic of plants in the family Chenopodiaceae. It effectively protects enzymes, especially photosystem II, from inactivation by high salt concentrations (Lambers, 2019).

3. Sugars and sugar alcohols — sucrose, sorbitol, mannitol, and others. They not only participate in osmotic regulation but also stabilise membranes under dehydrating conditions (Hopkins and Hüner, 2009).

Osmotic Hardening in Practice

Osmotic hardening can be observed in a simple experiment. If pea seedlings are gradually accustomed to increasing concentrations of polyethylene glycol (which creates osmotic stress but does not penetrate cells), they accumulate proline and sugars, lowering their water potential from -0.5 MPa to -1.5…-2.0 MPa. Hardened seedlings are able to survive osmotic stress that would immediately kill non-hardened plants (Schopfer and Brennicke, 2016; Taiz et al., 2023).

It is important to emphasise: osmotic hardening is not an increase in water content, but a reduction in the plant's own water potential. The plant becomes "thirsty" for water — it can extract it from the soil at a lower, more negative water potential. This is a fundamental difference from a "water storage" strategy — the plant does not become more succulent; it becomes more "absorbing".

2.4. A General View of Hardening

Thus, we have examined three different types of hardening — cold, heat, and osmotic stress. At first glance, they have little in common. But looking deeper, we can identify common principles:

1. Activity, not passivity: hardening is a metabolic reorganisation requiring synthesis of new proteins and energy expenditure.

2. Signal nature: the environment provides a signal (temperature, osmotic potential, presence of denatured proteins), and the plant launches a protective programme.

3. Shared defence systems: the same mechanisms (accumulation of osmolytes, synthesis of chaperones, membrane remodelling) are engaged under different stresses.

4. Preparation for the future: hardening is not just a response to ongoing damage, but a pre-emptive measure.

5. Reversibility: after the stressor is removed, hardening is gradually lost, and the plant returns to a "comfortable" state (except for cases of stress memory, which we will discuss later).

It is precisely this last point that is especially important for understanding practical applications: hardening seedlings before transplanting to open ground is a controlled use of these mechanisms.

But the most remarkable and practically important property of hardening is still ahead — cross-adaptation, the ability of one type of stress to increase resistance to another, completely different factor. We will devote the next part of our lecture to this phenomenon.

3. Why Do Different Stresses Help Each Other?

We have come to the most interesting and, from a practical standpoint, the most important question of our lecture. We have examined three types of hardening — cold, heat, and osmotic. At first glance, they have nothing in common: different signals, different primary damage, different defence mechanisms. And yet... a plant hardened to cold often withstands drought better. A plant that has experienced mild drought turns out to be more resistant to salinity. A plant that has undergone heat hardening may better tolerate water deficit. This phenomenon is called cross-adaptation, or cross-hardening.

The main question we must clarify: why do different stresses help each other? The answer to this is the key to understanding the entire modern physiology of plant stress.

3.1. The Common Denominator: All Stresses → Oxidative Stress

Paradoxically, the first and main answer lies on the surface: virtually all abiotic stresses (heat, cold, drought, salinity, excess light, heavy metals) lead to the same secondary effect — production of reactive oxygen species (ROS) (Taiz et al., 2023; Hopkins and Hüner, 2009).

What are ROS? They are oxygen molecules with enhanced reactivity: superoxide anion (O2^{•-}), hydrogen peroxide (H2O2), hydroxyl radical (OH^{•), and singlet oxygen (}1O2). They form wherever electron transfer occurs with disruption of the normal process: in the electron transport chains of chloroplasts and mitochondria, in peroxisomes, and at the plasmalemma (Schopfer and Brennicke, 2016).

How is this related to stresses?

Cold — slows down enzymatic reactions of photosynthesis but does not stop light absorption. As a result, electrons "get stuck" in the electron transport chain and are transferred to oxygen, forming ROS.
Heat — denatures proteins, uncouples transport chains, which also leads to electron leakage to oxygen.
Drought and salinity — cause stomatal closure, reduce CO_2 entry, and light energy not used for carbon fixation goes into ROS formation.
Excess light — the classic case: more energy is absorbed than can be used, and ROS are formed as a "dump" of the excess.

Thus, ROS are a universal secondary signal and a universal damaging factor under any stress (Taiz et al., 2023; Lambers, 2019).

3.2. ROS as a Signal: One Language for Different Messages

Here lies the key to understanding cross-adaptation. ROS are not just metabolic "waste". At moderate concentrations, they serve as signalling molecules. Their appearance in the cell is a message: "Attention! Something has gone wrong! Somewhere electron flow is disrupted! Activate protective programmes!" (Taiz et al., 2023).

Remarkably, the same signal (an increase in H₂O₂ or superoxide) can result from very different disturbances: thermal protein denaturation, membrane cooling, stomatal closure under drought. The cell "understands" this signal and launches a common defence programme aimed primarily at neutralising ROS — antioxidant defence (Schopfer and Brennicke, 2016; Kuznetsov and Dmitrieva, 2006).

Thus, the universal ROS detoxification system (superoxide dismutase, catalase, ascorbate-glutathione cycle) is activated under any stress. This is why a plant acclimated to one stress turns out to be better prepared for another — it already has its antioxidant defence "switched on" (Pessarakli, 2020; Taiz et al., 2023).

3.3. Common Signalling Pathways: Ca²⁺ and MAPK Cascades

But signalling via ROS is only the first level. Behind it come more complex and specific signalling cascades, and here too we find surprising similarities between different stresses.

Calcium Signalling (Ca²⁺)

Almost all stressors cause an increase in cytosolic Ca²⁺ concentration. Cold, heat, drought, salinity, mechanical damage — all of them, in one way or another, activate Ca²⁺ channels. The causes may differ: changes in membrane fluidity, activation of mechanosensitive channels, release of Ca²⁺ from intracellular stores. But the result is the same — a calcium spike (Taiz et al., 2023).

Ca²⁺ serves as a secondary messenger, activating Ca²⁺-dependent protein kinases (CDPKs). These kinases, in turn, phosphorylate numerous target proteins, including transcription factors, ion channels, and metabolic enzymes. Notably, the same set of CDPKs is activated under very different stresses (Lambers, 2019).

MAPK Cascades

Another universal signalling pathway is the MAPK cascades (mitogen-activated protein kinases). These are three-step pathways: MAPKKK → MAPKK → MAPK. They are activated not only by ROS and Ca²⁺, but also by the stress signals themselves. Importantly, the same MAPK cascade can be activated by different stressors (Taiz et al., 2023).

For example, in plants, MAPK cascades activated simultaneously by heat, drought, and salinity have been well studied. As a result, common transcription factors are phosphorylated and activated, which switch on defence genes common to all these stresses. The cell does not create a separate "molecular switch" for each stress — it uses a common signalling language, and specificity of response is achieved through fine-tuning (Schopfer and Brennicke, 2016).

3.4. Abscisic Acid (ABA): The Master Stress Hormone

Abscisic acid (ABA) deserves special attention. This phytohormone is often called the "stress hormone". Its concentration rises sharply under drought, salinity, cold, and also under some other stresses.

ABA is a universal mediator of stress responses (Taiz et al., 2023). It:

Closes stomata under water deficit.
Induces synthesis of LEA proteins and dehydrins.
Promotes osmotic adaptation.
Participates in triggering COR gene expression under cold.

And here again we see overlap: the same hormone — ABA — is activated by different stressors and launches similar defence programmes. Therefore, hardening to drought (which leads to ABA accumulation) can increase resistance to cold — a mechanism mediated by ABA (Schopfer and Brennicke, 2016; Taiz et al., 2023).

3.5. Common Effector Mechanisms: Protective Proteins and Osmolytes

Signalling pathways converge on the activation of common effector proteins — the molecules that directly carry out protective functions. And here cross-adaptation is most clearly visible.

Heat Shock Proteins as Universal Chaperones

We have already discussed HSPs as defence against heat. But HSPs are induced not only by heat. Cold, drought, salinity, heavy metals, even wounding — all these factors can activate the expression of some HSPs (Schopfer and Brennicke, 2016; Kuznetsov and Dmitrieva, 2006).

Why does this happen? Because any stress can cause protein denaturation — due to high temperature, due to dehydration (which changes ionic strength and macromolecular concentration), due to toxic ions. And the cell, "detecting" denatured proteins, activates chaperone synthesis — HSPs that help restore their structure (Taiz et al., 2023).

Thus, HSPs are not just heat shock proteins, but universal "emergency responders" to protein damage of any origin. Therefore, a plant that has "trained" its HSP synthesis system in response to heat shock will be better protected against protein damage caused by drought or salinity (Hopkins and Hüner, 2009).

LEA Proteins and Dehydrins: Protection against Dehydration

LEA proteins (late embryogenesis abundant) and dehydrins are another class of universal protective proteins. They are activated under dehydration of any origin: drought, salinity, cold (freezing), seed maturation (natural dehydration) (Pessarakli, 2020; Taiz et al., 2023).

These proteins possess unique properties: they are extremely hydrophilic, lack a rigid three-dimensional structure, and can bind large amounts of water. They act as "molecular shields", protecting other proteins and membranes from damage during water loss. Any stress that leads to dehydration activates synthesis of these proteins (Schopfer and Brennicke, 2016).

Therefore, hardening to drought, which leads to accumulation of LEA proteins, automatically increases resistance to cold (because during freezing, water also leaves the cell) and to salinity (which also creates osmotic water deficit) (Hopkins and Hüner, 2009).

3.6. Mechanisms of Synergy: Preparing for the Future

Now we can formulate the answer to the main question: why do different stresses help each other?

A plant encountering the first stress reorganises its metabolism. This reorganisation includes:

1. Accumulation of "standby" protective proteins (HSPs, LEA, antioxidant enzymes) that remain in the cell for some time after the stress ends.

2. Strengthening of ROS detoxification systems (increased activity of superoxide dismutase, catalase, ascorbate peroxidase).

3. Accumulation of compatible osmolytes (proline, glycine betaine, sugars) that create a margin of safety in case of dehydration.

4. Changes in membrane properties (increased proportion of unsaturated lipids), making them less sensitive to both heat and cold.

5. Activation of common signalling pathways (ABA, MAPK, calcium signalling) which remain in a state of "heightened readiness".

When the second stress arrives, even if it is completely different in nature, the plant is already prepared: the defence systems are already partially activated, and the response to the new challenge occurs significantly faster and more effectively (Taiz et al., 2023; Schopfer and Brennicke, 2016).

3.7. Limits of Cross-Adaptation: Are There Boundaries?

It is important to understand that cross-adaptation is not unlimited. Firstly, it is effective only for stresses that cause similar secondary disturbances — that is, stresses that in one way or another lead to ROS stress, protein denaturation, or dehydration. Drought and salinity are close "relatives"; cold and drought also share a common component (dehydration). However, say, excess light and salinity are less directly connected, although here too we see overlap (both lead to ROS stress).

Secondly, hardening to one factor may weaken resistance to another if their metabolic requirements contradict each other. For example, hardening to heat may "shift" the temperature optimum of physiological processes, making the plant less resistant to sudden cold. Such an effect results from different optimal temperatures for enzyme function, leading to imbalance under abrupt condition changes.

And finally, most importantly: hardening requires energy. Accumulation of protective proteins and osmolytes costs resources that could otherwise go to growth and reproduction. Therefore, hardening always comes at the expense of productivity. This is the very "price of resistance" we have already mentioned (Hopkins and Hüner, 2009; Taiz et al., 2023).

3.8. A Metaphorical Model

To solidify understanding, let us propose a simple model. Imagine that the cell has a "standby team" of antioxidant enzymes and chaperones — this is a "rapid response brigade". Under any stress (fire, flood, earthquake — as an analogy for stresses), exactly this brigade is activated. Upon a second blow, even if it is of a different type, the brigade is already in place and ready for action. Moreover, after the first stress, the brigade's numbers may have been increased — additional protective proteins have been accumulated, reserves of osmolytes created.

The key point is that nature did not create a separate "team" for each type of disaster — it built a universal system that responds to any homeostatic disturbance. This is evolutionary wisdom: maintaining a universal system is cheaper than maintaining dozens of specialised ones. And this is precisely what makes cross-adaptation possible.

Section conclusion: different stresses help each other because they activate common signalling pathways (ROS, Ca²⁺, MAPK, ABA) and common defence systems (antioxidants, HSPs, LEA proteins, osmolytes). Hardening to one factor "trains" these common systems, and they remain in a state of heightened readiness when another challenge arises. This gives the plant a significant advantage in an unstable, fluctuating environment.

In the next section, we will examine how long this state of heightened readiness persists — and move on to the concept of stress memory.

4. Stress Memory

We have reached the last, and perhaps most intriguing, section of our lecture. We have established that hardening is a reversible metabolic reorganisation that increases resistance to stress. But what happens after the stressor disappears? Does the plant "forget" about the trials it has endured? It turns out, no. Plants possess an amazing ability to retain "memories" of stress — a phenomenon called stress memory (stress imprint) (Taiz et al., 2023).

Here we encounter a fundamentally important refinement: if hardening is a current state of increased resistance that is maintained only as long as the stressor acts, then stress memory is the ability to respond faster and more effectively to repeated stress even after the primary acclimatory changes have already subsided (Schopfer and Brennicke, 2016; Taiz et al., 2023).

4.1. What Is Stress Memory? Definition and Criteria

Stress memory is the ability of a plant to retain information about a stress it has experienced for some time after the stress has ceased, and to use this information for accelerated and enhanced activation of defence responses upon re‑exposure to the same or a similar stressor (Lambers, 2019; Pessarakli, 2020).

For stress memory to be spoken of, two key conditions must be met:

1. A time gap — between the first and second stress exposure there must be a period sufficient for the primary acclimation (the increased resistance associated with the continued presence of protective proteins and metabolites) to have subsided.

2. Accelerated or enhanced response — upon repeated stress, the plant must respond faster (in terms of gene activation, protein synthesis, physiological changes) or more strongly (greater amplitude of the protective response) compared to the first exposure.

Thus, stress memory is not simply "I haven't recovered from last time". It is an actively maintained state of "readiness", in which some regulatory systems remain in an altered state even in the absence of stress.

4.2. Timescale of Stress Memory

It is important to distinguish types of stress memory by their duration. Three main timescales can be identified (Schopfer and Brennicke, 2016; Taiz et al., 2023):

1. Short-term memory (hours – a few days)

This is the fastest form of memory. It is based on the retention in the cell of modified proteins and metabolites that were synthesised during the first stress. For example, after a heat shock, heat shock proteins and active forms of antioxidant enzymes remain in the cell. If a new stress occurs within 1–2 days, these proteins are already present — meaning protection is activated faster.

Short-term memory also includes the state of priming — when signalling molecules (e.g., ROS) remain at a slightly elevated basal level, and upon repeated exposure they reach the threshold concentration for activating defence genes more quickly (Taiz et al., 2023; Lambers, 2019).

2. Medium-term memory (weeks – months)

This form of memory already requires changes at the level of gene expression and regulatory networks. For example, in plants that have experienced drought, an altered profile of DNA methylation or histone modifications associated with promoters of stress-inducible genes persists for several weeks. This keeps these genes in a "half-open" state, accessible for rapid activation (Taiz et al., 2023).

An important example is cold memory. In winter cereals, after cold acclimation has ended, elevated levels of CBF transcription factors and some COR proteins persist for some time. If a second cold spell occurs after a few weeks, these plants respond faster than those that have not experienced cold (Schopfer and Brennicke, 2016).

3. Long-term memory (months – years, sometimes transgenerational)

The most remarkable form of stress memory can persist for long periods and even be inherited (though not through classical mutations). This is epigenetic memory — changes in chromatin structure (DNA methylation, histone modifications) that persist after stress ends and can be transmitted during cell division and even to the next generation via seeds (Taiz et al., 2023).

For example, in plants that have experienced drought, some stress-inducible genes may remain hypomethylated for a long time, ensuring their easier activation upon repeated drought. Cases are known where plants whose maternal plants were exposed to stress show enhanced resistance from early developmental stages (Taiz et al., 2023).

However, as textbook authors rightly caution, we should not delve into epigenetics as a separate discipline. Our task is to show that mechanisms of long-term regulation exist and are not reducible to simple gene "on-off" switches.

4.3. Mechanisms of Stress Memory: From Molecules to Regulatory Networks

On what, then, does stress memory rely, if protective proteins have already degraded and osmolytes have been consumed? Several interrelated mechanisms are at its core.

Protein and Enzyme Modifications

Some enzymes may remain in a phosphorylated state or in another post-translationally modified form even after the stressor has ceased (Pessarakli, 2020). For example, MAPK cascades may retain some residual activity, keeping the system "primed". Such modifications are relatively short-lived but can persist for days or even weeks.

Changes in Transcriptional Regulation

Stress can alter the accessibility of gene promoters to transcription factors. If during the first stress some transcription factors (e.g., CBF, DREB) bound to chromatin and "opened" it for transcription, this open chromatin may persist for some time. This phenomenon is called chromatin priming (Taiz et al., 2023). Upon repeated stress, transcription factors bind faster, and mRNA synthesis begins without delay.

Epigenetic Changes

As already mentioned, this is the most long-lasting form of memory. Changes in DNA methylation and histone acetylation can persist after stress and even be transmitted during cell division (Schopfer and Brennicke, 2016). Importantly, epigenetic changes can affect not only individual genes but entire regulatory networks, allowing the plant to "reprogram" its response to future stresses on a more global scale.

4.4. Examples of Stress Memory in Plants

Heat Memory

The phenomenon of heat priming is well studied. If a plant is subjected to a mild heat shock (e.g., 30–35 °C), and then after a few days to a severe one (42 °C), it shows significantly higher survival than a plant not subjected to the first shock (Schopfer and Brennicke, 2016; Hopkins and Hüner, 2009). The effect can persist for up to 5–7 days and is associated with residual levels of HSPs and an altered status of HSFs.

Drought Tolerance

In many species, drought tolerance is enhanced after a preliminary mild water deficit. Plants "trained" to drought accumulate proline and other osmolytes, and this effect can persist for several weeks. Moreover, after stress removal, such plants retain elevated expression of genes encoding LEA proteins and dehydrins (Taiz et al., 2023; Lambers, 2019).

Cold Memory in Winter Cereals

Winter cereals (wheat, rye, barley) undergo cold hardening in autumn, which provides them with frost resistance in winter. But even in spring, after thawing, they retain a "memory" of cold — during return frosts, they respond faster and with less damage than spring cereals that have not undergone hardening (Schopfer and Brennicke, 2016).

Memory in Woody Plants

In perennial plants, stress memory can persist for years. For example, larch or pine that has survived drought may have altered anatomy (narrower tracheids, thicker cell walls) for the next several years, reducing the risk of embolism during subsequent droughts (Lambers, 2019).

4.5. Why Is Stress Memory Not Always a Blessing?

Like any adaptation, stress memory requires resources. Maintaining altered chromatin, maintaining elevated enzyme levels — all this requires energy and plastic substances. Therefore, in the absence of repeated stresses, memory gradually erodes (which is energetically advantageous) (Taiz et al., 2023).

If a plant is constantly "reminded" of stress, but without actual stress (e.g., grown at a slightly elevated temperature without hardening), this can lead to reduced productivity — energy is spent on maintaining defence systems but not used for growth. In agronomy, this is well known: excessive hardening of seedlings can slow their growth, although it will increase resistance to field conditions (Schopfer and Brennicke, 2016).

4.6. Relationship between Hardening and Stress Memory

Now we can connect all four sections of our lecture into a unified picture:

1. Evolutionary adaptation — a "hard" strategy, fixed in the genes, which does not change during the plant's life.

2. Physiological acclimation (hardening) — a "flexible" strategy, allowing the plant to change its phenotype in response to current environmental conditions.

3. Cross-adaptation — a consequence of the fact that many stresses use common signalling pathways and defence systems, allowing the plant to use "experience" from one stress to prepare for another.

4. Stress memory — the ability to retain a "trace" of a stress experienced, in order to respond faster and more effectively to repeated exposure.

Thus, stress memory is not a separate mechanism, but rather a continuation of acclimation in time. It is what remains of hardening after the stressor itself has disappeared. It allows the plant to "remember" adverse conditions and be prepared for their possible return (Taiz et al., 2023).

4.7. A Metaphorical Model

If we return to our analogy of the "rapid response brigade", stress memory is not just an enlarged brigade. It is an improved alert system. After the first alarm, the dispatching service "remembers" the nature of the signal and next time will respond to it faster — even if the brigade has already returned to normal mode. It is not just about "keeping extra resources in reserve", but about "training" the detection and alert system.

Section conclusion: Stress memory is a real, experimentally confirmed phenomenon that allows plants to respond more quickly to repeated stresses. It can last from several days to several years and is based on changes in gene expression, chromatin structure, and regulatory networks. It is not merely a "delayed effect of hardening", but a separate level of regulation that gives the plant an evolutionary advantage in an unstable environment.

In the concluding, fifth section, we will examine how knowledge of these mechanisms is applied in practice — from growing seedlings to strategies for adapting agricultural crops to climate change.

5. Practical Significance

We have travelled from fundamental concepts to complex mechanisms of stress memory. But plant physiology is not only an academic science. Understanding how plants harden, remember stress, and use cross-adaptation has direct practical significance — from a small vegetable garden to global agriculture. In this concluding section, we will examine how knowledge of hardening is applied in agronomy, horticulture, and breeding, and discuss a paradoxical conclusion: overly comfortable conditions are not always beneficial.

5.1. Seedling Hardening: A Classic Example

Perhaps the best-known example of using hardening in practice is preparing seedlings for transplanting to open ground. Every experienced gardener knows: if seedlings grown in warmth and a greenhouse are planted directly into the bed, they can suffer severely from temperature fluctuations, wind, and direct sunlight. That is why seedlings are "hardened" — gradually accustomed to outdoor conditions.

What happens at the physiological level? Plants grown under optimal conditions are in a state of "relaxed" physiology: they have thin cuticles, a high proportion of unsaturated lipids in membranes (making them fluid but vulnerable to cooling), low levels of osmolytes and protective proteins. Upon abrupt transfer to the field, they experience multiple stress: cold (at night), excess light (during the day), wind (increasing transpiration). This can lead to photoinhibition, loss of turgor, and even death (Hopkins and Hüner, 2009; Schopfer and Brennicke, 2016).

Gradual hardening is a simulation of the acclimation process:

1. Temperature reduction — activates COR protein and dehydrin synthesis, increases the proportion of unsaturated lipids.

2. Increased illumination — activates antioxidant systems and the xanthophyll cycle for protection against photoinhibition.

3. Reduced watering — stimulates osmotic adaptation and ABA accumulation.

As a result, the seedlings go into the field already "prepared" — with more rigid membranes, a reserve of protective proteins and osmolytes, and an adapted photosynthetic apparatus. They establish faster and produce higher yields (Taiz et al., 2023). This is precisely why even a few days of gradual adaptation can dramatically change the fate of the crop.

5.2. Moderate Stress as a Tool for Increasing Resistance

The paradox of hardening: to make a plant stronger, it needs to be "troubled" a little. This principle underlies many agronomic practices.

Hardening Irrigation (Drought Priming)

In arid regions, intermittent irrigation is practiced — the plant receives water not continuously but at intervals. This stimulates osmotic adaptation (accumulation of proline, sugars), development of a deeper and more branched root system, and increased stomatal sensitivity to ABA. As a result, even short-term droughts that would otherwise significantly reduce yields are tolerated much more easily (Lambers, 2019).

Temperature Priming

In greenhouse operations, short-term temperature increases (heat shock) are sometimes used to activate HSPs and increase resistance to subsequent temperature fluctuations. This is especially relevant for crops that will later be transplanted to open ground or subjected to transport. Moderate heat hardening can also increase resistance to cold — a classic example of cross-adaptation (Schopfer and Brennicke, 2016).

Salt Priming

Under soil salinity conditions, pre-sowing seed treatment with weak salt solutions is used. Seeds germinating in the presence of small NaCl concentrations activate the SOS pathway and accumulate compatible osmolytes. As a result, seedlings are more resistant to subsequent salinity (Taiz et al., 2023; Pessarakli, 2020).

5.3. Why Are "Overly Comfortable" Conditions Not Always Beneficial?

This conclusion sounds paradoxical: how can one complain about excessive care? But from a plant physiology perspective, constant existence under ideal conditions reduces the ability to adapt.

Imagine a plant that has grown its entire life at optimal temperature, ideal watering, balanced nutrition, and protection from pests. Such a plant has:

Membranes rich in unsaturated fatty acids (more fluid, but also more vulnerable to cooling).
Antioxidant systems operating at a minimal level (no signal for their activation).
Stomatal apparatus not "trained" for rapid closure under water deficit.
Poorly developed root system (no stimulus for enhanced growth).
Minimal levels of ABA and other stress hormones.

Such a plant, once placed in field conditions with inevitable fluctuations in temperature, humidity, and light, proves helpless. It lacks protective proteins, osmolytes, and well-practiced signalling pathways (Taiz et al., 2023; Lambers, 2019).

This is why modern agronomy increasingly focuses on controlled stress — creating conditions in which plants experience moderate but manageable adverse effects. This allows them to "train" their defence systems without reducing yields to a critical level.

5.4. Using Cross-Adaptation in Breeding and Agronomy

Understanding cross-adaptation opens up broad opportunities for optimising crop production:

Resource Savings

If one stress (e.g., moderate drought) can increase resistance to another (e.g., cold or salinity), then "training" treatments can be applied that prepare the plant for a whole range of field stresses. This is especially valuable in regions with unstable climates, where drought, heat, and return frosts can alternate within a single season (Hopkins and Hüner, 2009).

Selection of Tolerant Genotypes

Breeders can use acclimation tests for early identification of tolerant genotypes. Plants capable of rapid and effective hardening have an advantage in unstable conditions. Importantly, it is necessary to evaluate not only the ability to withstand stress but also the ability to remember — the speed of response to repeated stress (Taiz et al., 2023; Pessarakli, 2020).

Predicting Cultivar Performance

Knowledge of hardening mechanisms allows prediction of how particular cultivars will behave under specific climatic conditions. For example, cultivars with high osmotic adaptation potential are better suited for arid regions; cultivars with an efficient HSP system are better for regions with sharp temperature fluctuations (Schopfer and Brennicke, 2016).

5.5. Hardening and Modern Challenges: Climate Change

Global warming is leading to an increase in extreme weather events: droughts, heat waves, abnormal frosts. Under these conditions, the ability of plants to harden and retain stress memory becomes critically important (Taiz et al., 2023).

Acclimation potential — the ability of a cultivar or species to adapt to changing conditions — is high in some species (e.g., winter cereals, many perennials) and low in others (e.g., tropical crops). Breeding for improved acclimation ability is a promising direction for adapting agriculture to climate change.

Furthermore, knowledge of stress memory mechanisms allows the development of irrigation and protection strategies that take into account not only current weather but also the "stress history" of the plants. For example, if plants have already experienced mild drought, they may better withstand a subsequent heat wave — and this can be used when planning agronomic measures (Lambers, 2019).

5.6. Ethical Aspect: Hardening vs Plant Welfare

In discussions about plant rights, the question is sometimes raised: is deliberate hardening (moderate stress) a form of mistreatment? From a physiological standpoint, short-term moderate stress is not only not harmful but beneficial for the plant, as it activates its own defence systems, increasing overall viability.

This resembles physical training in humans: moderate exercise strengthens the body, while its absence leads to deconditioning. In their natural environment, plants always experience stresses — and they are adapted to this. An absolutely stress-free existence is not natural for them and does not promote their long-term health (Taiz et al., 2023).

5.7. Final Summary: Hardening as the Foundation of Sustainable Crop Production

Let us bring together all the practical conclusions of our lecture:

Principle	Practical application
Acclimation is an active reorganisation	Seedling hardening, seed priming
Cross-adaptation is based on common signalling pathways	One stress (e.g., drought) can prepare for another (heat)
Stress memory allows faster response to repeated stresses	Accounting for stress history when planning agronomic measures
Overly comfortable conditions reduce resistance	Moderate stress as part of cultivation technology
Hardening requires resources but confers an advantage	Balance between productivity and resistance

Lecture Conclusion

We have travelled a long path — from defining acclimation to practical recommendations. The key conclusion we must take away is: the plant is not a passive recipient of environmental conditions, but an active regulator of its own resistance. It is capable not only of responding to stress, but also of remembering it, using experience from one stress to prepare for another, and "training" to increase its hardiness.

Hardening is not merely protection against a specific factor. It is a systemic reorganisation of the organism that makes it more resistant to a whole range of adverse conditions. And it is precisely this ability that allows plants to survive and even thrive in the most extreme corners of our planet.

For agronomists and plant growers, this knowledge is not merely an academic interest. Managing hardening is a real tool for increasing resistance and productivity of crops under an unstable climate. Moderate stress, controlled and dosed, can become a "medicine" against helplessness in the face of extreme weather events.

And perhaps the main paradox we have realised today is: to make a plant stronger, one must sometimes allow it to face difficulties. And in this lies a profound wisdom that nature has honed over millions of years.

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